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• Published: 28 August 2024

Risk assessment and categorization of terrorist attacks based on the Global Terrorism Database from 1970 to 2020

• Zonghuang Xu   ORCID: orcid.org/0000-0003-1952-9601 1   na1 ,
• Yao Lin 2   na1 ,
• Hongyu Cai 1 ,
• Wei Zhang 1 ,
• Jin Shi 1 &
• Lingyun Situ   ORCID: orcid.org/0000-0001-7552-6769 1  

Humanities and Social Sciences Communications volume  11 , Article number:  1103 ( 2024 ) Cite this article

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• Criminology
• Information systems and information technology
• Politics and international relations

Risk assessment and categorization of terrorist attacks can assist enhance awareness of terrorism and give crucial information support for anti-terrorism efforts. This study utilizes quantitative approaches for the risk assessment and categorization of terrorist attacks. A total of 210,454 terrorist attacks that occurred worldwide from 1970 through 2020 were collected in the Global Terrorism Database, and 22 indicators related to the risk of terrorist attacks were selected. Then, the moment estimation theory and four comprehensive evaluation models were utilized to identify the top 10 riskiest terrorist attacks in the world. Furthermore, the five clustering analysis methods and three evaluation criteria were performed for the risk categorization of terrorist attacks, and the visual analysis was carried out using the kernel density estimation method. The research results have identified the top 10 riskiest global terrorist attacks, which were led by the September 11 terrorist attack event, along with their downward counterfactual events. The spatial distribution of global terrorist attack risk is primarily composed of four “turbulent cores” in the region of Central Asia, Middle East & North Africa, South Asia, and Central America & Caribbean. This study also provided insights and recommendations for anti-terrorism efforts. It has realized the risk assessment and categorization of terrorist attacks, aiding in the swift identification of its risk levels, and holds immense significance for safeguarding global national security and societal stability under new circumstances.

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Introduction.

Terrorism poses a significant threat to global security and stability (X Sanchez et al., 2018 ). There have been at least 210,000 terrorist attacks globally since the end of the 1960s. According to the 2024 Global Terrorism Index (GTI), there were almost 3350 terrorist attacks globally in 2023, killing over 8000 people, and resulting in a 56% increase in the average number of people killed per attack (Institute for Economics and Peace, 2024 ). While the extent and impact of terrorism have fluctuated over time, it remains an important challenge that governments and international organizations seek to counter through intelligence, security efforts, and addressing root causes (Benmelech et al., 2012 ; Perliger and Pedahzur, 2016 ). In recent years, terrorism has shown characteristics such as decentralized actors, civilian targets, complex forms, politicized goals, diversified means, and regional focus. It also exhibits trends such as the coexistence of internationalization and localization, the convergence of small-scale and grassroots activities, and the realization of the concept of cyberterrorism. In some countries or regions, terrorist attacks may either merge with separatist organizations, infiltrate by religious extremist groups, or transform into “lone wolf” attacks (Atran, 2021 ; Luo and Qi, 2021 ; Manzanero et al., 2021 ; Shin et al., 2023 ; Vicent et al., 2021 ). Without eliminating the soil of hatred, injustice, and inequality, it is difficult to completely eradicate terrorism. Religious extremism, income inequality, and other factors may trigger terrorist attacks. Facing potential terrorist threats, intelligence gathering and early warning measures are crucial, but the anti-terrorism strategies of different countries or regions may vary in emphasis. Typical targets of terrorist attacks include transportation infrastructure, chemical plants, energy facilities, and even virtual online platforms (Al-Dahash et al., 2022 ; Stewart, 2018 ; Zuijdewijn and Sciarone, 2021 ).

At present, the assessment and categorization of the extent of risk caused by terrorist attacks and other catastrophic events are mostly done subjectively (Bridgelall, 2022 ; Etaywe, 2022 ). Authoritarian organizations or departments commonly pick several key factors and impose mandatory categorization rules (Gaibulloev and Sandler, 2021 ; Gibbs, 2020 ; Hasisi et al., 2020 ). The risk of a terrorist attack is connected not only to deaths and property damages, but also to the time, location, target, and a variety of other elements (Duru et al., 2021 ). Furthermore, the incidence of terrorism is highly complicated and is influenced by a variety of circumstances, including quantitative ones such as politics, economics, and society, as well as non-quantifiable factors such as religion, history, and culture (Frykberg, 2006 ; Hao et al., 2019 ; Hu et al., 2019 ; Jünger and Gärtner, 2021 ). Therefore, the categorization technique is overly subjective and one-sided, and it is impossible to address the core issue of assessing the degree of risk caused by terrorist attacks. The existing quantitative study on terrorist attacks around the world is mainly limited to the following issues: (i) The majority of the description is still qualitative, with only a few quantitative analyses (Li et al., 2021 ; Treistman, 2021 ). (ii) In terms of research time and area, the time span is brief or limited to a single country or region. (iii) In terms of indicator selection, most of them have only a few study indicators that are fairly simple. (iv) In terms of research methodologies, researchers typically employ an analytic hierarchy process, fuzzy comprehensive evaluation, and K -mean clustering analysis to conduct research, which is subjective and solitary, and lacks evaluation of research results (Davis and Zhang, 2019 ; Reid, 1997 ; Wang et al., 2021 ). Therefore, quantitative analysis and research on terrorist attacks are still in their infancy, and there is still plenty of potential for exploration.

We probe into the risk assessment and categorization of terrorist attacks through quantitative approaches. First, we obtained the terrorist attacks that occurred worldwide from 1970 through 2020 based on the Global Terrorism Database (GTD) and chose 22 indicators related to the risk of terrorist attacks. Then, based on the theory of moment estimation, we combine subjective and objective weighting methods to generate optimal weights and utilize the linear weighted evaluation (LWE), fuzzy comprehensive evaluation (FCE), technique for order preference by similarity to ideal solution (TOPSIS) method, and the particle swarm optimization projection pursuit evaluation (PSO-PPE) method to identify the top 10 riskiest terrorist attacks in the world. Furthermore, the fuzzy c -means (FCM), clustering using representative (CURE), density-based spatial clustering of applications with noise (DBSCAN), clustering in QUEst (CLIQUE) and the Gaussian mixture model (GMM) clustering analysis methods were performed for the classification of terrorist attacks, and the visual analysis was carried out using the kernel density estimation (KDE) method. Finally, we provide three insights and recommendations for anti-terrorism efforts. This work is critical for ensuring national security and social stability in the changing scenario. As a result, an in-depth study of data connected to terrorist attack risk can assist people in better understanding terrorism and give vital information to aid in anti-terrorism and preventative efforts.

We summarize our core contributions are: (i) A more holistic assessment of terrorist attack risk that moves beyond unilateral metrics like casualties and economic damages. We obtained terrorist attacks that occurred worldwide from 1970 through 2020 based on the GTD and selected 22 indicators related to the risk of terrorist attacks. (ii) Robust model evaluation through combining multiple assessment techniques. Based on the LWE, FCE, TOPSIS, and PSO-PPE methods, the normalized comprehensive scores of the four models were averaged to provide the overall comprehensive evaluation value. It addresses the shortcomings of previous related studies, which relied on unilateral assessments of economic losses and casualties from terrorist attacks, and the evaluation methodologies were too subjective and solitary to compare the assessment findings. (iii) Comprehensive risk categorization validated by clustering evaluation criteria and spatial-temporal visualization of risk patterns globally over five decades. This study selects the FCM clustering approach to categorize terrorist attacks based on the FCM, CURE, DBSCAN, CLIQUE, and GMM clustering methods and the outcomes of the three clustering evaluation parameters, which makes the results scientific and reasonable. The findings provide novel insights to enhance understanding of terrorism dynamics and tendencies, with the potential to inform policies and practices for promoting security and stability amidst the evolving terrorist threat landscape.

The remainder of this paper is organized as follows: Section “Methodology” describes the methodology, which includes the datasets, indicator correlation analysis, optimal weight method, and the risk assessment and categorization methods. Section “Results” presents the results of analysis. Section “Conclusion” concludes and provides three insights and recommendations for the current global terrorist attack scenario, anti-terrorism efforts, and stability maintenance.

Methodology

The internationally recognized terrorism databases currently available include the Global Terrorism Database (GTD), the International Terrorism: Attributes of Terrorist Events (ITERATE) dataset, the Terrorism in Western Europe: Events Database (TWEED), the RAND Database of Worldwide Terrorism Incident (RDWTI), and the World Incidents Tracking System (WITS). Among them, the GTD has compiled 210,454 terrorist incidents worldwide from 1970 through 2020 (START, 2024 ), making it one of the most comprehensive terrorism databases available and a widely utilized resource for terrorism research and analysis. However, the GTD also has certain limitations: (i) Like most terrorism datasets, the GTD suffers from selection bias as it relies on media reports and open-source information, which may fail to capture the full scope of terrorist activities, especially in areas with limited media coverage or restricted information flow. Underreporting and spatial–temporal biases are also challenges faced by the GTD, as data availability and quality may vary over time and location. (ii) The GTD only captures event-level information about the terrorist attacks themselves. It does not directly consider the broader societal, political, economic, ideological, and other environmental factors that could either enable or constrain terrorism. These contextual factors influencing terrorist activities are not included in the database. Despite these limitations, the GTD remains a valuable resource for quantitative analysis of global terrorism, providing a large structured dataset over an extended time period and broad geographical areas. To detect and evaluate irrelevant data, do a multidimensional analysis of the number of terrorist attacks per year, the weapon type, the attack region, the attacking country, the attack type, and the target/victim type on the obtained data. Then, using the quantification of non-numerical data as the key, we do data cleaning, transformation, classification, and other processing on the data to produce the indicator system of terrorist attack risk.

Data cleaning

We discovered data that was irrelevant to the study during the data exploration and analysis procedure. We concluded that the data follows the following rules: Doubtterr is 1, “unknown” data, the number of records with null values, and the number of records with missing key attributes. The following are the data rules and data cleaning rules:

Doubtterr is 1

In the suspected terrorism (doubtterr) indication, if the ambiguous and uncertain event is a terrorist attack, such an event in GTD will take the value “yes” in this variable. 1 = “Yes” suspects that an event is a terrorist attack; 0 = “No” does not suspect that an event is a terrorist attack, and the database will be recorded as “−9” if this variable is not included in the data collecting procedure while coding. We do not investigate these objective criteria and remove the Doubtterr is 1 and −9.

“Unknown” data

The “unknown” rule for deleting data includes Geocoding Specificity (specificity), Attack Type (attacktype1), Weapon Type (weaptype1), Target/Victim Type (targtype1), Property Damage (property), Extent of Property Damage (propextent), City (city), Name of Entity (corp1), Perpetrator Group Name (gname), Mode for Claim of Responsibility (claimmode), Number of Perpetrators (nperps), Number of Perpetrators Captured (nperpcap), Hostages or Kidnaping Victims (ishostkid), and International-Logistical (INT_LOG).

Number of null records

Latitude (latitude), Longitude (longitude), Total Number of Fatalities (nkill), Total Number of Injured (nwound), Extent of Property Damage (propextent), Name of Entity (corp1), Hostages or Kidnaping Victims (ishostkid), and First Perpetrator Group Suspected/Unconfirmed? (guncertain1) are all included in the total number of null records.

Indicator construction

This article selects as many indicators as possible relevant to the degree of risk caused by terrorist attacks and eliminates certain factors that have no influence on the risk. Second, because the text variables’ descriptions of event characteristics are repeated in other fields, such as the event summary field, which specifies the “time, location, person, content, process, and purpose” of the event, these variables are mirrored in other fields. Finally, variable fields with less than 20% completeness should be removed from the GTD database. For example, the second and third weapon type field is about 98% empty. Consequently, removing this variable directly has minimal effect on the statistics. As a result, the indicator system of the risk influencing factors of terrorist attacks is obtained in Table 1 .

Indicator correlation analysis

We conduct an independent analysis of 22 secondary indicators. The correlation analysis methods primarily comprise the Pearson coefficient, Spearman coefficient, and Kendall coefficient (FitzGerald et al., 2023 ). The Pearson coefficient is appropriate for indicators that follow the normal distribution. Evidently, the sample data does not follow the normal distribution. The Kendall correlation is more robust and efficient than the Spearman correlation in general. As a result, for correlation analysis, we employ the Kendall coefficient. The formula is as follows:

where \(\tau\) represents the Kendall correlation coefficient between the two indicators. n is the number of terrorist attacks. \({n}_{{\rm {c}}}\) and \({n}_{{\rm {d}}}\) are the number of concordant and discordant pairs. The greater the connection between the two indicators, the bigger the Spearman correlation coefficient.

Optimal weight method based on moment estimation theory

At present, the subjective weighting method primarily relies on the experience and subjectivity of experts to determine the weight and ranking of the evaluation indicators. The objective weighting method determines the weight of the indicators using a mathematical theory approach based on the relationship between the original data, which represents the link between the weight of the evaluation indicators and the change in the original data.

Based on the moment estimation theory of nine indicator-related variables, the subjective and objective weighting methods are combined to achieve the optimal weight. The method considers subjective non-negligibility of events, the internal relationship of each indicator to be assessed, and the weight gradient across time.

Subjective weighting method

Analytical hierarchy process.

The analytic hierarchy process (AHP) is a method for subjectively and statistically analyzing the hierarchical nature (Cranmer et al., 2021 ). It divides the important decision factors into three layers: the goal layer, the criteria layer, and the scheme layer. It determines the benefits and drawbacks of the decision-making scheme based on the experienced judgment of decision-makers. This is the foundation for qualitative and quantitative analysis. The specific procedure is as follows: (i) Build a hierarchy model. (ii) Construct a judgment matrix A. (iii) Hierarchical ranking and testing for consistency. (iv) Hierarchical general ranking and consistency testing.

Order relationship analysis method

The order relationship analysis (ORA) method is a subjective weighting method based on the analytic hierarchy (Miao et al., 2011 ). The expert’s judgment value on the ratio of a specific indicator to the relevance of all indicators is primarily represented by assessing the elements of each row of the matrix. The specific procedure is as follows: (i) Determine the importance of each of the nine evaluation indicators. (ii) Determine the relative significance of neighboring indications. (iii) Using order synthesis, determine the structural weight of each indication.

Objective weighting method

Entropy weight method.

Information entropy is a measure of information uncertainty. The entropy weight (EW) method is a weighting method that generates the indicator weight coefficient based on the effect of each indicator’s relative change degree on the overall system (Xu, 2023 ). The specific procedure is as follows: (i) Construct a decision matrix X . (ii) Indicator standardization: homogeneity of diverse indicators. (iii) Calculate the characteristic proportion of i evaluation object under indicator j . (iv) Determine the entropy value of indication j . (v) Calculate the coefficient of variation in indicator j . (vi) Determine the weight coefficient of indication j .

CRITIC method

Another objective weighting method for indicators is the CRITIC weight system (Wang et al., 2024 ). The objective weight of the indicator is determined mostly by the conflict between the contrast intensity and the evaluation indicator. The specific procedure is as follows: (i) Standardization of data X ij . (ii) Determine the linear correlation coefficient between indicators X i and X j . (iii) Determine the quantity of data included in indication j . (iv) Determine each indicator’s normalized weight.

Moment estimation theory

The subjective and objective weights are used to generate the optimal weight for the assessment indicators to match the higher education evaluation system (Cui et al., 2021 ). The subjective weight set of each indicator may be determined using a subjective weighting method based on the concept that subjective weight as

Similarly, objective weighting methods are employed to power the assessment indicator and determine the objective weight as

Assume that k samples are drawn from the subjective weight population and l – k samples from the objective weight population. There are l weight samples for each assessment indicator. If the relative significance coefficients of the subjective and objective weights are α and β , respectively, for the integrated combination weight of each evaluation indicator, the divergence between w j and its subjective and objective weights must be as minimal as feasible. Then the integrated combination weight optimization model is

where \(0\le {w}_{j}\le 1,1\le j\le n\) .

