Opportunity

The first hypothesis suggests that militants go where it is easier to operate. It may be that choices are informed by domestic constraints (i.e., that the preemptive capability of Western security agencies on home turf is so high that Western jihadists go abroad instead). This hypothesis rests on the underlying assumption that the individual does not care much where he engages.

To probe the plausibility of this hypothesis, we can ask whether it makes sense to argue that it is much easier for a Western jihadist to operate abroad than at home. Much suggests that it does not. Planning an attack in the West is risky, but so is foreign fighting; it involves crossing international borders, navigating unfamiliar territory, dodging heavy-handed local police, and dealing with unknown new contacts. There is a considerable risk of being arrested, tortured, wounded, or killed in the search for access to a conflict zone.Footnote ¹¹ Although domestic counterterrorism measures have proliferated post-9/11, the same is true at the international level. States share more intelligence and cooperate more on counterterrorism. International travel is better monitored and ID papers harder to forge. Western states take more direct action, in the form of espionage, special operations, drone attacks, and the like, to dismantle jihadi networks in the Muslim world. One might even argue that domestic fighting can be easier than foreign fighting, because there is anecdotal evidence of jihadists who attacked at home after finding foreign fighting impractical.

The picture changes if we assume that jihadists care about the duration of the fighting experience. A key difference between domestic and foreign fighting is that in the West, attackers rarely get more than “one shot” before being killed or arrested, whereas they can fight longer if they reach a conflict zone. There is anecdotal evidence of a “Hemingway effect” associated with foreign fighting; that is, some recruits are motivated by the search for adventure (Sageman Reference Sageman2004). For these individuals, duration presumably matters more, making opportunity a major factor informing theirtheater choice. However, we do not know the proportion of foreign fighters for whom this is the case, because adventurism is not a motivation that activists usually declare.

Another indication that countermeasures matter is the chronological evolution of the foreign fighter-domestic fighter ratio. The opportunity hypothesis implies that the relative prevalence of foreign fighting is correlated with the level of constraints on domestic fighting relative to that on foreign fighting. It is reasonable to assume that, after 9/11, international counterterrorism improved relatively more than domestic counterterrorism because the improvement potential was larger. We should thus expect to see the relative popularity of foreign fighting decline after 9/11, which is exactly what we did see. However, higher constraints on foreign fighting are likely not the whole story; shifting preferences may also have something to do with the ratio change.

In sum, physical obstruction may account for some of the variance in theater choices, but it is likely not the most important factor.

Training

A second hypothesis is that recruits prefer operating in the West, but train abroad first to increase capability. Here, foreign fighting is an instrumental strategy in which militants forsake short-term tactical opportunities for the deferred gratification of a more destructive operation in the future. This view is common among analysts and policy makers today, and it underpins the widespread assumption that foreign fighters are domestic terrorists in the making.Footnote ¹²

It is true that good fighting skills require physical practice and that military know-how is more widely available in the conflict zones of the Muslim world than in the West. Moreover, the data presented later in this article suggest that foreign fighters really do make more lethal domestic operatives. However, does it make sense to argue that training abroad is necessary for a domestic operation? Probably not. For one, a domestic attack need not be very destructive or complicated, as al-Qaida itself has told its followers (see the later discussion). Second, certain forms of military instruction are available in the West, either offline from experienced operatives or online from instruction manuals. Third, the instruction offered abroad is not necessarily of the type required for attacks at home. Testimonies by foreign fighters (al-Amriiki Reference Al-Amriiki2012; Collins Reference Collins2002; Nasiri Reference Nasiri2006) indicate that most recruits receive only basic paramilitary training while abroad; only a handful learn bomb making, countersurveillance, and other skills needed to operate in Western cities. Fourth, the benefits of training abroad are counterbalanced by substantial costs, in the form of risks associated with foreign fighting.

