Early theorizing suggested that terrorists were fundamentally different to criminals, leading many suggested that terrorists were irrational and/or undeterrable (e.g., Miller, 2013). However, there is now good evidence that they engage in rational choice making, and weigh potential rewards against potential risks, just like ordinary criminals (Carson et al., 2020; Clarke & Newman, 2006; Dugan et al., 2005; Gill et al., 2020; Perry & Hasisi, 2015; Perry et al., 2017). In fact, it could be argued that this is the point at which terrorism and crime appear most similar. Evidence for rational choice making in terrorism can be observed in how terrorism responds to situational prevention efforts. Here, increasing the difficulty of successful attacks, and of apprehension, has been found to be associated with reductions in the risk of terrorism, following patterns that closely mirror crime (e.g., Dugan et al., 2005; Perry et al., 2015).
Most deterrence studies relating to terrorism follow the above noted criminological studies on policies as proxies for deterrence, perhaps being limited to this type of analysis due to the availability of data. These studies generally rely on time-series analyses in which specific interventions are treated as dummy variables. Methodologically these approaches can be quite problematic, making it is difficult to extrapolate deterrent effects from them (Charles & Durlauf, 2013). Nevertheless, the results from these studies can still be informative. In attempting to synthesize the results, indiscriminate policies like widescale crackdowns appear to have no effect on future terror attacks, and if they do, it is more likely to be a backlash effect, whereby terrorism increases. This has been found across a variety of contexts (Argomaniz & Vidal-Diez, 2015; Hsu & McDowall, 2020; LaFree, Dugan & Korte, 2009). Conversely, in many of these very studies, policies conceptualized as representing 'soft' approaches are more likely to be associated with decreases in the risk of terrorism.
There are a very limited number of studies testing the likelihood of arrest in a similar way found within general deterrence research. The first such study, conducted by Le Vine and Salert (1996) examined how a small number of annual arrests (between 11 and 17) of Palestinian Liberation Organization (PLO) terrorists impacted the number of attacks carried out by the group outside of Israel (international attacks). The study did find evidence that higher arrest rates (number of arrests per incident) displayed a small deterrent effect. In another study focused on Israel, Kaplan et al (2006) found that increased arrests reduced the likelihood of suicide bombings. It should be noted however, that for both studies, data for arrests was derived from open-sources and their dependent variables were both limited to very specific types of terrorism activity. Conversely, in Hussain's (2010) doctoral thesis which focused on Pakistan, official data sources were relied upon for arrest data. Here it was found that increased terrorism related arrests had a deterrent effect on a broader range of terrorism.
Other deterrent factors, primarily those pertaining to punishment severity, have rarely been examined with respect to terrorism. In a study of terrorism offenders in Israel, Hasisi, Weisburd, Carmel and Wolfowicz (2020) found that longer sentence lengths were associated with a reduced risk of later terrorism offending. However, there was no indication as to how severity of punishment impacted terrorism offending rates overall. It is also important to note that despite perceptions that terrorism cases are associated with stiff sentencing, there is actually evidence of relatively short sentences being associated with terrorism offending (Amirault et al., 2016; Harms, 2017; Hasisi et al., 2020). However, as stated, it is not known how variation in punishment severity impacts terrorism.
What is evident is that to date, studies on deterrence and terrorism have not applied the same theoretical and methodological frameworks that have been successful elsewhere. Resultingly, there remains a debate as to whether terrorism is likely to respond to the risk of punishment and severity of punishment similarly or not from other types of crime. Some have argued that because terrorists are willing to die for their cause or group, deterrence is not a relevant factor. However, others reject this proposition as representing a narrow understanding of terrorism, and such notions may be limited to the relatively rare incidents of suicide bombings (Hafez, 2007). Whilst it may be true that terrorists are willing to die, much of the 'new terrorism' experienced in the west involves actors engaging in extensive planning for how to survive. And some of the world's most famous terrorists, such as Osama bin Laden, demonstrate that evading capture and punishment is just as important to them as to other types of criminals (Fisher, 2007). Moreover, the willingness to die for a cause or in the commission of a crime is in no way unique to terrorists. Many criminals who engage in violent and other types of crime may prefer to 'get away with it' whilst at the same time being prepared to accept the possibility capture or death (Gómez et al., 2022).
There are additional issues with how the criminal justice systems in Western countries treats terrorism that could lead to differential deterrent effects. For example, compared to other forms of crime, the rate of release without charge may be quite high. According to statistics from the UK's Home Office, some years have seen more than 50% of those arrested under the Terrorism Act (2000) be released without charge (HMG, 2018). Our own review of these statistics shows that, not surprisingly, the rate seems to increase commensurate with the volume of arrests. Similarly, in the US, while arrests for terrorism have increased, and have been viewed as having led to a decrease in successful attacks, the rate of declination, in which charges are dropped, has increased. This may indicate that authorities are identifying terrorism threats early enough to prevent them, but too early to have gathered sufficient evidence for successful prosecution (Brooks, 2011). Additionally, conviction rates for terrorism cases may be substantially lower than for ordinary crime, although they have increased significantly in more recent years (Addison, 2002; Shields et al., 2016). These numbers have improved as terrorism cases have increasingly been tried as criminal-cases (Shields et al., 2016).
Another issue to consider is how terrorism differs from other forms of crime. Unlike high-volume crimes, such as general violent crime, terrorism has very low base-rates and a relatively small proportion of the population are even at risk of offending (Gill et al., 2016; Sageman, 2021). For high-volume crime, police resources are often limited, and when crime increases to a certain level, it can overwhelm the system, drastically impacting the likelihood of capture and punishment and thereby, deterrent effects (Bun et al., 2020). Conversely, relative to its low base-rate and small pool of offenders, extensive police resources are dedicated to combatting the phenomenon, with counter-terrorism spending in Western countries having increased significantly since 2001 (Lum et al., 2008). This means that issues of reverse causality that may impact the relationship between crime and the probability of arrest, are less likely to be present in the case of terrorism.
Another way in which terrorism differs from high-volume crime is with respect to the 'dark figure'. Generally speaking, violent crime rates are a function of victim or witness reports. As non-reporting increases, so too does the dark figure. This can artificially increase arrest rates. Of course, it has long been understood that arrest rates are lower when calculated against victimization surveys than police-reported crime (Cordner, 1989; Skogan, 1976). It can also be the case that higher crime and lower arrest rates further discourage victims to report crimes (Bowles et al., 2009; Levitt, 1998). Conversely, terrorism rates are the result of police detection or administrative recording, and are not sensitive to non-reporting, at least not in the same way that crime is. Whilst it is true that citizen reporting of possible terrorism threats is associated with their disruption (Hasisi, Perry and Wolfowicz, 2020), this represents something fundamentally different than reports for actual violent crimes having been committed coming from victims or other citizens, which are then logged as crime incidents. As such, an analysis of terrorism is less likely to suffer from these common issues.
In the absence of any rigorous tests of the deterrence theory model, it is still only possible to theorize about how the criminal justice system may impact terrorism. As described above there is both theoretical and empirical evidence to support assumptions that it will respond in ways that are at least somewhat similar to crime. On the other hand, terrorism does have some inherent characteristics upon which it differs from crime that give rise to assumptions that it may respond differently.