The l samples are drawn from two different populations. The anticipated value of w sj and w tj for each evaluation indicator is

Then, for each indicator, the significant coefficients of subjective and objective weights are

Similarly, using the moment estimation theory for the evaluation indicators in the multi-indicator decision matrix, we can obtain

With the aim that the less H ( w j ) for each evaluation indicator, the better, the optimization model indicated in Eq. ( 4 ) may be converted as

To convert the multi-objective optimization model into a single-objective optimization model, the linear weighting approach with equal weights is

The optimal weight based on various subjective and objective evaluation indicators may be determined by solving Eq. ( 9 ). The importance coefficients based on the subjective and objective weights may be calculated using the model as α and β . The optimal combination of evaluation indicator weights can be determined by solving nonlinear programming.

Risk assessment model

Linear weighted evaluation model.

The most widely used simple integrated evaluation model is the linear weighted evaluation (LWE) model (Hu et al., 2022 ), which essentially finds the weighted sum of each indicator for each terrorist attack once the indicator weights are specified. The formula is as follows:

where \({f}_{i}\) is the weighted comprehensive evaluation value of the i th terrorist attack.

Fuzzy comprehensive evaluation model

The fuzzy comprehensive evaluation (FCE) model is a decision-making approach that expresses evaluation outcomes using fuzzy sets. Furthermore, it can efficiently and thoroughly assess systems impacted by numerous factors by employing mathematical membership theory to convert qualitative evaluation into quantitative evaluation (Wu and Zhang, 2021 ). The specific procedure is as follows: (i) Determine the evaluated object’s factor set U and comment set V . (ii) Determine the membership degree vector and form the membership matrix R . (iii) Determine the evaluation indicator’s weight vector W . (iv) Synthesize the FCE model results vector. (v) Analyze the FCE model results.

TOPSIS method

The technique for order preference by similarity to ideal solution (TOPSIS) method arranges the evaluation items using the evaluation problem’s positive ideal and negative ideal solutions (Zhang et al., 2024 ). Positive ideal solutions are virtual best objects, and each indicator value is the best indicator value among all evaluation objects. While the virtual worst object is another virtual worst object, and each indicator value is. It is the worst value of this indication among all evaluation objects. Each assessment item is graded by its pros and drawbacks by calculating the distance between it and the positive ideal solution and the negative ideal solution. The specific procedure is as follows: (i) employ the vector programming approach to obtain the conventional decision matrix. (ii) Construct a weighted canonical matrix C . (iii) Determine the positive and negative ideal solutions. (iv) Calculate the distance between each evaluation item to the positive and negative ideal solutions. (v) Calculate the relative proximity of each evaluation item and the ideal solution (comprehensive evaluation indicator). (vi) Sort the evaluation objects by superiority and inferiority using the comprehensive evaluation indicator.

Assume that the positive ideal solution’s j th attribute value is \({c}_{j}^{+}\) and the negative ideal solution’s j th attribute value is \({c}_{j}^{-}\) , then the distance between each evaluation item to the positive and negative ideal solutions is

Therefore, the relative proximity of each evaluation item and the ideal solution is calculated as

where \({f}_{i}\in [0,1]\) , and when \({f}_{i}\to 1\) , it indicates that the evaluation object is getting closer to the optimal level.

Particle swarm optimization projection pursuit evaluation method

The projection pursuit evaluation (PPE) method is an efficient statistical approach for dealing with multi-factor complicated problems, and it may be immediately applied to nonlinear and non-normal issues. Its primary premise is to project high-dimensional data into low-dimensional space in a certain combination, and by determining the best projection value, the evaluation value achieved is more referential. The particle swarm optimization (PSO) algorithm is primarily based on adaptive weight computation, which is integrated with the history and present ideal individual location, and it gradually approaches the optimal. We employ a novel algorithm, the PSO-PPE method (Du et al., 2023 ). This approach combines the PSO algorithm and PPE method, which not only speeds up PSO convergence but also solves the problem of discrete random variables. The specific procedure is as follows: (i) Build a linear projection function. (ii) Constructs the projected indicator function. (iii) Optimize the projection direction. (iv) Establish the solution model based on the PSO algorithm.

The objective function of the PPE method is as follows:

where S ( a ) is the sample standard deviation of the projected eigenvalue \({z}_{i}\) , the larger S ( a ), the more evenly distributed each value is. B ( a ) is the local density of the projected eigenvalue. \({r}_{ij}\) is the distance between projected eigenvalues, and its value represents the degree of dispersion. The density window width parameter R has a value that depends on the sample data structure. \(I(R-{r}_{ij})\) is the unit step function.

Risk categorization model

Clustering methods.

The clustering analysis approach is critical in the mining and processing of big data from terrorist attacks. The partitioning clustering, hierarchical clustering, density-based clustering, grid-based clustering, and model-based clustering are the current dominant clustering analysis approaches (Ali et al., 2023 ; Bhattacharyya et al., 2022 ; Yi et al., 2023 ; Yu and Gui, 2021 ). The findings of our investigation into the properties and applicability of different clustering approaches are listed in Table 2 .

To quantitatively characterize the global terrorist attacks from 1970 through 2020, we employ the representative approaches corresponding to the five clustering models: FCM, CURE, DBSCAN, CLIQUE, and GMM.

Clustering evaluation

We evaluate five clustering effects using three internal assessment criteria: the Silhouette coefficient, the Calinski–Harabaz Index, and the Davies–Bouldin Index.

Silhouette coefficient (SC) (Peter, 1987 )

For a given terrorist attack event, the SC is as follows:

where a is the mean distance to the other events in the same cluster, and b is the mean distance to the events in different clusters that are closest to it. The higher the SC, the more effective the clustering effect.

Calinski–Harabaz Index (CHI) (Caliński and Harabasz, 1974 )

CHI is the ratio of the sum of squared distances between the central events of each cluster and the dataset’s central event (closeness inside a cluster) to the total of squared distances between events in a cluster and the cluster’s central event (separation of the dataset). The formula is as follows:

where B k is the between-clusters dispersion mean and W k is the within-cluster dispersion. C q is the set of the terrorist attack events in cluster q , and c q is the center terrorist attack event of cluster q . C E denotes the center of cluster E . n q denotes the number of events in cluster q . The higher the CHI, the more effective the clustering effect.

Davies–Bouldin index (DBI) (Davies and Bouldin, 1979 )

DBI calculates the total of the average distances within the class of any two categories divided by the distance between the two cluster centers to get the maximum value. The formula is as follows:

where s i the average distance between each terrorist attack in a cluster and the cluster’s centroid. d ij represents the distance between the centroids of clusters i and j . The lower the DBI, the more effective the clustering effect.

Kernel density estimation

The Kernel density estimation (KDE) is a method for investigating hotspots in space and time (Campedelli et al., 2021 ; Onat, 2019 ). It is used to determine the density of points around elements. It is believed that the probability of a specific geographic event varies depending on where it occurs in space. The kernel density estimation method was utilized to quantify the risk of terrorism in a certain area, considering the temporal and geographical patterns of the agglomeration area. Its approach is based on searching within a circle with a specified radius utilizing the center of each discrete point grid and finding the density values of the remaining grid elements. It is simply a surface interpolation method using discrete sample points. The specific procedure is as follows: (i) Create a search radius’s moving bandwidth and superimpose its center point with the centers of discrete point cells. (ii) Using the kernel density function, compute the density contribution value of each spatial point to each grid in the bandwidth. (iii) Assign a value to each grid’s density value, and that value is the sum of the contribution values of each spatial point inside the grid’s search radius to the grid density. (iv) Output the density value of each grid. The formula is as follows:

where \({\widehat{f}}_{h}(x)\) is the probability density function for estimating kernel density; h is the bandwidth, which is a smoothing parameter; x is the sample points of terrorist attacks; \({K}_{h}\) is the Kernel function described in Silverman’s work (Kemp and Silverman, 1987 ).

Data exploration and analysis

As seen in Fig. 1 , the GTD data enables rich multidimensional exploration of patterns in terrorist attacks globally from 1970 through 2020 across dimensions like time, weapons, geography, tactics, and targets. This allows us to discern important patterns and statistics about how terrorist attacks have manifested historically. Figure 1a shows the number of attacks per year has generally increased over the 50-year period, indicating a growth in the frequency of terrorism incidents globally, with a notable upward trend starting in the early 2000s. This aligns with the shift in the global terrorism landscape following the 9/11 attacks, which marked a pivotal moment in the evolution of terrorist tactics, motivations, and geographic distribution. Figure 1b and e show that explosives, firearms, and bombings/explosions are by far the predominant weapons and tactics used by terrorists, which highlights critical vulnerabilities around systems/policies governing flows of explosive materials and arms that need to be secured. Figure 1c and d reveal that terrorist attacks have been highly concentrated in specific regions like the Middle East, South Asia, and to a lesser extent Sub-Saharan Africa, and at the country level, Iraq, Afghanistan, Pakistan and India top the list. This geographic mapping of historical hot spots is valuable for prioritizing and allocating counter-terrorism resources. Finally, Fig. 1f indicates that terrorists primarily target private citizens/property as well as military and police forces, this insight on typical targeting can inform protective measures for hardening/securing potential high-risk target sets.

a The number of terrorist attacks per year. b Weapon type analysis. c The region where the attack occurred. d The country where the attack occurred. e Attack type analysis. f Target/victim type analysis.

Figure 2 clearly shows that the correlation between the 22 secondary indicators is extremely low, showing that the correlation is not great, and the indicators are independent of each other, indicating that the quality of the selected indicators is relatively high.

The correlation coefficient diagram of the secondary indicator.

Optimal weight results

The significance coefficients based on the subjective weight and the objective weight may be calculated using the optimal weight method as α  = 0.4711 and β  = 0.5289. The optimal combination of evaluation indicator weights can be determined by solving nonlinear programming in Table 3 .

From Table 3 , in the risk of terrorist attacks, Casualties and Consequences are the most important, with a weight of 38.12%. This is followed closely by the Incident Location, Perpetrator Information, and Target/Victim Information, with weights of 11.07%, 9.43%, and 9.31%, respectively. The GTD ID and date and Attack Information rank fifth and sixth, with weights of 8.35% and 7.81%, respectively. The importance of Incident Information, Additional Information and Sources, and Weapon Information are relatively low, accounting for only 6.57%, 5.70%, and 3.63%, respectively.

Top 10 riskiest terrorist attacks in the world

According to the four risk assessment models, the comprehensive score of each terrorist attack is obtained. We average the normalized comprehensive scores of the four models to acquire the overall comprehensive evaluation value, and then the overall comprehensive score is ranked from high to low in terms of risk. Simultaneously, the related incidents are treated as the same incident, and the top 10 riskiest terrorist attacks from 1970 through 2020 are listed in Table 4 .

Table 4 shows that the top two are 200109110005, and 200109110004, which are the September 11 terrorist attack event with significant casualties. American Airlines Flight 11 crashed into the North Tower of the World Trade Center Complex in New York, killing approximately 3000 people. More than 16,000 others were injured. Two buildings fell, and seven others were partially damaged and collapsed. It is also widely acknowledged as the most serious event of the current terrorist attack. The remaining top 10 terrorist attacks have resulted in a great number of casualties, and the effects have left a significant impact. As a result, in some ways, it demonstrates that the top 10 riskiest terrorist attacks in the world from 1970 through 2020 we have received have a high degree of dependability.

Downward counterfactual events of the top 10 riskiest global terrorist attacks

The top 10 riskiest terrorist attacks identified in this study were significant events that resulted in substantial loss of life and property damage. However, it is crucial to recognize that these events did not occur in isolation and were often preceded by other related plots or attempts that were interdicted or failed due to various reasons (Woo, 2011 ). Therefore, the 10 most dangerous terrorist attack events obtained in this study can also be found through a downward counterfactual search for extreme events, as these events all generate their own accompanying set of downward counterfactuals (Woo, 2019 ). Thus, we conducted a Kendall correlation analysis on terrorist attacks, and obtained the highly correlated events for the top 10 riskiest global terrorist attacks in Table 4 . Subsequently, based on the Incident Summary and Perpetrator Group Name in the original dataset, we identified the downward counterfactual (DC) events of the top 10 riskiest global terrorist attacks, as shown in Table 5 .

Table 5 presents the downward counterfactual events for the top 10 riskiest terrorist attacks globally. The downward counterfactual events refer to related plots or attempts that were interdicted or failed due to various reasons but were highly correlated with the main events in terms of their characteristics, perpetrators, or goals. The downward counterfactual events share similar characteristics with their respective main events, such as involving suicide bombings, car bombings, or attacks targeting specific locations or groups. For each of the top 10 riskiest terrorist attacks, Table 5 lists the corresponding downward counterfactual events, along with their event IDs, correlation coefficients, and brief overviews. To illustrate, we take the September 11 terrorist attack event as an example. Table 5 lists its three major related downward counterfactual (precursor) events as follows:

199808070002 (August 7, 1998)—Nairobi, Kenya, US Embassy Bombings

This event, with a correlation coefficient of 0.9038, involved suicide attackers detonating a vehicle bomb outside the United States Embassy in Nairobi, Kenya. The attack, perpetrated by Al-Qaeda, resulted in 224 fatalities. This event demonstrated Al-Qaeda’s capability and willingness to carry out coordinated attacks against U.S. targets abroad, foreshadowing the scale of the 9/11 attacks.

199302260001 (February 26, 1993)—World Trade Center Bombing

With a correlation coefficient of 0.8734, this event involved a car bomb detonated by Islamist terrorists in the underground parking garage below Tower One of the World Trade Center in New York City. Although less severe than 9/11 attacks, with 6 fatalities and over 1000 injuries, this attack directly targeted the World Trade Center complex and highlighted its vulnerability to terrorist attacks. It served as a precursor to the subsequent, more devastating 9/11 attacks on the same site.

200107240001 (July 24, 2001)—Bandaranaike International Airport Attack

This event, with a correlation coefficient of 0.8404, involved 14 militants of the Liberation Tigers of Tamil Eelam (LTTE) launching a suicide attack on the Bandaranaike International Airport in Colombo, Sri Lanka. While not directly linked to Al-Qaeda, this attack demonstrated the use of suicide tactics and the targeting of transportation infrastructure, which were also features of the 9/11 attacks.

These precursor events share several commonalities with the 9/11 attacks, such as the involvement of Al-Qaeda or similar terrorist groups, the use of suicide tactics and vehicle bombings, and the targeting of symbols of power or transportation infrastructure. They provide insights into the evolution of terrorist strategies and capabilities leading up to the unprecedented scale and impact of the 9/11 attacks.

Risk categorization of terrorist attacks

We solve the five clustering techniques in MATLAB software while also obtaining the results of three clustering evaluation parameters for each method under varied cluster numbers and eps neighborhood (for DBSCAN) is shown in Fig. 3 .

a Silhouette Coefficient (SC); b Calinski–Harabasz Index (CHI); c Davies–Bouldin Index (DBI).

From Fig. 3 , we can see that the clustering effect of FCM is better when k  = 4, the clustering effect of CURE is better when k  = 2, and the clustering effect of CLIQUE and GMM is better when k  = 3. Furthermore, the clustering effect of DBSCAN is better when ε  = 0.45. We eventually select the FCM clustering approach to categorize terrorist attacks based on the five clustering methods and the outcomes of the three clustering evaluation parameters and then visualize the classification results using ArcMap 10.7 as shown in Fig. 4 .

The risk categorization results of global terrorist attacks from 1970 through 2020.

Figure 4 shows that the global distribution of terrorist attacks at various levels was unequal from 1970 through 2020. There were 412 Level-I events, with the majority taking place in the Middle East & North Africa, South Asia, Central America & Caribbean, and East Asia. 931 Level-II events occurred primarily in South Asia, Central America & Caribbean, and the Middle East & North Africa. 2224 Level-III events were mostly found in South Asia, the Middle East & North Africa, North America, and Sub-Saharan Africa. While 6599 Level-IV events were mostly concentrated in South Asia, the Middle East & North Africa, Central America & Caribbean, Southeast Asia, and Western Europe.

Spatial distribution of terrorist attacks

The kernel density estimation in the ArcGIS spatial analysis tool is used in this study to visualize the spatial and geographical distribution of global terrorist attacks. The bandwidth in Eq. ( 20 ) is 50 km, and the cell size of the output raster data is 0.005 degrees, resulting in a kernel density map of terrorist attacks in countries throughout the world from 1970 through 2020, shown in Fig. 5 .