Perhaps for these reasons, several prominent jihadi leaders and ideologues have actively discouraged foreign fighting.Footnote ¹³ Interestingly, the discouragement has tended to come from precisely those ideologues who most vehemently advocate operations in the West. Abu Mus'ab al-Suri, a prominent pro-al-Qaida ideologue, cautioned in 2004 against training on what he called “open fronts” (i.e., the major war zones in the Muslim world), because foreign travel is costly and risky (Lia Reference Lia2008: 534–35). More recently, the English-language jihadi magazine Inspire repeatedly encouraged prospective militants in the West to act at home and not to go abroad for training (Ibrahim Reference Ibrahim2010: 55). In 2010 and 2011, the prominent U.S-Yemeni ideologue Anwar al-Awlaki, then based in Yemen, persuaded several Western recruits via e-mail to act at home rather than abroad; in one encrypted message, he told the UK-based Rajib Karim, “[Hearing] that you intend on making hijrah [emigration], I immediately wanted to contact you to tell you that my advice to you was to remain in your current position” (Berger Reference Berger2011). Even some senior leaders in al-Qaida Central in Pakistan have encouraged Western Muslims to engage in “individual jihad” at home. For example, in 2011, Adam Gadahn, the American convert-turned-al-Qaida spokesman, said,

Two other indicators further undermine the training hypothesis. One is the proportion of foreign fighters who actually return. My tentative data indicate that only about one in nine foreign fighters returned for an attack in the West (see the later section on “estimating the veteran effect”). If training was the main purpose of travel, we would expect more people to come back. Death or capture abroad might explain some, but far from all, of the missing returnees.

A second indicator is the foreign fighters’ declared initial intentions for going abroad. In the biographies of the 107 foreign-fighters-turned-domestic-fighters in the JPW dataset—the subset of foreign fighters most likely to have intended to eventually return—I found only 2 individuals who explicitly stated they went abroad to train for a future domestic attack. The likelihood of underreporting prevents us from making too much of this observation, but it is worth noting, especially in light of the fact that the same sample contains 45 cases of individuals declaring that the initial purpose of the trip was to join a foreign insurgency (see Table 4).

TABLE 4.

Declared initial purpose of first trip abroad by foreign-turned-domestic fighters

The available evidence thus stacks up against the training hypothesis. Some foreign fighters may well have left in search of training for a domestic attack, but the need for training does not plausibly account for the full extent of the popularity of foreign fighting.

Norms

A third hypothesis is that militants prefer going abroad because they view foreign fighting as more legitimate than domestic fighting. In this hypothesis, involvement by Western Islamists in militancy depends on perceived religious sanction, and the predominant view among Islamist religious authorities is that fighting in established conflict zones is more legitimate than attacks in the West. Given the distribution of views of religious authorities, most Western jihadis find fighting in established conflict zones the legitimate avenue of their new beliefs. Note that this is not an argument about a static ideology determining the behavior of its followers. The strength of the norm against out-of-area attacks may vary in time and space, and a given individual's perception of the same norms is sensitive to personal disposition and context.

Let us begin by asking what Islamist religious authorities say about where war can or should be waged. Although no codified Islamic laws of war exist, there are views on the conduct of war that can be considered mainstream. The legal tradition notably reflects norms of proportionality and discrimination: One should not cause unnecessary death or destruction, and one should not target noncombatants (Kelsay Reference Kelsay2007: 97–124; Peters Reference Peters1996: 145–47). Moreover, in the theological literature, the default understanding of military jihad is one in which two conventional armies—one Muslim, the other non-Muslim—fight each other on a contiguous piece of territory (Cook Reference Cook2005: 122–23). Differences of clerical opinion arise mainly on dilemmas confronting regular armies, such as whether to shell enemy units that use human shields or how to treat prisoners of war (Aboul-Enein and Zuhur Reference Aboul-Enein and Zuhur2004; Morabia Reference Morabia1993: 227–30). Throughout the ages, Islamic scholars have always declared certain forms of political violence as illegitimate, especially those that deliberately target civilians (Abou el Fadl Reference Abou El Fadl2001). In the modern era, mainstream Islamic scholars have repeatedly condemned terrorism (irhab; Jackson Reference Jackson2001; Misri Reference Misri2005; Schwartz Reference Schwartz1991).Footnote ¹⁴ Of course, norms against terrorism are diluted by definitional problems; certain activities considered terrorism by Western governments are viewed as legitimate resistance by some Islamic clerics (Kelsay Reference Kelsay2007: 140–41). Generally speaking, however, the more conventional a military technology, the more legitimate it is considered in Islamic law. When waged by a Muslim rebel group on home territory against a non-Muslim army, insurgency is thus considered more legitimate than international terrorism.

