Since the terrorist attacks of 9/11, a number of economists and other social scientists have been studying terrorism. Khusrav Gaibulloev and Todd Sandler summarize the findings in a review article written for the Journal of Economic Literature (June 2019, pp. 275-328, not freely available online, but many readers should have access via subscriptions through their library).  Here, I\’ll hit some high spots of their five main themes, and I\’ll skip the citations, but the paper itself has vastly more detail.

\”First, terrorism has altered in form after the rising dominance of religious fundamentalist terrorism in the 1990s and the augmented security measures in the West after 9/11. These considerations have changed the lethality, location, and nature of terrorism over time …\”

Examples include a shift in the nature of groups most likely to engaged in terrorist activities, along with a decline in transnational and a rise in domestic terrorism.

\”Prior to the 1990s, most terrorist groups were left wing or nationalist/separatist. The rapid rise of religious fundamentalist terrorist groups started in the 1990s with al-Qaida and its Islamic extremist affiliated groups. Unlike the leftists who generally wanted to limit casualties and collateral damage, the religious fundamentalists wanted to maximize carnage, as 9/11 and the March 11, 2004 Madrid commuter bombings demonstrate. During the 1990s, the religious fundamentalists assumed a dominant influence among terrorist groups. … [T]he number of transnational terrorist incidents have fallen by about
40 percent since the start of the 1990s; however, each incident was much more likely
to involve casualties since then.\”

\”Second, terrorist groups respond rationally to their environment to ensure their survivability and visibility. In so doing, they adopt novel institutional forms and adjust their attack portfolios in response to counterterrorism actions.\”
The theme of this research is that groups considering terrorist activity often have a range of other possible actions to pursue: peaceful protest, violent protest, guerrilla attacks, even an attempt to take over territory and in effect engage in civil war. If terrorism is the choice, will it take the form of kidnapping, hostage-taking, bombing, or mass shooting? In addition, groups considering terrorism will take context into account. Strong state or weak state? Are there other terrorist groups already in action, which can make it easier for new terrorist groups to begin and less likely that new groups will be caught? Do the terrorists have a reasonably safe refuge, perhaps in another country, to which they can retreat between attacks? 
The fact that terrorist groups evolve an dmake these kinds of choices has consequences. For example, there is some evidence to support the hypothesis that as governments have made greater efforts to  protect official installation and people from terrorist attacks, one result has been a rise in terrorist attacks aimed at civilian targets. There is also evidence to support an argument that terrorist groups sometimes try attract more supporters by outbidding\” each other to engage in more prominent acts of violence. 
Another finding in this literature is that the older-style political terrorist groups were more likely to break up in internal disagreements and easier to infiltrate. The newer religious-based terrorist groups
\”rely on kinship, long-term friendships, and worship for recruiting purposes. Such ties are very tight and make it extremely difficult for the authorities to infiltrate these groups. Additionally, these ties provide a aense of camaraderie among members that facilitates volunteers for dangerous and even
deadly operations …\”

\”Third, counterterrorism policies have had mixed success. Targeted governments often work at cross-purposes, relying too much on attack-deflecting defensive measures and too little on proactive offensive measures, especially when the same terrorist group targets multiple countries. Frequently, well-intentioned counterterrorism policies may have unintended consequences as terrorists or governments strategically react to one another’s actions. More thought needs to be given to countermeasures that offset terrorists’ actions, such as service provision, that win them a constituency.\”

\”After 9/11, the sustained War on Terror is seen to have apparently little long-term effect on global terrorism. … Furthermore, enhanced border security since 9/11 caused transference of attacks from North America and Europe to the Middle East, Africa, and Asia, consistent with the earlier defensive game theory model.\”

What are some possible steps that could be taken in addition to defensive measures? In situations where a terrorist group is providing services to a local population, opponents of the terrorists could seek to establish alternative sources of those services. On the other hand, a policy that involves sending more aid to areas that originate terrorism will send send some mixed messages! Cooperating to limit flows of money and materiel to terrorists can be helpful. \”The literature also shows that
directed proactive measures—e.g., assassination of militant leaders or house demolitions—are effective …\”

\”Fourth, terrorism has myriad causes. The alleged relationships between terrorism and globalization, terrorism and poverty, and terrorism and regime type are much more nuanced than believed after 9/11.\” 

It\’s difficult for most of us to get a grip on what leads a person to commit terrorist activity, and so it can be easy to make up reasons that seem at least a little plausible–and then just to assert for some people, these reasons are sufficient to drive some people to terrorism. The evidence hasn\’t been kind to such assertions.

For example, consider the argument that poverty leads to terrorism. One of the first research papers on this subject was published in the Fall 2003 issue of the Journal of Economic Perspectives, where I work as Managing Editor: Krueger, Alan, B., and Jitka Malečková.  \”Education, Poverty and Terrorism: Is There a Causal Connection?\”(17:4, 119-144). Looking at the Palestinian population and terrorism, they found that those with high levels of education (and thus presumably higher incomes) were quite likely to support terrorism, and that a sample of members of Hezbollah\’s military wing had higher education levels than the population average.  More broadly, the evidence suggests that very poor countries don\’t typically have a lot of terrorism, because physical survival is a bigger concern, and high-income countries have relatively less terrorism. The countries with higher levels of terrorism are in a middle range.

Or consider the possible connections between terrorism and regime change. One might argue that democracies are more vulnerable to terrorism, or that democracies offer other outlets for dissent. One might argue that autocracies have less room for dissent other than terrorism, or that autocracies are more likely to clamp down ferociously on terrorism. There are lots of hypotheses, and the evidence is weak for any of them \”the relationship between regime type and transnational terrorism is an empirical question. Findings in the empirical literature on this relationship are mixed and generally
unconvincing.\” But some studies suggest that when a country is moving away from autocracy and toward a nascent democracy, the risk of terrorism may rise.

Yet another argument is that globalization may be connected to terrorism, because it allows money, people, supplies, and most of all grievances to spill across national borders. But the research doesn\’t show any connection that countries with more global ties are more likely to face issues with transnational terrorism.

\”Fifth, as a general rule, terrorism has had little direct negative impact on the economic growth or GDP of targeted industrial countries, despite some large-scale attacks. Any impact is felt by a few terrorism-fragile sectors, and this impact is transitory and small relative to the economy. Larger macroeconomic effects may plague small terrorism-ridden countries.\”

Of course, this statement doesn\’t in any way diminish the costs of terrorism; it merely points out that in high-income countries, terrorism doesn\’t affect growth of GDP,

(Full disclosure, the Journal of Economic Literature is published by the American Economic Association, just like the Journal of Economic Perspectives where I labor in the fields as Managing Editor.)

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