The spatial distribution of terrorist attack risk from 1970 through 2020.

Figure 5 illustrates the spatial and geographical distribution of global terrorist attacks, which comprises four “turbulent cores” in the region of Central Asia, Middle East & North Africa, South Asia, and Central America & Caribbean.

The region of Central Asia

The region of Central Asia, particularly Afghanistan and Pakistan, is heavily impacted by Islamic traditions. Terrorists may readily gain the sympathy of local zealots by portraying themselves as national heroes and religious apologists. Terrorist organizations such as the Taliban in Afghanistan, Al-Qaida, and the Islamic State frequently exhibit multinational and cross-border features. In this region, there are three terrible forces of ethnic separatist, religious extremism, and violent terrorism are merging and converging. Both the breadth and intensity of destruction are expanding and intensifying.

The region of Middle East & North Africa

The region of Middle East & North Africa concentrated on countries such as Syria, Palestine, Arab, and Israel, is one of the world’s most vital security zones. Ethnic conflicts, religious factions, border disputes, ethnic conflicts, wars here, economic backwardness, and other causes have contributed to the region’s rise in terrorism. Terrorist organizations here include the “Hamas” organization, the “Kurdistan Workers’ Party,” the Egyptian Mujahideen organization, and the Pakistani People’s Jihad organization in Palestine, which has contributed to the development of terrorism in the region globally.

The region of South Asia

The region of South Asia concentrated on countries such as India, Nepal, and Bangladesh, also features racial and ethnic variety, as well as a convoluted political history. For a long period during the Cold War, the region was caught in a geostrategic battle between the United States and the Soviet Union, and the ethnic question was never satisfactorily addressed. Since the Cold War’s end, the development of ethnic conflicts in South Asia has been the primary source of societal instability.

The region of Central America & Caribbean

The region of Central America & Caribbean concentrated on countries such as Colombia, Peru, Honduras, Nicaragua, and El Salvador. The fundamental reason is because the region has long been impoverished, the government is impotent, and numerous crimes are common. Furthermore, governmental instability, numerous criminal groups, and societal instability all have a role in the occurrence of terrorism in this region.

In addition to the four “turbulent cores”, terrorism in the past five decades has expanded globally, affecting regions such as the Americas and Europe. The “September 11” terrorist attacks took place at the World Trade Center in New York, USA. This catastrophe resulted in over 10,000 deaths, injuries, and property damages. According to a United Nations study, the terrorist assault cost the United States 200 billion yuan in economic losses.

The globalization of the international economy, politics, and social life in the 21st century has aided the growth of the global economy while also strengthening the interdependence of countries and regions, causing certain local, domestic, or regional problems to become global ones. As a result, in the context of globalization, the fundamental causes of terrorism, terrorist organizations’ activities, and the suffering caused by terrorist attacks take on a global hue. Terrorism is becoming more related to the interests of many countries and has become a global concern. Terrorism’s effects and consequences are more widespread. The interconnectedness of countries is growing, as is the relationship of interest, and the effect of terrorism is becoming more widespread.

This study organizes the GTD database’s recorded data from 1970 through 2020, constructs a scientific index system and analytical model, to realize the risk assessment and categorization of terrorist attacks, theoretically enriching the terrorism study material, and reaches the following conclusions:

The number of terrorist attacks has steadily increased year after year in the past five decades, with the highest frequency occurring in the Middle East & North Africa, as well as in South Asia and other regions of the world. Among the countries where the attacks occurred, Iraq had the most significant terrorist attacks, implying that most of the countries that were targeted the most were highly inhabited. The most common weapon types globally are Explosives and Firearms. Bombing/Explosion and Armed Assault are the most prevalent forms of attacks across the world, while terrorist organizations mostly target Private Citizens & Property.

The LWE, FCE, TOPSIS, and PSO-PPE methods were utilized to assess the risk of terrorist attacks and the top 10 riskiest terrorist attacks from 1970 through 2020, represented by the September 11 terrorist attack event, and along with their downward counterfactual events. It addresses the shortcomings of previous related studies, which relied on unilateral assessments of economic losses and casualties from terrorist attacks, and the evaluation methodologies were too subjective and solitary to compare the assessment findings.

The FCM, CURE, DBSCAN, CLIQUE and GMM, cluster analysis methods were performed for the classification of terrorist attacks, and we used three evaluation criteria, such as SC, CHI, and DBI, to evaluate the clustering effect, and the visual analysis was carried out using the KDE method. The results demonstrate that the spatial and geographical distribution of global terrorist attacks, which comprises four “turbulent cores” in the region of Central Asia, the Middle East, South Asia, Central America, and the Caribbean.

According to the research, terrorist operations are still mostly concentrated in the Middle East & North Africa, South Asia, and Sub-Saharan Africa, and these regions are primarily represented by the Islamic State as international terrorist groups. The worldwide community has been fighting the Islamic State terrorist forces together in recent years, which has degraded their influence to some extent. Under this strain, they have begun to extend outside to other regions, such as South Africa, North America, Eastern Europe, and Southeast Asia, to survive and grow their power, spreading the forces being fought, and the worldwide terrorist situation remains very critical. In this regard, we have proposed the following constructive recommendations for the four “turbulent cores” in light of the current global situation of terrorist attacks:

First, strengthen border security: Improve intelligence sharing and coordination among regional governments to prevent the movement of terrorists and weapons across borders. Second, invest in economic development: Create economic opportunities in impoverished areas to reduce the appeal of terrorist organizations, especially among youth. Finally, promote dialog among religious leaders: Foster an environment where religious leaders can promote peaceful interpretations of Islam and counter extremist propaganda.

First, support political transitions: Encourage and support political transitions to democracy in countries experiencing conflict to reduce the grievances exploited by terrorist groups. Second, address root causes of conflict: Invest in conflict resolution initiatives to address the root causes of conflict and build peace. Finally, strengthen regional cooperation: Enhance intelligence sharing and joint military operations among regional governments to combat terrorist networks.

First, address governance gaps: Strengthen governance, rule of law, and provision of public services in areas with terrorist activity to build trust in government institutions. Second, invest in economic development: Focus economic development efforts on areas with high poverty and unemployment rates to address the socio-economic grievances driving terrorist recruitment. Finally, promote interfaith dialog: Encourage dialog and understanding among different religious communities to counter extremist propaganda.

First, target transnational criminal networks: Strengthen law enforcement efforts to dismantle transnational criminal networks that facilitate terrorist activities. Second, address underlying socio-economic issues: Tackle underlying issues such as poverty, inequality, and lack of opportunity through targeted economic and social development programs. Finally, enhance regional cooperation: Improve coordination among regional governments on border security, intelligence sharing, and joint operations against terrorist groups.

While this study offers valuable insights into the risk assessment and categorization of terrorist attacks, it is important to acknowledge several weaknesses and limitations: (i) Although the GTD is a comprehensive dataset, it has inherent weaknesses like selection biases, underreporting, and variability in data quality across time and regions. Using just this single data source restricts the analysis. (ii) Incident-focused data: The GTD only captures details about terrorist attack incidents themselves. It lacks direct data on broader contextual factors like social, political, economic, and ideological environments that enable or constrain terrorism. Future research could explore integrating the GTD data with other structured and unstructured data sources (e.g. economic indicators, opinion polls, social media) to capture broader environmental contexts and downstream impact indicators. Exploring the use of new open-source indicators (such as GDELT, ACLED, etc.) to supplement the limited data also can be pursued, in order to conduct cross-validation and expand perspectives. Furthermore, investigating the root causes, motivations, and underlying sociopolitical dynamics behind terrorism would provide valuable insights for developing more effective counter-terrorism strategies and policies.

Data availability

The data that support the findings of this study are openly available in the START Consortium at https://www.start.umd.edu/gtd/contact/download .

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Acknowledgements

This work is supported by the National Social Science Foundation of China (No. 21BTQ012) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Nos. KYCX24_0102 and KYCX23_0082). In addition, the authors want to express gratitude to the editors for the editing assistance and thank the reviewers for their valuable comments and suggestions on this paper.

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Zonghuang Xu, Hongyu Cai, Wei Zhang, Jin Shi & Lingyun Situ

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Z.H.X. and Y.L. wrote the main manuscript text. Z.H.X., Y.L., and J.S. designed and performed research and experiments; Z.H.X. and Y.L. research and experiments; Z.H.X., Y.L., H.Y.C., W.Z., and L.Y.S. analyzed the data and results. All authors reviewed the manuscript. Z.X., Y.L., and J.S. revised the manuscript. All authors critically reviewed the manuscript and approved the final manuscript.

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Xu, Z., Lin, Y., Cai, H. et al. Risk assessment and categorization of terrorist attacks based on the Global Terrorism Database from 1970 to 2020. Humanit Soc Sci Commun 11 , 1103 (2024). https://doi.org/10.1057/s41599-024-03597-y

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Article Contents

1. introduction, 2. on terrorism, 3. event data sets, 4. domestic and transnational terrorism: some data plots, 5. counterterrorism, 6. findings of the articles of the special issue, 7. concluding remarks, acknowledgements, terrorism and counterterrorism: an overview.

• Article contents
• Figures & tables
• Supplementary Data

Todd Sandler, Terrorism and counterterrorism: an overview, Oxford Economic Papers , Volume 67, Issue 1, January 2015, Pages 1–20, https://doi.org/10.1093/oep/gpu039

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This introduction sets the stage for the articles collected in this special issue of Oxford Economic Papers . It begins by introducing essential concepts including domestic terrorism, transnational terrorism, defensive actions, proactive countermeasures, and guerrilla warfare. Three terrorist event databases, used by seven of the articles, are briefly introduced. These data sets are then used to display some stylized facts about domestic and transnational terrorism during the past four decades. Next, some essential strategic distinctions are drawn between defensive and proactive measures in the case of transnational terrorism when multiple countries are confronted by a common terrorist group. These strategic concerns vanish for domestic terrorism as a central government is able to internalize potential externalities. Finally, the key findings of the articles in the special issue are highlighted in two tables.

Terrorism is the premeditated use or threat to use violence by individuals or subnational groups to obtain a political or social objective through the intimidation of a large audience beyond that of the immediate noncombatant victims ( Enders and Sandler, 2012 , p.4). The two essential ingredients of terrorism are its violence and its political or social motive. Terrorists tend to employ shockingly violent acts, such as beheadings, downing of commercial airlines, bombings in public markets, and armed attacks in public places, to intimidate an audience. Their unpredictable and horrific attacks are meant to make everyone feel at risk even though the true likelihood of falling victim to a terrorist incident is rather minuscule, roughly equivalent to that of drowning in one's bathtub ( Mueller, 2006 ). Terrorists seek to circumvent normal channels for political change by traumatizing the public with brutal acts so that governments feel compelled to either address terrorist demands or divert public funds into hardening potential targets. Terrorist campaigns are more prevalent in liberal democracies, where the government's legitimacy hinges on its ability to protect the lives and property of its citizens ( Eubank and Weinberg, 1994 ).

The four airplane hijackings on 11 September 2001 (9/11) are terrorist acts since the perpetrators were members of al-Qaida, a subnational terrorist group, bent on pressuring the USA to remove its troops from Saudi Arabia, which was al-Qaida's primary political goal at the time. These skyjackings intimidated a global audience, caused huge temporary losses to the major stock exchanges ( Chen and Siems, 2004 ), and created $80–90 billion in direct and indirect damages ( Kunreuther et al ., 2003 ). Even though stock exchanges recovered lost values in just over a month, the death of almost 3,000 people caused rich industrial countries to allocate more resources to counterterrorism, shook insurance markets, and made an indelible impression on virtually the entire world. Heinous terrorist incidents continue to capture headlines with recent newsworthy incidents involving al-Shabaab's armed attack on the Westgate Mall in Nairobi, Kenya, on 21 September 2013; Chechen separatists’ suicide bombings of a train station and a trolley in Volgograd, Russia, on 29 and 30 December 2013, respectively; and Boko Haram's kidnapping of more than 200 female students in Chibok, Nigeria, on 14–15 April 2014. These and countless other incidents since 9/11 indicate that the government must allocate resources in an effective and measured manner to counterterrorism activities so that terrorists cannot circumvent legitimate political processes or cause significant economic losses. These losses may involve reduced foreign direct investment ( Enders and Sandler, 1996 ; Abadie and Gardeazabal, 2008 ), lower economic growth ( Abadie and Gardeazabal, 2003 ; Eckstein and Tsiddon, 2004 ; Gaibulloev and Sandler, 2008 , 2011 ), less trade ( Nitsch and Schumacher, 2004 ), reduced tourism ( Enders et al ., 1992 ; Drakos and Kutan, 2003 ), or lost values of stock and bond indexes ( Kollias et al ., 2013 ). Economic impacts of terrorism are greatest in small terrorism-plagued countries and developing countries ( Keefer and Loayza, 2008 ; Sandler and Enders, 2008 ). Modern industrial economies can insulate themselves through judicious fiscal and monetary policy, rapid counterterrorism responses, and the transference of economic activities ( Enders and Sandler, 2012 ). The latter involves economic activities moving from terrorism-prone sectors and regions to safer areas, which advanced, diversified economies allow. Thus, economic activity may switch from the tourism sector to other sectors when the former is targeted. In Spain, economic investment switched from the Basque Country to other Spanish provinces because of Euskadi ta Askatasuna attacks ( Abadie and Gardeazabal, 2003 ).

Modern-day econometric methods—time series, panel, and discrete-choice models—lend themselves to the quantification of these economic losses as shown in this special issue by Choi (2015) , Egger and Gassebner (2015) , and Younas (2015) . Additionally, game-theoretic models can display counterterrorism interactions among terrorists and governments as in the contributions in this issue by Carter (2015) and Kaplan (2015) . In fact, game theory is an excellent tool to study interactions among targeted governments, between rival terrorist groups, between a terrorist group and its sponsoring state, and among the media, the terrorist group, and the public. 1

The purpose of this article is to provide the requisite background to the studies in this special issue of Oxford Economic Papers . This task requires a fuller discussion of the notion of terrorism and its two primary subdivisions—domestic and transnational terrorism—in Section 2. The three event data sets employed in empirical studies, including seven of the eight articles in this issue, are briefly presented in Section 3. In Section 4, two of these data sets are used to display some recent trends and aspects of domestic and transnational terrorism during the last four decades. Essential concepts of counterterrorism are then presented in Section 5, where proactive measures are distinguished from defensive actions. Key findings of the four terrorism and four counterterrorism articles contained in this special issue are highlighted in two summarizing tables in Section 6. Concluding remarks follow in Section 7.

I now return to the definition of terrorism, given at the outset of this article. Any definition of terrorism involves much debate ( Hoffman, 2006 ; Enders and Sandler, 2012 ). The research community is converging to a consensus based on an operational definition on which to construct event data sets to test theoretical propositions. The article puts forward a definition that is consistent with that used by the main event data sets and relied on by researchers. Also, this definition possesses the main ingredients that are agreed on by economists, political scientists, and political economists.

The three stakeholders in this definition are the perpetrators, the victims, and the audience. By limiting terrorism to subnational agents including individuals or a ‘lone wolf’, my definition rules out state terror in which a government terrorizes its own people. The definition, however, does not rule out state-sponsored terrorism where a government clandestinely assists a terrorist group through various means, including supplying weapons, safe haven, intelligence, training, funding, or safe passage ( Mickolus, 1989 ; Bapat, 2006 ). There was a lot of state sponsorship of terrorism during the final decade of the Cold War with groups such as the Abu Nidal Organization serving as a terrorist group for hire ( Hoffman, 2006 ). 2 The most controversial element of my definition is the victim, since some definitions exclude combatants, so that attacks against an occupying army, such as US forces in Iraq or Afghanistan, are not viewed as terrorism. Generally, an attack against peacekeepers, such as the 23 October 1983 suicide bombing of the US Marines barracks at Beirut International Airport, is considered an act of terrorism. The barracks’ bombing had the political objective of removing peacekeepers from Lebanon, which happened in February 1984. Attacks against US soldiers and their dependents stationed in Germany constitute terrorist incidents, because these targeted individuals were noncombatants when attacked. ‘Audience’ refers to the collective that terrorists seek to intimidate through their wanton brutality. With sufficient and sustained intimidation, the audience will apply pressures on the besieged government to concede to the terrorist group's political demands or alternatively to take decisive action to annihilate the group. 3 In the latter case, the Italian authorities dismantled the Italian Red Brigades in the 1980s.