However, as with many issues in Sunni Islam there exists a range of legal opinion. At the radical end of the spectrum, some clerics, such as the Saudis Hamud al-Shu' aybi, Nasir al-Fahd, and Abd al-Rahman al-Jarbu‘, have declared mass casualty operations in the West to be entirely legitimate (Hegghammer Reference Hegghammer2010). These pronouncements are in addition to the many lay activist-ideologues linked to al-Qaida, such as Usama bin Ladin (Lawrence Reference Lawrence2005), Abu Mus'ab al-Suri (Lia Reference Lia2007), and Anwar al-Awlaqi (al-Malahim Reference Al-Malahim2010). Ideologues who condone attacks in the West typically stress three arguments: (1) the Prophet Muhammad allowed, in certain circumstances, for the killing of civilians and for collective punishment; (2) civilians in democracies are responsible for their governments’ policies; and (3) non-Muslims are killing Muslim civilians, so Muslims can reciprocate. Mainstream clerics reject this position, typically by arguing that (1) the situation of Muslims today is not analogous to that of the Prophet; (2) the intentional targeting of civilians, as opposed to unintentional collateral damage, is categorically forbidden; and (3) violence in the West is not permitted in the first place due to the existence of international treaties (March Reference March2011).

Even at the radical end of the spectrum of clerical opinion, there is some debate about attacks in the West, whereas almost no clerics question the legitimacy of geographically limited insurgency. For example, Abdallah Azzam, the influential Palestinian cleric described as “the mentor of Usama bin Ladin,” never called for nor organized any out-of-theater attacks during his eight years of fighting the Soviets in Afghanistan in the 1980s (Hegghammer Reference Hegghammer2011). More recently, the UK-based Syrian ideologue Abu Basir al-Tartusi surprised many of his jihadi followers in 2005 by publicly condemning the London bombings (Al-Tartusi Reference Al-Tartusi2005; Paz Reference Paz2005b). According to Nesser (Reference Nesser2011a: 181), even the most fiery jihad clerics in Europe in the late 1990s and early 2000s, such as Abu Qatada al-Filastini, Abu Hamza al-Masri, and Umar Bakri Muhammad, “seem to have had certain reservations about launching terrorist attacks in Western countries.” Nesser (185) adds that “although Abu Hamza justified violent jihad in any country, including non-Muslim host states, he remained mainly focused on supporting insurgencies and liberation wars in Yemen, Chechnya, and Kashmir until he was arrested in 2004.”

It is, of course, difficult to measure with precision the relative prevalence and influence of different ideological positions. Relying on behavioral indicators brings endogeneity problems, and quantitative content analysis is difficult because of the vastness of the universe of texts. Because of the lack of better methods, we can rely on qualitative assessments by academic specialists on religious literature (Bonner Reference Bonner2006; Cook Reference Cook2005; Firestone Reference Firestone1999; Khadduri Reference Khadduri1966; Sivan Reference Sivan1998), virtually all of whom maintain that most Muslim clerics condemn out-of-theater attacks on non-combatants. It follows that clerics who approve of foreign fighting most likely outnumber those condoning domestic fighting.

This distribution of views appears to be reflected in the broader Muslim population. Unfortunately, no major surveys explicitly ask about views on the relative legitimacy of terrorism and insurgency. However, a few surveys distinguish between support for al-Qaida and support for local insurgent groups such as Hamas and Hizballah (Pew 2010), and the results suggest that the latter two enjoy considerably more support than the former among non-Palestinian and non-Lebanese Muslims. Similar findings appear in qualitative surveys of young Muslims in the West. A study based on interviews with a cross-section of 70 young Muslims in Canada concludes that “it is possible—and common—to believe violent jihad in Western countries is unacceptable, while simultaneously believing violent jihad in Muslim majority countries is acceptable” (Bartlett, Birdwell, and King Reference Bartlett, Birdwell and King2010: 96). Another study based on semi-structured interviews with Islamists in Europe reports widespread support for Muslim insurgents on the one hand and skepticism toward violence in the West on the other (Kühle and Lindekilde Reference Kühle and Lindekilde2010: 55–58). The general attitude is reflected in the following statement by a young British Muslim interviewed by Neumann and Rogers (Reference Neumann and Rogers2007: 79):

Social desirability bias is obviously a concern in this type of data, but the bias should not be overestimated for several reasons. Some informants in the same study did admit to condoning attacks in the West. Moreover, support for violence against Western troops abroad is also considered controversial by mainstream Western society, yet most informants declared their support for it. Third, some Islamists have expressed similar views while not in an interview situation. For example, Karim Bourti, a French Algerian actively involved in recruitment to Chechnya and Afghanistan, was captured on hidden camera in 2002 saying that “those who decided [in the mid-1990s] to strike France had made a mistake both strategically and from the point of view of dogma” (Sifaoui Reference Sifaoui2003: 53).