There are some crucial distinctions to draw between terrorism and related concepts. For instance, there is the distinction between terrorism and crime. A kidnapping for ransom is a criminal act of extortion when the kidnappers are not pursuing or financing a political agenda. If a political motive is tied to the kidnapping, then it is a terrorist incident even with ransom demands being made. The hijacking of a commercial airliner by a deranged person is a crime but not terrorism. In the absence of a political motive, an armed attack by a student on fellow students or teachers is a criminal action. Next consider an insurrection, which ‘is a politically based uprising intended to overthrow the established system of governance and to bring about a redistribution of income’ ( Sandler and Hartley, 1995 , p. 307). Leaders of insurrections recruit from the peasantry and general population in the hopes of challenging the government's hold on power ( Grossman, 1991 ). Successful rebel operations can generate new recruits and may ideally cause the government to impose draconian measures on its citizens, which subsequently create more support for the insurgency. If a tipping point is attained, then the government may be sufficiently challenged to lose its power to the rebels.

In distinction to insurrections, guerrilla warfare generally involves a band of rebel forces (e.g., the Revolutionary Armed Forces of Colombia [FARC], Abu Sayyaf in the Philippines, or Shining Path in Peru) that controls a sector of the country, from which to dispatch its operatives to confront government forces. Some guerrilla wars take place in urban centers. In contrast to most terrorist groups, guerrilla groups are larger in number and organized like a military force. Some guerrilla groups engage in terrorist acts, such as the three just-mentioned groups, to raise funds to secure their operations and pursue their political aims. For example, FARC kidnaps government officials and others for ransoms. Unlike an insurrection, guerrilla groups are not bent on overthrowing the government or engaging in propaganda to gain popular support ( Hoffman, 2006 ). Shining Path and FARC apply threats and harsh measures to gain the compliance of the people in the territory that they control. Guerrilla groups rely on surprise and cover to harass numerically superior government forces. Terrorism is a tactic employed by both insurrections and guerrilla movements. As a consequence, many guerrilla groups are listed as terrorist groups despite their control of territory. Often, countries with jungle cover or mountainous terrain provide remote areas where guerrillas can conduct training and operations. In this special issue, Carter (2015) is interested in the interaction between a guerrilla group and the government, as the former chooses between terrorism and the control of territory and the latter chooses between defensive counterterrorism actions and proactive military responses to influence the group's decision.

2.1 Domestic versus transnational terrorism

Domestic terrorism is homegrown and home-directed, and represents the most common form of terrorism. For domestic acts of terrorism, the perpetrators, victims, and audience hail from the venue country, where the attack takes place. Domestic terrorist incidents include Timothy McVeigh's bombing of the Alfred P. Murrah Federal Building in Oklahoma City on 19 April 1995 or Eric Rudolph's anti-abortionist bombing of Centennial Olympic Park in Atlanta, Georgia, on 27 July 1996. Civil wars often involve numerous domestic terrorist attacks before and during the conflict by the adversaries ( Findley and Young, 2012 ). 4 These terrorist acts are more apt to be domestic when an intervention by a third party from outside the country is not involved. Boko Haram's kidnapping of more than 200 female students is a domestic terrorist incident, which involves victims and perpetrators from the venue country of Nigeria. Boko Haram is an Islamic jihadist terrorist group that controls territory in the northeast portion of Nigeria. Given the country's limited military capabilities, its government sought some assistance from the USA in terms of military advisors and intelligence in addressing the significant threat that Boko Haram poses. At times, Boko Haram crosses into Chad. If one or more of the schoolgirls are moved into a neighboring country, then the kidnapping becomes a transnational terrorist incident. In general, poor countries may request foreign assistance if they cannot properly confront an indigenous terrorist group that may attack at home or abroad ( Azam and Thelen, 2010 ; Fleck and Kilby, 2010 ; Bandyopadhyay et al ., 2011 , 2014 ; Young and Findley, 2011 ).

Terrorism is transnational when an incident in the venue country concerns perpetrators or victims from another country. If a terrorist attack in the UK is perpetrated by terrorists from Yemen, then the incident is one of transnational terrorism. When a terrorist attack in France harms Dutch citizens, the attack is transnational. If one or more victims or perpetrators are not citizens of the venue country, then the terrorist attack is transnational. The kidnapping in January 2002 and subsequent murder of US reporter Daniel Pearl in Pakistan is classified as a transnational terrorist incident. The same is true of the near-simultaneous bombings of the US embassies in Kenya and Tanzania on 7 August 1998. Terrorist attacks against another country's embassy, even when perpetrated by citizens of the venue country, are transnational terrorist events because an embassy's grounds represent foreign soil. Similarly, terrorist attacks against international organizations’ personnel or property are considered to be transnational terrorist acts. An important transnational terrorist incident is the August 2006 plot to use liquid explosives to blow up 10 or more transatlantic flights departing the UK for the USA and Canada. A skyjacking originating in one country that is diverted to another country for political purposes is a transnational terrorist event. If a politically motivated hijacked plane has citizens from more than one country, the event is transnational terrorism even if the flight is domestic and ends in the country of origin. On 9/11, the four skyjackings are transnational terrorist acts since the victims were citizens from upward of 80 nations and the perpetrators were foreigners. The kidnapping of US journalist James Foley in Syria on 22 November 2012 and his beheading on 19 August 2014 is a transnational terrorist act. The victim was American, whereas the murderer is allegedly a British citizen from the Islamic State in Iraq and Syria (ISIS), also known as Islamic State (IS).

Transnational terrorist incidents frequently imply transnational externalities—for example, perpetrators from one country impose uncompensated costs on the victims of another country. If a country provides safe haven to a transnational terrorist group that attacks other countries’ interests, then transnational externalities ensue. 5 The Taliban in Afghanistan had given safe haven to Osama bin Laden's al-Qaida, which planned and executed the events of 9/11. When the Taliban would not surrender bin Laden to the USA following 9/11, the USA led an invasion of Afghanistan on 7 October 2001 ( Enders and Sandler, 2012 ). In this extreme case, the transnational externality resulted in a military invasion with the intent to defeat the Taliban and al-Qaida. Transnational externalities also arise from counterterrorism policies of targeted countries, which result in inefficient levels of these policies ( Sandler and Lapan, 1988 ; Sandler and Siqueira, 2006 ; Bueno de Mesquita, 2007 ). Actions by one targeted country to secure its borders and ports of entry may merely transfer the attack abroad, where borders are more porous (see Section 5). Since 9/11, few transnational terrorist incidents occur on US soil but 35% to 40% of such incidents involve US people or property in other countries ( Enders and Sandler, 2006 , 2012 ).

2.2 Some historical considerations of transnational terrorism

Hoffman ( 2006 , pp.63–5) traces the modern era of transnational terrorism to the 22 July 1968 hijacking of an Israeli El Al flight en route from Rome to Athens by three armed members of the Popular Front for the Liberation of Palestine (PFLP) terrorist group. This skyjacking was motivated by the intention of the PFLP terrorist to trade its hostages for Palestinian prisoners held by Israel. This event is noteworthy for a number of reasons. First, through its protracted 40-day negotiations, the Israelis were forced to negotiate with the Palestinian terrorists, which the Israelis had hitherto vowed they would never do ( Hoffman, 1998 , p.68). Second, the media coverage demonstrated to terrorists worldwide that such incidents could garner worldwide attention for their cause. Not surprisingly, transnational terrorist attacks increased greatly in numbers during the years following this incident (see the figures in Section 4). Third, there was evidence of state sponsorship after the diverted plane landed in Algiers as Algerian forces secured the hostages and held some Israeli hostages until 1 September 1968 when a deal was concluded ( Mickolus, 1980 , pp.93–4). Fourth, Israel eventually traded 16 Arab prisoners from the 1967 Arab-Israeli War for the remaining Israeli hostages. This trade showed terrorists that hostage taking could yield significant concessions.

Transnational terrorist groups were primarily nationalists/separatists or leftists (socialists) during the late 1960s until the late 1980s ( Rapoport, 2004 ). Even the Palestinian terrorists were secular until the end of the 1980s with the rise of Hamas, Palestinian Islamic Jihad, and other groups. After the mid-1990s, the religious fundamentalists came to dominate and increased the carnage ( Enders and Sandler, 2000 ; Gaibulloev and Sandler, 2014 ). The phenomenon of religious-based transnational terrorism is not novel and can be traced back to the Sicarii or Zealots, a Jewish sect that conducted a terror campaign against the Romans and their Jewish collaborators in Judea from CE 48 to 73 ( Rapoport, 1984 ; Bloom, 2005 ). Sicarii terrorists engaged in daytime assassinations in public places that typically resulted in the death of the assassin. As such, their dagger attacks were an early form of suicide terrorism, since the perpetrator had little chance of escape. 6 From 1090 to 1256, the Islamic Assassins opposed Sunni rule in Persia and Syria, with the intent to set up their own community and state of believers in the region ( Bloom, 2005 ). Although the Assassins’ terrorist campaign was on a much smaller scale, their goal was similar to that of ISIS. Like the Sicarii, the Assassins relied on politically motivated assassinations, performed with a dagger. Perpetrators usually sacrificed their own lives by making no efforts to escape after the deed.

In the beginning of the 1980s, the first terrorist event data set—International Terrorism: Attributes of Terrorist Events (ITERATE)—was made available to researchers. ITERATE only includes transnational terrorist attacks. Coverage starts in 1968, the beginning of the modern era of transnational terrorism, and runs until the end of 2012, with annual updates in August ( Mickolus et al ., 2013 ). ITERATE codes many variables—for example, incident date, country start location, country end location, attack type, target entity, terrorist group, perpetrators’ nationalities, number of deaths, number of injuries, victims’ nationalities, logistical outcome, US victims, state sponsorship, and scene of attack—in its Common File of over 40 variables. In addition, there is a Fate File indicating the fate of the terrorists—for example, the number of terrorists captured, the number of terrorists sentenced, and their length of incarceration. There is also a Hostage File, which has invaluable observations used by researchers to analyze logistical and negotiation success of hostage taking ( Santifort and Sandler, 2013 ). If an attack is completed as planned, then it is a logistical success. For hostage missions, securing one or more hostages is deemed a logistical success. The Hostage File of ITERATE is currently updated through 2010. Finally, there is a Skyjacking file with additional observations, and variables on skyjacking missions such as the duration of the incidents, airline involved, and negotiation strategies used. ITERATE, like the other event data sets, relies on the news media—print, broadcast, and digital—for the observations of its variables.

An initial focus of empirical studies was on transnational terrorism because ITERATE was the most extensive data set available throughout the 1980s and 1990s. Its lengthy series of daily data were ideal for time-series studies, which dominated the research landscape except for a few survival studies, the first being Atkinson et al .'s (1987) study of the duration of hostage-taking incidents. Today, panel studies are prevalent including those in this special issue— Berrebi and Ostwald (2015) , Choi (2015) , Egger and Gassebner (2015) , Gries et al . (2015) , and Younas (2015) .

Another competing event data set, modeled after ITERATE, is the RAND (2012) data set, which currently codes incidents for 1968–2009 and is not being updated. Gaibulloev (2015) uses the RAND event data in conjunction with Jones and Libicki's (2008) classification of terrorist groups’ ideologies in his study of groups’ location decisions. For 1968–97, RAND event data only include transnational terrorist attacks; after 1998, RAND data distinguish between transnational and domestic terrorist attacks in a manner consistent with my early definitions. Compared to ITERATE, RAND data code fewer variables and, for transnational terrorist incidents, have more limited coverage than ITERATE as demonstrated by Enders (2007) .

The third event data set, germane to this special issue, is that of the Global Terrorism Database (GTD), which records both domestic and transnational terrorist incidents ( La Free and Dugan, 2007 ; National Consortium for the Study of Terrorism and Response to Terrorism, 2013 ). Although GTD recorded both kinds of incidents, until 2013 it did not distinguish between the two kinds of incidents. Enders et al . (2011) devised a five-step procedure for distinguishing between domestic and transnational terrorist incidents in GTD for 1970–2007 and made their breakdown available to researchers. This division is now applied to 2008–2012 (see Enders et al ., 2014 ). A breakdown of terrorism into its two components is essential because the two types of terrorism may affect economic variables and counterterrorism differently—for example, economic growth or foreign direct investment is more influenced by transnational terrorism ( Gaibulloev and Sandler, 2008 ). Moreover, the influence of other variables on domestic and transnational terrorism may differ ( Sandler, 2014 ; Choi, 2015 ). The Enders et al . (2011) procedure does a much better job in distinguishing between the two types of events than recent GTD efforts, based on the authors’ method without attribution. GTD has tens of thousands of unclassified incidents compared to Enders et al . (2011) .

There are some things to note about GTD. First, it has changed its coding conventions a few times, most recently for the 2012 data. Coding was also changed after the National Consortium for the Study of Terrorism and Responses to Terrorism (2013) , based at the University of Maryland, took charge of the data around 2005. As shown by Enders et al . (2011) , there are periods of undercounting and overcounting of incidents, which can be addressed by these authors’ calibration methods. Second, GTD data for 1993 are very incomplete because in an office move by Pinkerton, which originated the database, the box containing 1993 fell off of the truck! Third, perpetrators’ nationalities are not identified. Fourth, GTD does not contain any hostage negotiation variables, making the study of hostage-taking incidents impossible with this database. Fifth, GTD coverage of some kinds of domestic incidents, such as kidnappings, is virtually nonexistent prior to the late 1990s ( Enders et al ., 2011 ).

Key aspects of the articles for the special issue

Notes: *denotes that Enders et al . (2011) breakdown of GTD into domestic and transnational terrorist incidents was used. ITERATE = International Terrorism: Attributes of Terrorist Events; GTD = Global Terrorism Database.

A variety of empirical techniques are applied by the six empirical articles. In his cross-sectional study of the location choice of terrorist groups, Gaibulloev (2015) relies on a conditional logit estimator. To account for the count nature of the dependent terrorism variable, Choi (2015) and Gries et al . (2015) apply a negative binomial panel estimator. Endogeneity between the dependent variable and one or more independent variables is addressed in various ways: the Berrebi and Ostwald (2015) article employs instrumental variables (IV), consisting of lagged domestic terrorist attacks in neighboring countries, whereas the Younas (2015) article uses system generalized method of moments (SGMM) to augment his feasible generalized least squares (FGLS) estimates. The Gries et al . (2015) article addresses endogeneity with lagged values of the independent variables and also SGMM estimates. The Choi (2015) article primarily uses lagged independent variables to partly handle the endogeneity concern.

ITERATE transnational terrorist incidents and total casualties per year, 1968–2012

GTD terrorist incidents per year, 1970–2012

ITERATE attacks against US interests per year, 1968–2012

Annual number of casualties per attack

This special issue contains articles on terrorism and counterterrorism. Given my discussion of terrorism and its concepts, it is now instructive to introduce some basic concepts and concerns that are associated with the practice of counterterrorism.

Quite simply, counterterrorism corresponds to actions to ameliorate the threat and consequences of terrorism. These actions can be taken by governments, military alliances, international organizations (e.g., INTERPOL), private corporations, or private citizens. Counterterrorism comes in two basic varieties: defensive and proactive measures.

Defensive countermeasures protect potential targets by making attacks more costly for terrorists or reducing their likelihood of success. When, however, successful terrorist attacks ensue, defensive actions also serve to limit the resulting losses to the target. Defensive measures have generally been reactive, instituted after some successful or innovative terrorist attacks. In the USA, airline passengers are now required to remove their shoes when being screened, following the innovative, but fortunately unsuccessful, attempt by Richard Reid to bring down American Airlines flight 63 en route from Paris to Miami on 22 December 2001 with explosives hidden in his shoes. Before the installation of metal detectors to screen passengers at US airports on 5 January 1973, there were on average over 25 skyjackings each year in the USA ( Enders et al ., 1990 ). After their installation, attempted US skyjackings dropped to fewer than four a year. The success of these metal detectors in US airports led to their installation worldwide over the next six months. Following the downing of Pan Am flight 103 over Lockerbie, Scotland, on 21 December 1988 and the downing of UTA flight 772 over Niger on 19 September 1989, bomb-detecting devices were used to screen checked luggage. Defensive or protective counterterror actions may involve more than technological barriers. Other instances of defensive measures include target hardening, such as defensive perimeters around government buildings or embassies, or guards at key points of a country's infrastructure. Defensive measures can also take the form of issuing terrorism alerts, enacting stiffer penalties for terrorism offenses, enhancing first-responder capabilities, and stockpiling antibiotics and antidotes for biological and chemical terrorist attacks. This list of defensive actions is by no means exhaustive.

By contrast, proactive measures are offensive as a targeted government directly confronts the terrorist group or its supporters. Proactive measures may destroy terrorists’ resources (e.g., training camps), curb their finances, eliminate their safe havens, or kill and capture their members. In recent years, the Obama administration has relied on drone attacks to assassinate terrorist leaders and operatives. Proactive operations may assume myriad other forms, including a retaliatory raid against a state sponsor that provides resources, training, sanctuary, logistical support, or intelligence to a terrorist group. On 15 April 1986, the USA launched a retaliatory bombing raid on targets in Libya for its alleged support in the terrorist bombing of the La Belle discotheque in West Berlin on 4 April 1986, where 3 died and 231 were wounded, including 62 Americans ( Mickolus et al ., 1989 , vol. 2, pp.365–7). Another proactive measure takes the form of a preemptive attack against a terrorist group or a harboring country, such as the US-led invasion of Afghanistan four weeks after 9/11. A preemptive strike differs from a retaliatory raid because the former is more sustained and meant to severely compromise the capabilities of the terrorists to conduct future missions. Such strikes or raids concern transnational terrorism where a targeted country confronts the foreign threat. Other proactive measures include infiltrating terrorist groups, engaging in military action, conducting propaganda campaigns against the terrorists, and gathering intelligence to foil terror plots ( Kaplan, 2015 ). ‘Military action’ generally refers to operations by the host government against a resident terrorist group as in Carter (2015) . Actions that improve the economy, which in turn reduces grievances, can also be proactive ( Choi, 2015 ). Younas (2015) demonstrates empirically that increased globalization of a country's economy may also be proactive by limiting harmful and therefore grievance-causing effects of terrorism on economic growth. In this special issue, the four counterterrorism articles primary address aspects of proactive measures. Only Carter (2015) considers both defensive and proactive counterterrorism responses.

The game-theoretic literature on counterterrorism draws some fascinating strategic contrasts between defensive and proactive countermeasures. Suppose that two or more countries are confronted by the same Islamic jihadist terrorist group—for example, al-Qaida in the Arabian Peninsula (AQAP). In this transnational terrorism scenario, each at-risk country is inclined to work at cross-purposes by engaging in a defensive race in the hopes of transferring the terrorist attacks to other targeted countries ( Sandler and Lapan, 1988 ; Arce and Sandler, 2005 ). In the process, the countries engage in too much defense since the negative transference externality is not internalized. The only check on this adverse ‘defense race’ stems from the countries having large interests abroad, which may be hit when attacks are transferred abroad—recall Fig. 3 and attacks against US interests ( Bandyopadhyay and Sandler, 2011 ; Bandyopadhyay et al ., 2011 ).

Next consider proactive measures in these multicountry scenarios. Any country's actions to confront the common terrorist threat confer purely public (nonrival and nonexcludable) benefits to all potential target countries. As a result, there is too little proactive response as each country tries to free ride on the actions of other countries. Thus, defensive measures are strategic complements as one country's actions encourage those of other countries (i.e., reaction paths are upward-sloping), whereas proactive measures are strategic substitutes as one country's actions inhibit those of other countries (i.e., reaction paths are downward-sloping) ( Eaton, 2004 ; Sandler and Siqueira, 2006 ). Moreover, leadership can be shown to curb the defensive race, whereas leadership exacerbates the free-riding underprovision of offensive measures ( Sandler and Siqueira, 2006 ). Thus, there is no simple fix for these concerns among sovereign targeted nations.

For domestic terrorism, countries possess the proper incentives to choose defensive and proactive measures judiciously, because all associated costs and benefits are internalized ( Enders and Sandler, 2012 ). Thus, Kaplan's (2015) finding that the USA staffs about the right number of intelligence analysts to intercept terror plots is consistent with past game-theoretic findings regarding domestic counterterrorism optimality. Carter's (2015) article is also geared to domestic terrorism, whereas Choi (2015) and Younas (2015) may involve economic-based countermeasures for either type of terrorism.

Key findings of terrorism studies

Key findings of counterterrorism studies

This special issue contains many noteworthy advances in the study of terrorism. Gaibulloev (2015) is the first article to address terrorist groups’ base location choice; currently, the study of terrorist group survival is an active area of research. Berrebi and Ostwald (2015) is the initial study to examine the impact of terrorism on fertility. Younas (2015) and Choi (2015) refine the relationship between terrorism and growth; Egger and Gassebner (2015) show that terrorism has less of an effect on trade than conventionally believed. Kaplan (2015) investigates the social efficiency of intelligence staffing in intercepting terror plots. Carter's (2015) analysis is novel because the terrorist group must decide between terrorist attacks and holding territory in reaction to the government's countermeasures. The interface between terrorism and other forms of political violence is seldom studied. Finally, the Gries et al . (2015) study extends knowledge of the determinants of anti-American terrorist attacks, which comprise 35–40% of all transnational terrorism.

Terrorism remains an active research area in economics, political economy, and political science. Given the microeconomic and macroeconomic consequences of terrorism, its study is a fitting topic for a general economics journal such as Oxford Economic Papers . Following 9/11, industrial countries engaged in a huge reallocation of resources toward counterterrorism, whose efficacy can be best studied and understood with the theoretical and empirical tools drawn from economics.

The rise of ISIS and the considerable threat that its Western fighters pose for their home countries means that terrorism will remain a policy concern. AQAP's war on the USA and its allies is yet another terrorist threat. Just as Africa appears to be achieving sustained economic growth in select countries, significant terrorist challenges arise from Boko Haram and al-Shabaab in Nigeria and Kenya, respectively. Failed states—for example, Afghanistan, Iraq, Somalia, Syria, and Yemen—supply safe havens for terrorist groups that threaten Western interests. It is my hope that this special issue on terrorism and counterterrorism will stimulate further research in these two topics.

The present collection arises from a set of papers presented at the sixth conference on Terrorism and Counterterrorism Policy at the University of Texas, Dallas. Held between 21 May and 24 May 2014, this conference invited many leading contributors to the study of terrorism and counterterrorism. Past conferences resulted in special issues published in Economics &Politics (November 2009), Journal of Conflict Resolution (April 2010), Journal of Peace Research (May 2011), Public Choice (December 2011), and Southern Economic Journal (April 2013). While assuming full responsibility for any remaining shortcomings, this article has profited from comments from Anindya Banerjee, who also advised me on the special issue. Khusrav Gaibulloev read and commented on an earlier version. Finally, I thank all of this special issue's reviewers, most of whom are major contributors to the study of terrorism. These reviewers provided rigorous reviews in a timely fashion. All articles went through a careful and demanding prescreening process, followed by two rounds of reviews by two to three anonymous referees. I appreciate the understanding of those authors whose papers did not make it into the special issue.

On the game-theoretic literature on terrorism, see Bandyopadhyay and Sandler (2011) , Bandyopadhyay et al . ( 2011 , 2014 ), Sandler and Arce (2003) , Bueno de Mesquita (2005) , and Sandler and Siqueira (2009) .

The Abu Nidal Organization was headed by Sabri Khali al-Banna. Its notorious attacks included the simultaneous armed attacks on the Rome and Vienna airports on 27 December 1985, the armed attack on the Neve Shalom Synagogue in Istanbul on 6 September 1986, and the attempted hijacking of Pan Am flight 73 in Karachi, Pakistan, on 5 September 1986 ( Enders and Sandler, 2012 ). This group is credited with the first simultaneous terrorist attacks.

On alternative ways that terrorist groups end, see Carter (2012) , Gailbulloev and Sandler (2014) , and Phillips (2014a) .

On the relationship between civil wars and terrorism, see Sambanis (2008) .

How terrorist groups choose their home base is discussed in this special issue by Gaibulloev (2015) . On the definition of terrorist groups, see Phillips (2014b) .

In this special issue, Choi (2015) investigates suicide terrorism, which may be domestic or transnational in nature. Suicide terrorist attacks grew in numbers after 1988 ( Santifort-Jordan and Sandler, 2014 ).

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• A-Z Publications

Annual Review of Criminology

Volume 7, 2024, review article, open access, terrorism, political extremism, and crime and criminal justice.

• Joshua D. Freilich 1 , Steven M. Chermak 2 , Rachael A. Arietti 1 , and Noah D. Turner 3
• View Affiliations Hide Affiliations Affiliations: 1 Department of Criminal Justice, John Jay College of Criminal Justice, New York, NY, USA; email: [email protected] 2 School of Criminal Justice, Michigan State University, East Lansing, Michigan, USA 3 National Counterterrorism Innovation, Technology, and Education Center (NCITE), School of Criminology and Criminal Justice, University of Nebraska, Omaha, Nebraska, USA
• Vol. 7:187-209 (Volume publication date January 2024) https://doi.org/10.1146/annurev-criminol-022422-121713
• First published as a Review in Advance on August 08, 2023
• Copyright © 2024 by the author(s). This work is licensed under a Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. See credit lines of images or other third-party material in this article for license information

This review focuses on terrorism and extremist crimes, including ideologically motivated hate crimes. Research on these topics has become more rigorous in recent decades, and more scholars have engaged in original data collection. Our assessment found a burgeoning literature that increasingly includes the application of integrated theories, but gaps remain as few studies examine life-course and critical approaches. Our review of the policing of terrorism found a limited evidence base for counterterrorism initiatives. We also found that court/sentencing issues are understudied. We suggest improving data quality in these areas by creating a national data collection protocol on these crimes, enhancing the rigor of offender and victim self-report studies, and requiring more transparency from open-source research efforts. We propose that government agencies fund rigorous evaluations of policing strategies in the terrorism context. Finally, it is hoped that increased access to federal court documents will lead to more scholarly attention on sentencing issues.

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• Article Type: Review Article

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Five Trends Driving the Future of Terrorism

• Colin P. Clarke
• September 2, 2022
• National Security Program

In mid-May 2022, eighteen-year-old Payton Gendron approached a grocery store in a predominantly African-American neighborhood in Buffalo, NY. He opened fire with a Bushmaster XM-15 semi-automatic rifle , killing ten people. The attack was one of the deadliest racist massacres in recent American history.

The attack made headlines worldwide, the latest in a series of racially and ethnically motivated violent extremist attacks. As in the Christchurch mosque attack in New Zealand, Gendron live-streamed his armed assault and left behind a manifesto outlining a range of conspiratorial views and documenting extreme racial animus.

Beyond the tragedy itself, the attack was noteworthy for another reason—it was the quintessential example of how terrorist ideologies have become more diverse in recent decades, attacks have become more decentralized, terrorism itself has been democratized by new technologies, disinformation has been used to fuel hate, and terrorists have acquired the means to make attacks much deadlier. These trends in terrorism (diversity, decentralization, democratization, disinformation, and deadly) are shaping the future of violent extremism. 

For counterterrorism analysts, government officials, and policymakers, it is crucial to understand how terrorism is changing to know how to prioritize threats and allocate resources. Given China’s increasingly assertive foreign policy and Russia’s invasion of Ukraine, counterterrorism is no longer the principal national security focus of Western governments. As a result, those charged with preventing terrorist attacks need be smarter about how terrorists and terrorist groups are evolving. Moreover, it is essential to understand the grievances motivating political violence and terrorism in order to craft a comprehensive response.

First, the attack in Buffalo reflected the diverse ideologies that are motivating terrorism and violent extremism. Whereas the past two decades have mostly been about dedicating resources to combat Salafi-jihadism and groups like al-Qaeda and the Islamic State, the current terrorism landscape is far more varied. In Buffalo, Gendron’s motivation was a deep hatred of African-Americans, but even within the broader category of far-right extremism, there exists an intersectionality among prejudices that also include Islamophobia, anti-Semitism, or threats against the LGBTQ+ community.

The ideologies motivating terrorists is the most significant change in counterterrorism. What was mostly a monochromatic threat from Salafi-jihadist groups and those individuals they sought to inspire, has now become a kaleidoscope with new threats from white supremacists and neo-Nazis, shadowy anarchist elements, neo-Luddites, and the extreme fringe of violent incels (i.e., politicized involuntary celibates fueled by a hatred of women). Once-formidable ethno-nationalist terrorist groups, including the Provisional Irish Republican Army and the Basque group ETA, have all decommissioned their arms and faded into history—the same is true of radical left-wing militants like the Baader Meinhoff group in Germany and the Red Brigades in Italy. In South Asia, secessionist groups like the Liberation Tigers of Tamil Eelam have also been pacified.

Second, the shooting was decentralized . The attacker was a so-called lone wolf, who acted on his own without any external assistance or instigation. Lone wolves are difficult for law enforcement and intelligence agencies to detect, although the term “lone wolf” itself is something of a misnomer. Even in cases where a single attacker may be responsible, the individual almost always emerges from a broader ecosystem where they have radicalized, often after being exposed to extremist propaganda online.

Decentralization describes the concept of “leaderless resistance” pioneered by violent far-right extremists, but also the phenomenon wherein the leadership and upper echelon of large, bureaucratized terrorist organizations like al-Qaeda and the Islamic State are winnowed through relentless counterterrorism campaigns. In both cases, the result has been a shift to an affiliate or franchise model, where regional branches pursue a blend of local and global objectives.

Third, the attack was an example of the democratization of terrorism. In other words, the barriers to entry to engage in terrorism have been significantly lowered over the past decade. Today, an individual with an internet connection, a smartphone, and access to weapons, including do-it-yourself weapons like “ghost guns” and 3-D printed explosives , can now wreak havoc in society. In the case of Gendron, he used a Bushmaster XM-15 semi-automatic rifle, taking advantage of America’s gun culture and ease of access to lethal weaponry.

The means of terrorism, perhaps once restricted to a hard core of guerrilla fighters, is now widely available to billions of people around the globe. Innovations in encrypted communications and access to sophisticated weaponry have facilitated the rise of what the United States military has called “ super-empowered individuals ,” who have access to powerful, low-cost commercial technology and are highly connected, able to reach far beyond their geographic location.

Fourth, Gendron was fueled in many ways by disinformation . A review of the Buffalo attacker’s social media history shows an obsession with the so-called “Great Replacement Theory,” a conspiracy that posits that a shadowy cabal of elites is deliberately replacing native-born white people with immigrants in an act of what they label “white genocide.” Gendron spent considerable time on 4chan , where he consumed significant amounts of disinformation about the supposed contribution of Black people to the crime rate, IQ differences, and posts directly related to Holocaust denialism.

Disinformation is now ubiquitous, with foreign countries crafting disinformation campaigns that feed domestic terrorism elsewhere. Russia and China regularly amplify divisive narratives on social media in an effort to sow further dissent within the United States. Many of these narratives are related to racial issues and promote posts related to civil war and domestic uprisings. Conspiracies abound online, and the advent of so-called “deep fakes” and artificial intelligence have only served to further complicate the information environment. Disinformation also lives forever on the internet, enshrined in terrorist manifestos, and passed around by followers and supporters of violent extremist ideologies.

Fifth, the Buffalo attack was deadly . In the United States, between 1994 and 2013 , there were only three years in which more than eight individuals were killed in terrorist attacks during the entire year. This was due to the Oklahoma City bombing (1995), 9/11 (2001), and the Fort Hood attack (2009). The Buffalo attack resulted in ten dead, reflective of the increasing trend in lethality—between 2014 and 2021, the average number of fatalities per year was thirty-one, demonstrating the increasing lethality of terrorism. Gendron illegally modified his rifle so that high-capacity magazines could be loaded into it, increasing the lethality of his attack.