What about militants? The proposition that Western jihadists heed norms when deciding where to fight can be assessed by looking at declared motivations and motivation sequencing. If most foreign fighters say they intend to join insurgencies abroad and most militants acquire the motivation to fight abroad before the motivation to attack at home, then the norms hypothesis is strengthened.

Measuring declared intentions is, of course, problematic. We often lack reports of intentions, and when we do have them, there is no guarantee that the source is being truthful. That said, it would be unreasonable to dismiss declared intentions as completely irrelevant. Consider again the 107 foreign-fighters-turned-domestic-fighters in the JPW dataset. Their subsequent involvement in domestic operations makes them the least likely of all foreign fighters to have considered insurgency as the end purpose of their departure abroad. Yet, at least 45 of these 107 appear to have originally wanted to join insurgencies, and not train for domestic operations (see Table 4). Examples include Najibullah Zazi (2009 New York subway plot), who said he went to Pakistan with the intention of joining the Taliban in Afghanistan (Sulzberger and Rashbaum Reference Sulzberger and Rashbaum2010); Mohammed Siddique Khan (2005 London bombing) who “originally . . . wanted to join Taliban's war in Afghanistan (Silber and Bhatt Reference Silber and Bhatt2007: 48), and Fritz Gelowicz and Adem Yilmaz (2007 Islamic Jihad Union plot, Germany) who “told the court that they originally wished to join the Sunni resistance in Iraq” (Nesser Reference Nesser2011b: 474).

As far as motivation sequencing is concerned, we already know that it is more common to fight abroad before fighting at home than the other way around, and that very few foreign fighters expressed the intention to return for domestic operations before they went abroad. What about those who attacked at home without going abroad first? A closer look at their biographies reveals that several (at least 13 of 294) expressed an intention to fight abroad before plotting domestic attacks. For example, in the late 2000s the so-called North Carolina cluster went abroad in search of combat experience; only when they failed to join a battlefront did they contemplate home operations (U.S. v Boyd et al. 2009). So did, allegedly, Tarek Mehanna, Ahmad Abousamra, and their unnamed associate from the “Mehanna case” (U.S. v Mehanna and Abousamra 2009). Two other U.S.-based militants, Farooque Ahmad and Mohammad Osman Mohamoud, both first expressed a desire to go abroad for jihad, but were drawn into domestic plots by FBI informers (U.S. v Ahmed 2010). For Europe we generally have less information about individual trajectories, but we know, for example, that Jamal Ahmidan from the 2004 Madrid bombing “spoke incessantly about jihad and his desire to fight the Americans in Iraq” (Silber and Bhatt Reference Silber and Bhatt2007: 39) and that Samir Azzouz, a member of the Hofstad group, made a failed attempt to travel to Chechnya in 2002 (Crawford Reference Crawford2004).

Another point relevant to motivation sequencing is that the imagery used in typical al-Qaida recruitment videos is almost exclusively drawn from conflict zones, such as Palestine, Iraq, Afghanistan, Kashmir, Chechnya, Bosnia, and Somalia (Farwell Reference Farwell2010; Finsnes Reference Finsnes2010; Salem, Reid, and Chen Reference Salem, Reid and Chen2008). This suggests that in the early radicalization phase, recruits are exposed to more propaganda about foreign conflicts than about domestic operations. It also indicates that propagandists expect new recruits to be more sensitive to imagery from established war zones than to examples of operations in the West.