Terrorists and terrorist groups to seek large body counts, with mass casualty events fueling the propaganda value of an attack or series of attacks. The advent of technologies like drones, 3-D printing, and autonomous weapons has exacerbated the threat of large-scale attacks, while terrorists’ pursuit of weapons of mass destruction, including chemical, biological, radiological, or nuclear weapons, remains a nightmare scenario. Terrorism expert Brian Michael Jenkins once wrote, “terrorists want a lot of people watching, not a lot of people dead.” The quote reflected the zeitgeist in terrorism studies at the time, especially with the skyjackings perpetrated by Palestinian terror groups in the 1970s, that the overarching purpose of terrorism was psychological and as a mode of communication to express a grievance to a large audience. After the al-Qaeda attacks of September 11, 2001, Jenkins revised his axiom, now commenting , “Many of today’s terrorists want a lot of people watching and a lot of people dead.”

Witnessing the devastation and destruction wrought by the COVID-19 pandemic likely increased terrorists’ interest in acquiring the materials and know-how to conduct a bioterror attack. Such an attack could be conducted surreptitiously by a small group with catastrophic effects. There would be considerable challenges in managing the aftermath, which could include the contagion of humans or animals or contamination of food and water sources or medicines. There will also be major challenges posed by physical-to-digital conversion technologies—including gene sequencing technology and the ability to send genome sequences by e-mail. Being able to send these sequences by e-mail means that terrorists in far-reaching corners of the globe could potentially cooperate to harness the disruptive power of technologies like CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats). In some circumstances, CRISPR could be used to create viruses, crop-destroying plagues, or “killer mosquitoes” that spread disease. This is another area where barriers to entry are being lowered, offering more opportunities for nefarious individuals and small groups to do harm.

Following two decades of prosecuting the Global War on Terrorism, many Western countries are now pivoting toward great power competition as the pendulum swings away from a focus on violent non-state actors back toward state-based threats. Accordingly, what were once vast troves of money and resources allocated to counterterrorism have diminished, and states and governments will be largely unprepared to deal with what comes next. France recently withdrew its troops from Mali. Germany has also suspended its mission to the country , even as the Russian private military contractor Wagner Group has moved to fill the power vacuum. The United States and many of its Western allies are attempting to move beyond counterterrorism as an organizing principle, preparing to meet the challenges posed by near-peer competitors. Russia’s invasion of Ukraine could further accelerate this trend.

But counterterrorism and great-power competition are not mutually exclusive. Indeed, in many cases, a convergence between the two will further complicate the response and may also lead to an increase in state-sponsored terrorism, a throwback to the proxy conflicts waged during the Cold War. An increase in state-sponsored terrorism could spur a spike in terrorist groups motivated by ethnonationalism or separatism. Counterterrorism strategies must adapt to the range of traditional and emerging threats from a multiplicity of international and domestic adversaries.

Some attacks, like the one in Buffalo, NY, will comprise all of the aforementioned trends. Others will represent one or two of the trends. When these trends converge, counterterrorism practitioners will face significant challenges. Thinking through the implications of a more diverse, decentralized, democratized, and deadly terrorist threat environment fueled by disinformation could help respond to a short-list of future possible scenarios. Moreover, it could also frame policy responses for how law enforcement, governments, and policymakers can begin preparing for the continued evolution of terrorism, violent extremism, and political violence.

The views expressed in this article are those of the author alone and do not necessarily reflect the position of the Foreign Policy Research Institute, a non-partisan organization that seeks to publish well-argued, policy-oriented articles on American foreign policy and national security priorities. 

Correction: The article has been updated to reflect that the Baader Meinhoff group in Germany and the Red Brigades in Italy were left-wing terrorist groups, not ethno-nationalists. 

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Understanding Domestic Radicalization and Terrorism

In 2017, there were 10,900 terrorist attacks around the world that killed more than 26,400 people, according to the National Consortium for the Study of Terrorism and Responses to Terrorism (START). [1] The number of terrorist attacks per year in the United States in the post-September 11 era has increased from 33 in 2002 to 65 in 2017. [2] It is evident that the number of terrorist attacks and the lethality of these attacks are increasing at alarming levels within the United States and abroad, and terrorism is a pressing national issue that lies very much within a global context.

With the threat of terrorism on the rise and acts of terrorism occurring increasingly at a national and global level, it is imperative — perhaps now more than ever — that we ensure our resources are being directed to the most practical and evidence-based means of countering violent extremism. Understanding why and how people radicalize, as well as what can be done to prevent radicalization or intervene during the process, are key to countering violent extremism.

See “Defining Terrorism”

What makes this issue complex is that in the United States, terrorist acts are carried out by people who are motivated by a wide variety of ideological viewpoints, who have gone through different radicalization processes, and who have unique grievances or life experiences that lead them toward radicalization to terrorism. This, in turn, makes it difficult to target prevention and intervention efforts toward any one “vulnerable” population. Radicalization to terrorism can be motivated by extremist groups/ideologies, or it can occur at an individual level (commonly referred to as “lone wolf terrorism”). In the United States, terrorists are usually associated with one of the six most commonly known ideologies: right-wing extremism, left-wing extremism, environmental extremism, nationalist/separatist extremism, religious extremism, and single-issue extremism. [3] Further, the nature of radicalization and types of extremist attacks are dynamic, changing from year to year and from decade to decade. [4]

NIJ plays a vital role in funding research related to domestic radicalization and terrorism in the United States. As a federal leader in the field, NIJ’s work is complementary to that of the Department of Homeland Security (DHS) and Department of Defense, as well as international partners in Canada, the United Kingdom, Australia, and New Zealand, among others. Although the field of domestic radicalization and terrorism research as a whole is considered to be in its infancy (compared to other fields in criminology, such as gangs or violent crime), significant achievements have been made.

NIJ’s Terrorism and Radicalization Research

In response to the September 11 terrorist attacks, NIJ started working with the National Academies in 2002 to help craft a research agenda to move the field of terrorism research forward. NIJ funded projects that focused on:

• Developing terrorism databases for analysis.
• Improving the criminal justice response.
• Addressing potential high-risk terrorism targets.
• Examining the links between terrorism and other crimes.
• Studying the organization, structure, and culture of terrorism.

After investing in numerous projects, NIJ played a shared role in the development and longevity of numerous national and global terrorism databases (such as the American Terrorism Study and the Global Terrorism Database) and institutions (such as START at the University of Maryland, the Terrorism Research Center at the University of Arkansas, and the Triangle Center on Terrorism and Homeland Security at Duke University).

In its fiscal year 2012 appropriation, Congress directed NIJ to administer dedicated funding for “research targeted toward developing a better understanding of the domestic radicalization phenomenon, and advancing evidence-based strategies for effective intervention and prevention.” [5] Every year since then, NIJ has received similarly dedicated funding to carry out this mission. In its first five years, the goal of NIJ’s Domestic Radicalization and Terrorism research portfolio was to answer the following questions:

• What are the primary drivers of radicalization to violent extremism, and how do they vary across cohorts (e.g., by grievance, by age, by socioeconomic categories)?
• How is radicalization to violent extremism analogous to other forms of extreme violence, such as mass casualty events and gangs?
• Which policy choices or programmatic interventions prevent or reduce radicalization to violent extremism, induce disengagement from violent extremism, or support deradicalization and desistance from violent extremism?

The program aimed to answer these questions for the benefit of multiple stakeholders but considered criminal justice agencies and their community partners as the primary beneficiaries.

Between 2012 and 2019, NIJ made competitive awards for 34 projects through its annual domestic radicalization and terrorism solicitation. Three of the most common topic areas funded under these awards involve research surrounding:

• The drivers of radicalization.
• The role of the internet and social media in the radicalization and recruitment process.
• Program evaluations of extremism prevention and intervention programs.

Some of the most important findings to date come from a set of NIJ-commissioned papers that summarize key findings from approximately 15 NIJ-funded studies and an international conference. [6] For example, one paper explores research on risk factors associated with radicalization to violent extremism that was presented at an international conference in 2015. These risk factors include violent extremists in an individual’s social network, identity processes, violent extremist belief systems and narratives, group dynamics, connections with violent extremists and violent extremist material through the internet and social media, and grievances. [7]

Two papers on radicalization risk factors [8] and the radicalization process [9] emphasize the social nature of radicalization. Reasons for concern may include an individual’s associates and drastic changes in those relationships. The papers also found 16 potential risk factors associated with attempts to engage in terrorism by both group-based and lone-actor terrorists. Although lone actors tend to be more public with their grievances and their intent to do harm, [10] they share these common risk factors with group-based terrorists:

• Having a criminal history.
• Having mental health issues (or receiving a diagnosis of schizophrenia or delusional disorder).
• Being unemployed.
• Being single.
• Being a loner (or socially isolated).
• Having military experience.

Meanwhile, NIJ-funded researchers at Georgia State University compared the motive, weapon use, and behaviors of three types of terrorists — lone-actor terrorists (not related to or in contact with a terrorist group), solo terrorists (those who act alone but are related to a terror group or network), and mass murderers. They found that these individuals can be distinguished by the degree to which they interact with co-conspirators, their antecedent event behaviors, and whether (and the degree to which) they leak information prior to an attack. [11] NIJ has funded other comparative studies of extremists and gang members, as well as extremists and human traffickers, among others. [12]

NIJ has also funded program evaluations, most notably an evaluation of the first Muslim-led, community-based countering violent extremism (CVE) program. [13] Researchers at the University of Massachusetts Lowell found that peers were the most likely to notice early signs of radicalization, but they had a reduced likelihood of reporting due to a reluctance bias. The research team developed a metric tool called a “suite of measures” that pertains to various types of psychological processes, motivations, states, and social circumstances; the tool can be readily adapted to CVE program evaluations.

Although many of the questions NIJ originally sought to answer have been addressed to some degree, many uncertainties remain. For example, we have learned about many of the drivers of radicalization and the similarities and differences between terrorism and other forms of violent crime. We have also learned that a majority of recruitment and radicalization occurs via the internet and social media. However, one of the most important findings may be that there is no one-size-fits-all approach to these questions. There is no single checklist that determines whether someone is on the path to becoming a terrorist. Additionally, although there have been many takeaways from NIJ-funded evaluations of intervention and prevention programs, the implementation, replication, and evaluation of such programs are still lacking in this field.

Advancing Knowledge

Forthcoming awards and publications in NIJ’s portfolio will help further advance the field of terrorism and radicalization studies. NIJ-funded researchers at the University of Virginia are studying women who have been involved with violent extremism to identify strategies used by the Islamic State group to recruit and radicalize Western women. [14] Another project is using post-September 11 era geocoded, terrorism-related precursor data to identify where people radicalize versus where terrorist events actually take place. [15] This is especially important as some of NIJ’s previously funded research has found that approximately 60% of terrorists lived more than 30 miles away from their terrorist target. [16]

NIJ-funded researchers at the RAND Corporation are conducting interviews with the families and close friends of individuals who have radicalized. [17] These interviews will offer unique insights and perspectives from the people who were closest to the radicalized individual because they are the ones most likely to detect changes in behavior.

A recent NIJ grant will build on landmark studies from Australia and the United Kingdom to understand the dynamics of and barriers to community reporting in the United States. [18] NIJ hopes that the study resulting from this grant will bridge a large research gap by attempting to understand the triggers, thresholds, facilitators, and barriers to reporting terrorism involvement. Further, the results could potentially inform the broader issue of reporting about violence in general, as the study will compare perspectives about reporting involvement in terrorism versus involvement in nonterrorist mass violence. The potential impact on the field is expected to be high, as it would be the first study of its kind to allow for direct comparison of sentiments and issues surrounding reporting by family and close friends in the United States, the United Kingdom, and Australia.

Lastly, in 2017, NIJ began a new publication series called Notes From the Field. [19] This series serves as a platform for leading criminal justice practitioners to share promising practices and lessons learned on pressing issues. It is part of an effort by NIJ to better connect with and learn from law enforcement. Notes From the Field recently launched a series on terrorism prevention to help fill gaps in knowledge and remain on the forefront of the most cutting-edge research in the field.

NIJ’s Role at the Federal and International Levels

NIJ also plays a large role in advancing radicalization and terrorism research at the federal and international levels. The Institute is one of the leading federal agencies that fund research on this topic.

Through meetings, project collaborations, and working groups, NIJ has established successful working relationships with its national and international partners. For example, NIJ has coordinated with DHS and other federal partners through the Countering Violent Extremism Task Force. As a member of the Task Force’s Research and Analysis Working Group, NIJ shares research findings with the group and stays abreast of research priorities, gaps, and progress made across entities within the federal government. NIJ also organizes expert panels at conferences, such as the annual meetings of the International Association of Chiefs of Police and the American Society of Criminology, which helps to translate research findings directly into law enforcement practice and target and enhance future research efforts.

On an international level, NIJ is involved with the Five Country Research and Development (5RD) Network, which includes government agency representatives from Australia, Canada, New Zealand, the United Kingdom, and the United States. The group works to cooperate, collaborate, and exchange information to ensure efficiency and coordination of applied research and development work relevant to a variety of domestic security topics. The DHS Science and Technology research team leads the group, which first met in 2015 to discuss best practices and lessons learned from international partners in efforts to counter violent extremism. The 5RD participants communicate regularly to share information and coordinate efforts to develop new technologies to prevent crime, ensure security, and protect citizens.

The 5RD Network has commissioned systematic reviews of research and evaluation efforts from all five countries to build a “global evidence-base for terrorism prevention, policy, strategy, and activity.” [20] These systematic reviews will cover subtopics such as common factors leading to radicalization, online indicators of radicalization, the role of the media in the radicalization process, and how community support and societal connections influence the prevention of radicalization. Recognizing that the threat of terrorism transcends all borders and ideologies, NIJ is hopeful that the rigorous and high-quality scientific findings from these reviews will help guide policymakers and practitioners in their decision-making.

Looking Forward

NIJ’s focus for the future will be to continue funding rigorous evaluations, developing stronger baseline knowledge of radicalization processes, and informing policy and programming through research to better understand how and why people radicalize, and which programs and policies work best to prevent radicalization from occurring.

Further, NIJ hopes to address significant research gaps in the field, such as gaps in understanding disengagement and deradicalization processes and what programs can be developed and delivered to those incarcerated for terrorist offenses. While it is vital to understand the “push” (forcing) and “pull” (attracting) factors behind why individuals become terrorists to help inform prevention and intervention policies and programs, it is equally important to understand these factors to inform disengagement and deradicalization efforts. If we do not understand what makes terrorism attractive to certain individuals and how terrorism is unique from other forms of violence, we will not be able to prevent, intervene with, deradicalize, or reintegrate those susceptible to this phenomenon. Bridging this gap would not only complement NIJ’s previous investments, it would lead to future opportunities for rigorous comparative research initiatives in conjunction with other topics, such as mass shootings and gangs.

By understanding disengagement and deradicalization, NIJ hopes to inform policy and practice around programming for those convicted of terrorist offenses in confinement as well as post-release. In the United States, it is estimated that 275 individuals have been convicted of terrorism-related charges since the September 11 terrorist attacks. [21] Although some of these individuals are serving life sentences, many others are expected to be released. For example, as of December 1, 2018, 168 individuals have been charged with offenses related to the Islamic State group alone (not including those motivated by any other type of terrorist ideologies); their average sentence is 13.2 years. [22] A majority of these individuals will be released, often sooner than we would expect. Hundreds of others are incarcerated for terrorism offenses or terrorism-related charges across the country, and there is still much to be learned about how these individuals should be reintegrated back into society, what the recidivism rate for these people will be, and to what extent their ideologies have changed after incarceration. Questions about the effectiveness of prison programming, access to services, and success with reintegration remain. Those who commit terrorist offenses are a relatively new and niche population that has been under-studied; immediate research and programming attention are needed to keep up with the growing number of individuals being released on such charges. This is a widely recognized concern in the field of terrorism research, and NIJ hopes to play a key role in fulfilling this urgent research need.

Overall, NIJ funding has allowed for valuable contributions to the field of domestic radicalization and terrorism research. However, there are still many uncertainties around how to intervene before an individual radicalizes or mobilizes to violence. NIJ intends to remain engaged in combating the constantly evolving threats presented by violent extremism through soliciting rigorous research, engaging with stakeholders, and informing policymakers.

For More Information

Learn more about NIJ’s domestic radicalization and terrorism portfolio .

About This Article

This article was published in the NIJ Journal Issue Number 282.

This article discusses the following grants:

• “Across the Universe? A Comparative Analysis of Violent Radicalization Across Three Offender Types with Implications for Criminal Justice Training and Education,” grant number 2013-ZA-BX-0002
• “Evaluation of a Multi-Faceted, U.S. Community-Based Muslim-Led CVE Program,” grant number 2013-ZA-BX-0003
• “Social Media as a Platform for Crafting Gender-Specific Interventions for the Domestic Radicalization of Women,” grant number 2016-ZA-BX-K002
• “Research on Domestic Radicalization to Violent Extremism: Insights from Family and Friends of Current and Former Extremists,” grant number 2017-ZA-CX-0005
• “Innovative Methodologies for Assessing Radicalization Risk: Risk Terrain Modeling and Conjunctive Analysis,” grant number 2017-ZA-CX-0004
• “Community Reporting Thresholds: Sharing Information with Authorities Concerning Terrorism Activity,” grant number 2018-ZA-CX-0004

Sidebar: Defining Terrorism

The definition of terrorism/terrorists is often a contested issue, with academics, government entities, media outlets, and others using varying language to define this concept. For the purposes of soliciting applications for research, NIJ has defined terrorists as “those individuals who commit or provide support for the commission of ideologically motivated violence to further political, social, or religious goals.” NIJ’s focus has been on the radicalization process as it occurs in the United States, regardless of the location of any act of terrorism that may ensue from that process.

Return to text .

[note 1] National Consortium for the Study of Terrorism and Responses to Terrorism (START), “Global Terrorism in 2017,” START Background Report, College Park, MD: University of Maryland, August 2018.

[note 2] START, “American Deaths in Terrorist Attacks, 1995-2017,” START Fact Sheet, College Park, MD: University of Maryland, September 2018.

[note 3] START, “Ideological Motivations of Terrorism in the United States, 1970-2016,” START Background Report, College Park, MD: University of Maryland, November 2017.

[note 4] START, “Ideological Motivations of Terrorism in the United States, 1970-2016.”

[note 5] Consolidated and Further Continuing Appropriations Act, 2012 , Public Law 112–55, 125 Stat. 615, November 18, 2011.

[note 6] National Institute of Justice, “ Radicalization and Violent Extremism: Lessons Learned from Canada, the U.K. and the U.S. , ” Washington, DC: U.S. Department of Justice, National Institute of Justice, July 2015, NCJ 249947; Allison G. Smith, “ How Radicalization to Terrorism Occurs in the United States: What Research Sponsored by the National Institute of Justice Tells Us ,” Washington, DC: U.S. Department of Justice, National Institute of Justice, June 2018, NCJ 250171; Allison G. Smith, “ Risk Factors and Indicators Associated With Radicalization to Terrorism in the United States: What Research Sponsored by the National Institute of Justice Tells Us ,” Washington, DC: U.S. Department of Justice, National Institute of Justice, June 2018, NCJ 251789; and National Institute of Justice, “Overview of Research Sponsored to Support and Evaluate Programs Aimed at Countering Violent Extremism in the United States” (unpublished).

[note 7] National Institute of Justice, “ Radicalization and Violent Extremism: Lessons Learned from Canada, the U.K. and the U.S. ”

[note 8] Smith, “ Risk Factors and Indicators .”

[note 9] Smith, “ How Radicalization to Terrorism Occurs .”

[note 10] John G. Horgan et al., “ Across the Universe? A Comparative Analysis of Violent Behavior and Radicalization Across Three Offender Types with Implications for Criminal Justice Training and Education ,” Final report to the National Institute of Justice, grant number 2013-ZA-BX-0002, June 2016, NCJ 249937.

[note 11] Horgan et al., “ Across the Universe? ”

[note 12] National Institute of Justice, “ Research on Domestic Radicalization and Terrorism ,” January 16, 2020.

[note 13] Michael J. Williams, John G. Horgan, and William P. Evans, “ Evaluation of a Multi-Faceted, U.S. Community-Based, Muslim-Led CVE Program ,” Final report to the National Institute of Justice, grant number 2013-ZA-BX-0003, June 2016, NCJ 249936.

[note 14] National Institute of Justice, “ Social Media As a Platform for Crafting Gender-Specific Interventions for the Domestic Radicalization of Women ,” grant number 2016-ZA-BX-K002.

[note 15] National Institute of Justice, “ Innovative Methodologies for Assessing Radicalization Risk: Risk Terrain Modeling and Conjunctive Analysis ,” grant number 2017-ZA-CX-0004.

[note 16] Brent Smith, “ A Look at Terrorist Behavior: How They Prepare, Where They Strike ,” NIJ Journal 260, July 2008.

[note 17] National Institute of Justice, “ Research on Domestic Radicalization to Violent Extremism: Insights from Family and Friends of Current and Former Extremists ,” grant number 2017-ZA-CX-0005.

[note 18] National Institute of Justice, “ Community Reporting Thresholds: Sharing Information with Authorities Concerning Terrorism Activity ,” grant number 2018-ZA-CX-0004.

[note 19] National Institute of Justice, “ Notes From the Field .”

[note 20] Department of Homeland Security Science and Technology Directorate, “ Systematic Reviews of Terrorism Prevention Research ,” Washington, DC: Department of Homeland Security, September 19, 2018.

[note 21] Lorenzo Vidino and Seamus Hughes, “America’s Terrorism Problem Doesn’t End with Prison — It Might Just Begin There,” Lawfare (blog), June 17, 2018.

[note 22] Lorenzo Vidino and Seamus Hughes, ISIS in America: From Retweets to Raqqa, Washington, DC: The George Washington University, Program on Extremism, December 2015.

About the author

Aisha Javed Qureshi is a social science analyst at NIJ.

Cite this Article

Read more about:, related publications.

• NIJ Journal Issue No. 282

Related Awards

• Across the Universe? A Comparative Analysis of Violent Radicalization Across Three Offender Types with Implications for Criminal Justice Training and Education
• Evaluation of a Multi-Faceted, U.S. Community-Based Muslim-Led CVE Program
• Social Media As a Platform for Crafting Gender-Specific Interventions for the Domestic Radicalization of Women
• Research on Domestic Radicalization to Violent Extremism: Insights from Family and Friends of Current and Former Extremists
• INNOVATIVE METHODOLOGIES FOR ASSESSING RADICALIZATION RISK: RISK TERRAIN MODELING AND CONJUNCTIVE ANALYSIS
• Community Reporting Thresholds: Sharing Information with Authorities Concerning Terrorism Activity.

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The Escalating Terrorism Problem in the United States

Photo: MARK RALSTON/AFP via Getty Images

Table of Contents

Brief by Seth G. Jones , Catrina Doxsee , and Nicholas Harrington

Published June 17, 2020

Available Downloads

• Download the CSIS Brief 542kb
• Download the Methodology 68kb

The United States faces a growing terrorism problem that will likely worsen over the next year. Based on a CSIS data set of terrorist incidents, the most significant threat likely comes from white supremacists, though anarchists and religious extremists inspired by the Islamic State and al-Qaeda could present a potential threat as well. Over the rest of 2020, the terrorist threat in the United States will likely rise based on several factors, including the November 2020 presidential election.

On June 3, 2020, federal authorities arrested three individuals allegedly associated with the “boogaloo” movement, a loosely-organized group of extremists preparing for a civil war, for conspiring to cause violence in Las Vegas and possessing an improvised incendiary device. 1 Less than a week later, law enforcement officials near Richmond, VA, arrested Harry H. Rogers, a member of the Ku Klux Klan, for driving a vehicle into peaceful protesters. Around the same time, members of a Brooklyn anarchist group urged its supporters to conduct “rebellion” against the government. 2 Extremists from all sides flooded social media with disinformation, conspiracy theories, and incitements to violence in response to the protests following the death of George Floyd, swamping Twitter, YouTube, Facebook, and other platforms. 3 This CSIS brief examines the state of terrorism in the United States. It asks two sets of questions. First, what are the most significant types of terrorism in the United States, and how has the terrorism threat in the U.S. homeland evolved over time? Second, what are the implications for terrorism over the next year? To answer these questions, this analysis compiles and analyzes an original data set of 893 terrorist plots and attacks in the United States between January 1994 and May 2020. This analysis makes several arguments. First, far-right terrorism has significantly outpaced terrorism from other types of perpetrators, including from far-left networks and individuals inspired by the Islamic State and al-Qaeda. Right-wing attacks and plots account for the majority of all terrorist incidents in the United States since 1994, and the total number of right-wing attacks and plots has grown significantly during the past six years. Right-wing extremists perpetrated two thirds of the attacks and plots in the United States in 2019 and over 90 percent between January 1 and May 8, 2020. Second, terrorism in the United States will likely increase over the next year in response to several factors. One of the most concerning is the 2020 U.S. presidential election, before and after which extremists may resort to violence, depending on the outcome of the election. Far-right and far-left networks have used violence against each other at protests, raising the possibility of escalating violence during the election period. The rest of this brief is divided into the following sections. The first defines terrorism and its main types. The second section analyzes terrorism trends in the United States since 1994. The third examines far-right, far-left, and religious networks. The fourth section highlights the terrorism threat over the next year.  

DEFINITIONS

This analysis focuses on terrorism: the deliberate use—or threat—of violence by non-state actors in order to achieve political goals and create a broad psychological impact. 4 Violence—and the threat of violence—are important components of terrorism. Overall, this analysis divides terrorism into four broad categories: right-wing, left-wing, religious, and ethnonationalist. 5 To be clear, terms like right-wing and left-wing terrorism do not—in any way—correspond to mainstream political parties in the United States, such as the Republican and Democratic parties, which eschew terrorism. Instead, terrorism is orchestrated by a small minority of extremists. First, right-wing terrorism refers to the use or threat of violence by sub-national or non-state entities whose goals may include racial or ethnic supremacy; opposition to government authority; anger at women, including from the incel (“involuntary celibate”) movement; and outrage against certain policies, such as abortion. 6 This analysis uses the term “right-wing terrorism” rather than “racially- and ethnically-motivated violent extremism,” or REMVE, which is used by some in the U.S. government. 7 Second, left-wing terrorism involves the use or threat of violence by sub-national or non-state entities that oppose capitalism, imperialism, and colonialism; pursue environmental or animal rights issues; espouse pro-communist or pro-socialist beliefs; or support a decentralized social and political system such as anarchism. Third, religious terrorism includes violence in support of a faith-based belief system, such as Islam, Judaism, Christianity, and Hinduism, among many others. As highlighted in the next section, the primary threat from religious terrorists comes from Salafi-jihadists inspired by the Islamic State and al-Qaeda. Fourth, ethnonationalist terrorism refers to violence in support of ethnic or nationalist goals—often struggles of self-determination and separatism along ethnic or nationalist lines. In examining terrorism, this analysis does not specifically address several related phenomena. For example, it does not focus on hate crimes. There is overlap between terrorism and hate crimes, since some hate crimes include the use or threat of violence. 8 But hate crimes can also include non-violent incidents such as graffiti and verbal abuse. Hate crimes are obviously concerning and a threat to society, but this analysis concentrates only on terrorism and the use—or threat—of violence to achieve political objectives.  

TRENDS IN U.S. TERRORISM

To evaluate the threat posed by terrorism, we compiled a data set of 893 incidents that occurred in the United States between January 1994 and May 8, 2020. 9 (The link to the methodology can be found at the end of the brief.) These incidents included both attacks and foiled plots. We coded the ideology of the perpetrators into one of five categories: ethnonationalist, left-wing, religious, right-wing, and other (which included motivations that did not fit into any of the categories). All of the religious attacks and plots in the CSIS data set were committed by terrorists who ascribed to a Salafi-jihadist ideology. This section analyzes the data in two parts: terrorist incidents and fatalities. The data show three notable trends. First, right-wing attacks and plots accounted for the majority of all terrorist incidents in the United States since 1994. In particular, they made up a large percentage of incidents in the 1990s and 2010s. Second, the total number of right-wing attacks and plots has grown substantially during the past six years. In 2019, for example, right-wing extremists perpetrated nearly two-thirds of the terrorist attacks and plots in the United States, and they committed over 90 percent of the attacks and plots between January 1 and May 8, 2020. Third, although religious extremists were responsible for the most fatalities because of the 9/11 attacks, right-wing perpetrators were responsible for more than half of all annual fatalities in 14 of the 21 years during which fatal attacks occurred.  

ATTACKS AND PLOTS

In analyzing fatalities from terrorist attacks, religious terrorism has killed the largest number of individuals—3,086 people—primarily due to the attacks on September 11, 2001, which caused 2,977 deaths. 10 The magnitude of this death toll fundamentally shaped U.S. counterterrorism policy over the past two decades. In comparison, right-wing terrorist attacks caused 335 deaths, left-wing attacks caused 22 deaths, and ethnonationalist terrorists caused 5 deaths. To evaluate the ongoing threat from different types of terrorists, however, it is useful to consider the proportion of fatalities attributed to each type of perpetrator annually. In 14 of the 21 years between 1994 and 2019 in which fatal terrorist attacks occurred, the majority of deaths resulted from right-wing attacks. In eight of these years, right-wing attackers caused all of the fatalities, and in three more—including 2018 and 2019—they were responsible for more than 90 percent of annual fatalities. 11 Therefore, while religious terrorists caused the largest number of total fatalities, right-wing attackers were most likely to cause more deaths in a given year.  

TYPES OF TERRORISM

This section outlines the threat from right-wing, left-wing, and religious networks. In particular, it focuses on the threat from right-wing extremists because of the high number of incidents and fatalities they perpetrated, as highlighted in the previous section. It does not cover ethnonationalist networks, which are not a major threat in the United States today.

RIGHT-WING TERRORISM

There are three broad types of right-wing terrorist individuals and networks in the United States: white supremacists, anti-government extremists, and incels. There are numerous differences between (and even within) these types, such as ideology, capabilities, tactics, and level of threat. Adherents also tend to blend elements from each category. But there are some commonalities. First, terrorists in all of these categories operate under a decentralized model. The threats from these networks comes from individuals, not groups. 12 For example, anti-government activist and white supremacist Louis Beam advocated for an organizational structure that he termed “leaderless resistance” to target the U.S. government. 13

Second, these networks operate and organize to a great extent online, challenging law enforcement efforts to identify potential attackers. 14 Right-wing terrorists have used various combinations of Facebook, Twitter, YouTube, Gab, Reddit, 4Chan, 8kun (formerly 8Chan), Endchan, Telegram, Vkontakte, MeWe, Discord, Wire, Twitch, and other online communication platforms. Internet and social media sites continue to host right-wing extremist ideas such as the Fourteen Words (also referred to as the 14 or 14/88) coined by white supremacist David Lane, a founding member of the group the Order. The Fourteen Words includes variations like: “We must secure the existence of our people and a future for white children.” 15 Far-right perpetrators also use computer games and forums to recruit. 16 Third, right-wing extremists have adopted some foreign terrorist organization tactics, though al-Qaeda and other groups have also adopted tactics developed by right-wing movements. 17 In a June 2019 online post, a member of the Atomwaffen Division (AWD) stated, “the culture of martyrdom and insurgency within groups like the Taliban and ISIS is something to admire and reproduce in the neo-Nazi terror movement.” 18 Similarly, the Base—a loosely organized neo-Nazi accelerationist movement which shares the English-language name for al-Qaeda—uses a vetting process to screen potential recruits, similar to the methods of al-Qaeda. 19

Anti-government Extremists: The right-wing terrorist threat also includes anti-government extremists, including militias and the sovereign citizen movement. Most militia extremists view the U.S. government as corrupt and a threat to freedom and rights. 31 Other far-right anti-government groups mobilized to protect a perceived threat to individual gun ownership rights. Modern militias are organized as paramilitaries that conduct weapons training and other field exercises. 32 The Three Percenters are a far-right paramilitary group that advocates gun rights and seeks to limit U.S. government authorities. In August 2017, Jerry Varnell, a 23-year-old who identified as holding the “III% ideology” and wanted to “start the next revolution” attempted to detonate a bomb outside of an Oklahoma bank, similar to the 1995 Oklahoma City bombing. 33 Also, in January 2017, Marq Perez, who discussed the attack in Three Percenter channels on Facebook, burglarized and burned down a mosque in Texas. 34

Anti-government extremists, which sometimes blend with white supremacist movements, have used the slang word “boogaloo” as a shorthand for a coming civil war. Several popular Facebook groups and Instagram pages, such as Thicc Boog Line, P A T R I O T Wave, and Boogaloo Nation, have emerged spreading the boogaloo conspiracy. Police in Texas arrested 36-year-old Aaron Swenson in April after he attempted to livestream his search for a police officer that he could ambush and execute. 35 Prior to his arrest, Swenson had shared memes extensively from boogaloo pages.