A final indicator of the impact of norms is the pre-attack deliberations of domestic fighters. Occasionally, scholars can observe these deliberations in transcripts of secret recordings or intercepted communication made public during trials. The limited evidence available shows that operatives often doubt, discuss, and consult others about the permissibility, in Islamic legal terms, of their intended actions. Nesser notes that, in the 2000s, “there have been recurring discussions about security treaties [i.e., Islamic legal opinions regulating the use of force by the Muslim community against non-Muslim states] on radical websites, and captured terrorists possessed texts dealing with the topic.” For example, in 2003, Jason Walters, a member of the Dutch cell known as the Hofstad group, consulted a certain Imam Abdul-Jabbar van de Ven about the legality of attacking in the West (Silber and Bhatt Reference Silber and Bhatt2007: 57). Similarly, in 2009, a group of Somalis in Australia plotted an attack on a military facility in Australia.They were so unsure about the legitimacy of their operation that one cell member traveled all the way to Somalia to seek permission from a religious cleric (Munro Reference Munro2010). Thus, even the most radical activists seem to realize that domestic fighting is considered illegitimate by many.

The norms hypothesis also plausibly accounts for at least some of the decrease in the foreign-to-domestic fighter ratio (from 11:1 in the 1990s to 3:1 in the 2000s). We know that the normative consensus against attacks in the West weakened in the Islamist community from the late 1990s onward (Gerges Reference Gerges2006). In the early 1990s, virtually no Islamist figure publicly articulated arguments for attacks in the West. It was not until 1998 that al-Qaida published the first in a series of statements advocating attacks on Western civilians worldwide (Lawrence Reference Lawrence2005).Footnote ¹⁵ After 9/11, however, many more ideologues and some religious clerics declared it legitimate to attack in the West. The 2000s saw an exponential growth in the amount of Internet propaganda advocating this position. It seems plausible that the weakening of the normsagainst domestic fighting negatively affected the foreign-to-domestic-fighter ratio. This explanation is not incompatible with the one about higher physical constraints on foreign fighting described earlier. Most likely, both factors contribute to the ratio decrease, although it is difficult to disentangle the precise effect of each. In addition, there may be an interaction effect between norms and physical constraints, namely, that clerical views are sensitive to changes in counterterrorism practices. We saw earlier that al-Qaida clerics discouraged foreign travel partly on the basis of its risks and that they discouraged it more in the 2000s when it had become more difficult.

In sum, the norms hypothesis emerges as the strongest explanation for the prevalence of foreign fighters. A majority of Western jihadis choose foreign fighting over domestic fighting, most likely because they have come to view the former as more legitimate after observing the distribution of views among religious authorities. The preference for foreign fighting among Western jihadists as a group has weakened over time, because foreign fighting has become more difficult and more ideologues encourage domestic fighting. Still, the preference remains strong.

Return Rates

My data, with all its limitations, indicate that no more than one in nine foreign fighters returned to perpetrate attacks in the West (107 returnees against 945 foreign fighters). The return rate may well be even lower, because the foreign fighter estimate has a larger upside than that of returnees. On the one hand, this rate suggests that far from all foreign fighters are domestic fighters-in-the-making. On the other hand, a one-in-nine radicalization rate would make foreign fighter experience one of the strongest predictors of individual involvement in domestic operations that we know. The predictive power of other biographic variables—whether nationality, economic status, or any other biographical trait studied so far—does not come close (Bakker Reference Bakker2006; 2009; Egerton Reference Egerton2011; Sageman Reference Sageman2004; Venhaus Reference Venhaus2010). Unfortunately, however, the question of differential radicalization—why only some foreign fighters become domestic fighters—cannot yet be answered for lack of data.

Radicalization Processes and Mechanisms

Why would any foreign fighter return for an attack if he originally decided it was preferable to leave? A full answer to this question is beyond the scope of this study, but I sketch out two hypotheses for further testing. These hypothetical processes were generated inductively from observation of known cases of foreign fighters “going domestic.” There may be additional processes, but the two I propose seem to be more common.

The first, called “enlistment,” refers to a trajectory in which the foreign fighter is drawn into domestic fighting by a calculating second party. His own initial intention was to join a foreign insurgency, but he finds himself in a training camp operated by a group with plans to attack in the West. The recruit may or may not be aware of the group's international ambitions when joining. Over time he develops a sense of loyalty to the leadership and makes friends with fellow trainees. At some point he is asked about his general willingness to serve the organization by participating in an operation, the details of which may or may not be known to him. Peer pressure, a sense of loyalty, and/or a lack of exit options make him accept. When the order to deploy to the West arrives, he is so deeply invested in the organization and has so few ways out that he complies, even if the operation is of a type that he originally considered illegitimate.

There are numerous examples of this process, the most famous being the so-called Hamburg Cell of the 9/11 plot, whose members went to Afghanistan with the intention to train for jihad in Chechnya (9/11 Commission 2004: 165), only to be drawn into the 9/11 plot by al-Qaida after arriving there. Another example is Najibullah Zazi (2009 New York subway plot), who told the court,

In the second process, which may be called “socialization,” the foreign fighter's preferences change through the experience of military life. Here, the recruit is pushed toward domestic fighting by a gradual change in perceptions on the legitimacy of domestic fighting; no second-party manipulation is involved. The recruit arrives in the conflict zone, takes part in combat, and comes to see theological arguments constraining violence as impractical or naïve. Having participated in insurgency, he comes to view domestic fighting as just a small and permissible escalation. He decides to return, inclined to operate in the West, but without a specific plot in mind.

One example of this pattern is provided by the so-called Chechen Network, which plotted attacks on the Russian embassy in Paris in 2002, having previously trained in Chechnya but without having forged ties with a particular organization (Nesser Reference Nesser2011b: 274ff). The same appears to have been the case with the Frankfurt cell, which plotted an attack on the Strasbourg Christmas market in 2000, after training in Afghanistan (Nesser Reference Nesser2011b: 223ff). More recently, the “Stockholm bomber” Taimour Abdelwahhab trained in Iraq before perpetrating a botched suicide operation in late 2010, apparently on his own initiative (Reuters Reference Reuters2011).

The precise mechanisms by which individuals come to defy normative constraints on terrorism in these two processes remain to be identified. One possible mechanism is continued exposure to selective information about clerical views on terrorist tactics. In the field, the foreign fighter is surrounded exclusively by fellow militants. Moreover, he is exposed to a narrow range of clerical opinion on violence, because such opinions only reach him through the host organization's own ideologues, literature selected by the host organization, or fellow recruits. This is in contrast to the home setting, where he is exposed to a broader range of opinion, for example in the local mosque or among family and friends. Moderate voices might not convince him that domestic attacks are categorically forbidden, but they introduce uncertainty about the validity of legal claims made by radical ideologues.

Both of these hypothetical radicalization processes require further testing through process tracing. All we know for now is that some foreign fighters acquire the motivation to attack in the West after going abroad.

Impact

The JPW data suggest that the presence of foreign fighter returnees increases the effectiveness of attacks in the West (see Table 5). Whereas only 26% of all plotters are known to have foreign fighter experience, around 46% of all plots (49 of 106) included at least one veteran. For executed attacks, the rate is 58% (14 of 24), and for executed attacks with fatalities, it is 67% (8 of 12). Twenty-nine percent of plots with veterans reached execution and 16% caused fatalities. For plots without veterans the corresponding rates are 18% and 7%. Here again, precise figures must be taken with a pinch of salt given the data problems, but the available numbers tentatively suggest that the presence of a veteran increases—by a factor of around 1.5—the probability that a plot will come to execution, and it doubles the likelihood that the plot will kill people.

TABLE 5.

Effect of veteran participation on domestic plot effectiveness

* P refers to the probability that the plot will be executed or be lethal.

Other studies report similar results from different data. Sageman (Reference Sageman2010) examined 60 plots in the West by 46 networks between 1993 and 2008, finding that 26 of the networks included a veteran and that a veteran doubled the probability of the attack reaching execution. Cruickshank (Reference Cruickshank2010) examined the 21 most serious attack plots in the West between 2004 and 2010 and found that 12 plots involved a veteran and at least 37 of 131 plotters had been abroad. Gartenstein-Ross and Grossman (Reference Gartenstein-Ross and Grossman2009:14) noted that “over 40% of the sample traveled abroad for training or to fight jihad.” British authorities said in 2008 that three-quarters of post-9/11 UK plots can be “traced to Pakistan” (Coates and Page Reference Coates and Page2008). Clarke and Soria (Reference Clarke and Soria2010: 28) noted that “seven of the eight major terrorist plots in the UK included in their cells one or more individuals who had attended terrorist training camps; six of the eight of these had been in Pakistan.”

My tentative data thus corroborate existing evidence of a “veteran effect” that makes returnees more lethal operatives. It is important, however, not to select on the dependent variable and infer that outgoing foreign fighters are more dangerous as a group, because as we know, most foreign fighters do not return for domestic attacks.