Incels: Involuntary celibates, or incels, conduct acts of violence against women. The incel movement is composed of a loosely organized virtual community of young males. Incels believe that one’s place in society is determined by physical characteristics and that women are responsible for this hierarchy. Incels identify with the writings of Elliot Rodger, who published a 133-page manifesto, titled “My Twisted World.” 36 In October 2015, Christopher Harper-Mercer, inspired by Rodger, killed nine people at a community college in Oregon. 37 In November 2018, 40-year-old Scott Beierle killed two women in a yoga studio in Tallahassee, Florida, before committing suicide. 38

LEFT-WING TERRORISM

The far-left includes a decentralized mix of actors. Anarchists are fundamentally opposed to a centralized government and capitalism, and they have organized plots and attacks against government, capitalist, and globalization targets. 39 Environmental and animal rights groups, such as the Earth Liberation Front and Animal Liberation Front, have conducted small-scale attacks against businesses they perceive as exploiting the environment. 40 In addition, the far-left includes Antifa, which is a contraction of the phrase “anti-fascist.” It refers to a decentralized network of far-left militants that oppose what they believe are fascist, racist, or otherwise right-wing extremists. While some consider Antifa a sub-set of anarchists, adherents frequently blend anarchist and communist views. One of the most common symbols used by Antifa combines the red flag of the 1917 Russian Revolution and the black flag of nineteenth-century anarchists. Antifa groups frequently conduct counter- protests to disrupt far-right gatherings and rallies. They often organize in black blocs (ad hoc gatherings of individuals that wear black clothing, ski masks, scarves, sunglasses, and other material to conceal their faces), use improvised explosive devices and other homemade weapons, and resort to vandalism. In addition, Antifa members organize their activities through social media, encrypted peer-to-peer networks, and encrypted messaging services such as Signal. Antifa groups have been increasingly active in protests and rallies over the past few years, especially ones that include far-right participants. 41 In June 2016, for example, Antifa and other protestors confronted a neo-Nazi rally in Sacramento, CA, where at least five people were stabbed. In February, March, and April 2017, Antifa members attacked alt-right demonstrators at the University of California, Berkeley, using bricks, pipes, hammers, and homemade incendiary devices. 42 In July 2019, William Van Spronsen, a self-proclaimed Antifa, attempted to bomb the U.S. Immigration and Customs Enforcement detention facility in Tacoma, Washington, using a propane tank but was killed by police. 43

RELIGIOUS TERRORISM

While religious terrorism is concerning, the United States does not face the same level of threat today from religious extremists—particularly those inspired by Salafi-jihadist groups such as the Islamic State and al-Qaeda—as some European countries. 44 But Salafi-jihadists still pose a limited threat. In December 2019, Second Lieutenant Mohammed Saeed Alshamrani, a Saudi air force cadet training with the American military in Pensacola, Florida, killed three men and injured three others. He was inspired by al-Qaeda’s ideology, communicated with leaders of al-Qaeda in the Arabian Peninsula up until the attack, and joined the Saudi military in part to carry out a “special operation.” 45 In addition, leaders of al-Qaeda and the Islamic State continue to encourage individuals in the West—including the United States—to conduct attacks. 46 There are still perhaps 20,000 to 25,000 jihadist fighters in Syria and Iraq from the Islamic State and another 15,000 to 20,000 fighters from two al-Qaeda-linked groups: Hay’at Tahrir al-Sham and Tanzim Hurras al-Din. 47 Over the next several months, more jihadists may enter the battlefield after escaping—or being released—from prisons run by the Syrian Democratic Forces in areas such as al-Hol, located in eastern Syria near the border with Iraq. 48 In addition, there are still concerns about al-Qaeda and Islamic State groups operating in Yemen, Nigeria and neighboring countries, Somalia, Afghanistan, and other countries. In a May 2020 report, the United Nations concluded that al-Qaeda remains a serious threat and that the “senior leadership of Al-Qaida remains present in Afghanistan, as well as hundreds of armed operatives, Al-Qaida in the Indian Subcontinent, and groups of foreign terrorist fighters aligned with the Taliban.” 49  

THE RISING SPECTER OF TERRORISM

Our data suggest that right-wing extremists pose the most significant terrorism threat to the United States, based on annual terrorist events and fatalities. Over the next year, the threat of terrorism in the United States will likely increase based on several factors, such as the November 2020 presidential election and the response to the Covid-19 crisis. These factors are not the cause of terrorism, but they are events and developments likely to fuel anger and be co-opted by a small minority of extremists as a pretext for violence. First, the November 2020 presidential election will likely be a significant source of anger and polarization that increases the possibility of terrorism. Some—though not all—far-right extremists associate themselves with President Trump and may resort to violence before or after the election. As U.S. Department of Justice documents have highlighted, some far-right extremists have referred to themselves as “Trumpenkriegers”—or “fighters for Trump.” 50 If President Trump loses the election, some extremists may use violence because they believe—however incorrectly—that there was fraud or that the election of Democratic candidate Joe Biden will undermine their extremist objectives. Alternatively, some on the far-left could resort to terrorism if President Trump is re-elected. In June 14, 2017, James Hodgkinson—a left-wing extremist—shot U.S. House Majority Whip Steve Scalise, U.S. Capitol Police officer Crystal Griner, congressional aide Zack Barth, and lobbyist Matt Mika in Alexandria, VA. A few months earlier, Hodgkinson wrote in a Facebook post that “Trump is a Traitor. Trump Has Destroyed Our Democracy. It’s Time to Destroy Trump & Co.” 51 Tension on both the far right and far left has dramatically risen over the past several years. Second, developments associated with Covid-19—such as prolonged unemployment or government attempts to close “non-essential” businesses in response to a second or third wave—could increase the possibility of terrorism. Some far-right extremists, for example, have threatened violence and railed against federal, state, and local efforts to take away their freedoms by requiring face coverings in public indoor settings, closing businesses, and prohibiting large gatherings to curb the spread of the virus. In March 2020, Timothy Wilson, who had ties to neo-Nazi groups, was killed in a shootout with FBI agents who were attempting to arrest him for planning to bomb a hospital in Missouri. Though he had been planning the attack for some time and had considered a variety of targets, he used the outbreak of Covid-19 to target a hospital in order to gain additional publicity. On the far left and far right, some anti-vaxxers—who oppose vaccines as a conspiracy by the government and pharmaceutical companies—have threatened violence in response to Covid-19 response efforts. 52 Third, a polarizing event other than the presidential election—such as a school shooting or racially-motivated killing—could spark protests that extremists attempt to hijack. As highlighted in the introduction, extremists from all sides attempted to hijack the May and June 2020 protests in the United States as an excuse to commit acts of terrorism. In addition, far-right and far-left networks have used violence against each other at protests—such as in Berkeley, CA and Charlottesville, VA in 2017—raising concerns about escalating violence. All parts of U.S. society have an important role to play in countering terrorism. Politicians need to encourage greater civility and refrain from incendiary language. Social media companies need to continue sustained efforts to fight hatred and terrorism on their platforms. Facebook, Google, Twitter, and other companies are already doing this. But the struggle will only get more difficult as the United States approaches the November 2020 presidential election—and even in its aftermath. Finally, the U.S. population needs to be more alert to disinformation, double-check their sources of information, and curb incendiary language. Terrorism feeds off lies, conspiracies, disinformation, and hatred. Indian leader Mahatma Gandhi urged individuals to practice what he called “satyagraha,” or truth force. “Satyagraha is a weapon of the strong; it admits of no violence under any circumstance whatever; and it always insists upon truth,” he explained. 53 That advice is just as important as it has ever been in the United States. Seth G. Jones is the Harold Brown Chair and director of the Transnational Threats Project at the Center for Strategic and International Studies (CSIS) in Washington, D.C. Catrina Doxsee is a program manager and research associate with the Transnational Threats Project at CSIS. Nicholas Harrington is a research associate for the Transnational Threats Project at CSIS. The authors give special thanks to James Suber and Grace Hwang for their research assistance and helpful comments, including their involvement in building the terrorism data set. For an overview of the methodology used in compiling the data set, please see here . This brief is made possible by general support to CSIS. No direct sponsorship contributed to this brief.

CSIS Briefs   are produced by the Center for Strategic and International Studies (CSIS), a private, tax-exempt institution focusing on international public policy issues. Its research is nonpartisan and nonproprietary. CSIS does not take specific policy positions. Accordingly, all views, positions, and conclusions expressed in this publication should be understood to be solely those of the author(s).

© 2020 by the Center for Strategic and International Studies. All rights reserved.

Please consult the PDF for references.

Seth G. Jones

Catrina Doxsee

Nicholas harrington, programs & projects.

By: Bastian Herre , Veronika Samborska , Hannah Ritchie and Max Roser

Terrorism is the threat or use of violence to intimidate or coerce in the pursuit of political or ideological goals. It is usually understood to be done by non-state actors — individuals or organizations not part of the government.

Terrorism can take many forms, including bombings, armed assaults, hijackings, or hostage-taking. Its targets can also vary and can be aimed at civilians, state actors, or public infrastructure.

Globally, terrorist attacks and deaths are rare. But this varies a lot worldwide: in some countries, they are common and on the rise, while in others, there are few attacks or none.

Terrorism is also a big concern for people across the world. Attacks are rare, but often shocking, as they are meant to scare and subdue.

This page provides data and research on how common terrorism is, how it differs across countries, and whether it is becoming more or less frequent over time.

Key Insights on Terrorism

Terrorism is a rare cause of death globally.

Globally, around 20,000 people died from terrorism in 2019.

This meant it caused an estimated 1 in 2000 deaths that year.

This is shown in the chart, which lays out what people died from. You can find terrorism in the bottom right-hand corner.

Terrorism is not only a much rarer cause of death than non-communicable or infectious diseases; it is also a rare cause of violent death: more than 4 times as many people die in armed conflicts, and more than ten times as many die from homicides. 1

While the number of terrorism deaths can change a lot from year to year — in 2014, almost 45,000 people died in terrorist attacks, while in 2010, it was less than 8,000 — the average over the last decade was close to the 2019 number, at around 24,000 per year.

While terrorism is a rare cause of death globally, it is more common in some countries and regions .

What you should know about this data

• We rely on the Global Terrorism Database (GTD) 2 to measure terrorism deaths.
• The GTD defines a terrorist attack as the threat or use of violence to achieve a political, economic, religious, or social goal through intimidation or coercion by a non-state actor. Learn more in our technical article on the GTD .
• Because the data is based on news reports, attacks, and deaths may be underreported in countries with sparser media, especially during early years.
• Data for other causes of death comes from the Global Burden of Disease study by the Institute for Health Metrics and Evaluation (IHME), published in 2019.

Terrorism affects some countries much more than others

There are very large differences in the prevalence of terrorism across the world. Many countries experience few to no attacks and deaths. Others experience hundreds or thousands a year.

The map shows deaths from terrorism around the world. The distribution of terrorist attacks looks similar.

Most attacks happen in South Asia, the Middle East, and Africa. Deaths are even more concentrated in some regions. In them, terrorism deaths can account for several percent of all deaths in a year.

The countries most affected by terrorism can change frequently. In 2020, almost half of the people who died from terrorism were in Afghanistan, while in 2016, Iraq was the most affected, where a third of all terrorism deaths occurred.

Terrorism has increased in some parts of the world, but decreased in others

Terrorism trends have been very different across the world’s regions.

The chart shows terrorism deaths in various regions since 1970.

Importantly, because the estimates are based on news reports, attacks, and deaths may be underreported in countries with sparser media, especially during early years.

To help show the trends within regions, the axis scales between regions are not identical. But when comparing regions, you can see that terrorism deaths have been much more common in some than in others.

Global deaths from terrorism have been higher over the last decade than in the 1990s and 2000s – even when data collection had improved significantly. But they have fallen again in recent years.

This global trend is driven by increased terrorism deaths in South Asia, the Middle East and North Africa, and Sub-Saharan Africa. However, there has been a decline in deaths in more recent years in these regions, especially in the Middle East and North Africa.

In Western Europe, deaths from terrorism have declined in recent decades. A few deadly attacks, however, have meant that deaths have varied a lot from year to year.

Deaths from terrorism in North America are typically low – but the September 11 attacks in the United States in 2001 stand out. South America has seen a sustained decrease in terrorism.

The global and regional trends are broadly similar if we look at attacks instead of deaths.

Most terrorist attacks target private citizens, the military, and the police

Terrorist attacks have different targets, and some are much more common than others.

The chart shows the global number of terrorist attacks by the intended target.

Most attacks target private citizens, the military and police, and other government institutions. Attacks on religious and educational institutions, non-government organizations, and journalists are rare. Attacks on tourists are very rare.

This looks similar when we examine deaths from terrorism by the intended target 3 : most people globally die in attacks that target private citizens or their property, the military, and the police.

Many people worry about terrorism, even in countries where it is rare

In almost every country, concern about terrorist attacks is high. This is even true in countries where they are rare .

The chart shows the share of respondents in representative surveys that worried “very much” or “a great deal” about a terrorist attack.

In most countries, more than half of all people said they are worried about terrorism. In some, it was more than 80%.

When we compare the share of people worried about terrorism to how many people die from it, we see that concerns are also high in countries that rarely experience terrorism.

This means that terrorism’s reach goes much farther than the people who experience attacks themselves or have loved ones get injured or die in them.

The goal of terrorism is often to intimidate people. Surveys show that this is often successful.

• We rely on the World Values Survey (WVS) 4 to measure concerns about terrorism.
• The WVS provides data from representative surveys taken by tens of thousands of people in countries around the world.

Research & Writing

The Global Terrorism Database: how do researchers measure terrorism?

There are different approaches to measuring terrorism. Here is the approach of the Global Terrorism Database, a leading source of terrorism data.

Bastian Herre

Causes of death globally: what do people die from?

To make progress towards a healthier world we need to have a good understanding of what health problems we face today.

More Articles on Terrorism

Why do some terrorist attacks receive more media attention than others.

Hannah Ritchie and Max Roser

Airline hijackings were once common but are very rare today

Hannah Ritchie

Interactive Charts on Terrorism

Acknowledgements.

We thank Cam Appel for his contributions to previous versions of this topic page.

Armed conflicts can include terrorist attacks, but they do not necessarily seek to coerce and intimidate or targets civilians.

Homicides differ from terrorism because they are not committed in the pursuit of political or ideological goals.

START (National Consortium for the Study of Terrorism and Responses to Terrorism). 2021. Global Terrorism Database (GTD) [Data set] . University of Maryland.

Importantly, this includes bystanders who were not the intended target. So, an attack targeting members of the military may still kill more civilians who happened to be nearby.

Inglehart, R., C. Haerpfer, A. Moreno, C. Welzel, K. Kizilova, J. Diez-Medrano, M. Lagos, P. Norris, E. Ponarin & B. Puranen (eds.). 2022. World Values Survey: All Rounds - Country-Pooled Datafile. Madrid, Spain & Vienna, Austria: JD Systems Institute & WVSA Secretariat. Dataset Version 3.0.0. doi:10.14281/18241.17

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