Readers should be aware that Marc Sageman's new book on global Islamist terrorism is strongly disputed by part of what we might call the “terrorism elite” within the broader U.S. foreign policy elite. Indeed, Leaderless Jihad was savaged in Foreign Affairs, the elite foreign-policy journal, by Bruce Hoffman, a leading expert on terrorism and a frequent government consultant. Readers should also be aware that Sageman's book includes plagiarized passages. In yet another critical review, Chip Berlet, a leading expert on right-wing movements, noted two unattributed passages that had appeared in previously published works by other authors (the passages had been slightly reworded by Sageman). The publisher blamed this on Sageman's rush to beat a deadline and promised that those passages would be properly cited in subsequent printings of the book.

Readers should be aware of these things, but they may also conclude, as I have, that Leaderless Jihad is a very interesting and thoughtful study. Without minimizing the book's shortcomings, I would argue that it is one of the most important works available on the transnational movement that has employed violence against the U.S. and allied governments—and, in many cases, ordinary citizens of these states—in response to their policies in Islamic countries. (“Global Islamist terrorism,” however, is a misnomer insofar as there are disagreements in these circles—as Sageman himself points out—over the legitimacy of killing civilians.) This movement is not very large—he says it “consists at most of a few thousand people” (p. 28)—but it has had an enormous impact on world affairs and on the domestic politics of several countries.

The core of Sageman's analysis is data he has collected on more than five hundred participants in the transnational Islamist (or “jihadist”) movement from court records, captured documents, the media, and other “open-source” records. There are, however, some ambiguities with this data that Sageman ought to have clarified. For example, what exactly makes one a “participant” in this movement? What level of involvement is required? How many people in his sample perpetrated violence, including violence against civilians, and how many of these “terrorists” served in support roles? Also, how complete are the author's data? How much information on particular variables (e.g., participants' occupation or marital status) is reliable and how much is missing? Alas, he never presents his data in any standard tabular form.

In his previous book, Understanding Terror Networks (2004), Sageman at least listed each individual in his data set (172 at that time) by name, alias, and date and place of birth, also indicating the regional social-network “cluster” to which each belonged. A comparable table is missing from the book under review. Sageman's discussion of his data, moreover, is often imprecise. He repeatedly notes that he has data on “over 500” people, but never says how many precisely. Phrases like “about a fourth” and “about two-thirds of the terrorists” appear repeatedly, with no indication of the sample size for the variable under discussion. At one point, he writes, “In my sample of about five hundred, there were about four with hints that they experienced beliefs that were not based in reality” (p. 64). “About” four? What can this mean?

With these caveats in mind, what do Sageman's data tell us about the people involved in the global Islamist movement against the United States and its allies? His findings are sometimes surprising. Three-quarters are married, and two-thirds have children. (So much for the idea that they are sexually frustrated youth longing for virgins in heaven.) Only about a quarter, furthermore, were deeply religious as youth, and only 13% attended madrassas, or religious schools; the vast majority grew up in secular households and subsequently “found” religion. (Sageman describes transnational Islamism as a “born-again social movement.”) He concludes, in fact, that the “fervor” of these jihadists “results from their lack of religious training, which prevents them from evaluating their new [“Manichaean”] beliefs in context” (p. 60; emphasis added).

Sageman also suggests that this transnational movement is rooted in the Muslim diaspora. Of its participants, 84% are expatriates or second- or third-generation immigrants. They are not, for the most part, poor. The “vast majority” are middle class, although the most recent wave—following the U.S. invasion of Iraq—includes many from working-class backgrounds. More than 60% attended university, a much higher rate than their peers.

Sageman's findings about the socioeconomic backgrounds of these jihadists are strikingly similar to findings presented in Alan B. Krueger's What Makes a Terrorist, a smart, concise, and accessible book that is based on lectures given by Krueger at the London School of Economics in 2006. He reviews evidence on Palestinian suicide bombers, deceased Hezbollah militants, and Gush Emunim members, all of which indicates that they tended to come from better-educated and/or more affluent strata in their societies. But he probably goes too far when he claims that “most terrorist organizations are composed of people drawn from the elites” (p. 44). His data on Hezbollah militants, for example, indicate that preparatory school and university graduates are underrepresented among these activists. Krueger also notes that members of the Provisional Irish Republican Army were disproportionately working class, regarding this as some kind of anomaly. But it might have seemed less odd if he had examined the backgrounds of the rank-and-file membership of Hamas, the Liberation Tigers of Tamil Eelam, Peru's Sendero Luminoso, or Sunni and Shia militias in Iraq. As a rule, more broadly based movements that employ terrorism as one tactic among many tend to have more working-class (or peasant) members, while smaller, underground sects or networks dedicated more or less exclusively to violent tactics tend to have more middle-class and better educated members. (It is not surprising, then, that the loose networks that comprise Sageman's global Islamist movement would be overwhelmingly middle class.)

Krueger's data on “international” terrorists—activists who have killed people of a different nationality—provide some needed context for understanding Sageman's global Islamist terrorism. Krueger finds that 88% of recent international terrorist attacks took place in the perpetrators' country of origin. So not only is most terrorism local, but most international terrorism is also overwhelmingly local. Indeed, much if not most of Sageman's “global” Islamist violence has been carried out by militants who are operating close to home. For example, the suicide bombers who attacked London's subways in 2005 were British citizens. Terrorism by nonstate actors who have traveled long distances to wreak havoc—such as the 9/11 perpetrators—remains a very rare form of violence.

Sageman's analysis suggests, in fact, that the front line in the so-called war on terrorism is neither Iraq nor Afghanistan but Western Europe. His data indicate that alienated Muslim youth in Europe are the pool from which the current wave of Islamist terrorists are emerging; the threat to the West is no longer from the outside but from the inside—from disconnected networks or “bunches of guys” who are not under the control of Al Qaeda or any other group (hence, “leaderless”). This claim is the source of Hoffman's disagreement with Sageman; Hoffman and other terrorism experts argue that Al Qaeda's central leadership in Pakistan remains the more formidable threat and that Al Qaeda operatives are more active in Europe than Sageman believes.

Sageman presents an interesting four-pronged model of “radicalization” in order to account for the transformation of European Muslim youth into terrorists. The four “prongs” are moral outrage with Western policies towards Muslims; a conviction that the West is in fact waging a coordinated war against Islam; personal experiences—including discrimination in school or at work—that resonate with this outrage and conviction (such experiences are much more common among Muslims in Europe, Sageman argues, than in the United States); and involvement in social networks of like-minded youth (e.g., friendship or kin groups, Muslim student associations, study groups linked to radical mosques, etc.). These four factors do not radicalize youth in the same temporal order, although he suggests that ideological commitment typically follows from embeddedness in social networks, rather than the other way around.

Is Sageman's model of radicalization conclusive? It is certainly plausible, but it is only sparsely illustrated with actual case histories. I suspect it overpredicts organized violence. Does every Muslim student group—or angry social circle within such groups—conclude that violence or terrorism is their best hope? How many angry youth have the wherewithal to carry out terrorist acts, even if they reach this conclusion? That said, I also suspect that Sageman has correctly identified the general social sites from which Islamist terrorism is arising today in Europe, however sporadically. I fear, however, that certain authorities will use his model to portray every association or social circle of Muslim youth in Europe as a potential national-security threat. Nothing could be further from Sageman's intentions, but we all know about the road to hell.

Krueger's data on international terrorists, incidentally, contrast rather sharply with Sageman's discussion of the emergent terrorist threat in Europe. Krueger's data reflect the non-European origins of most international terrorism in recent years. To be sure, international terrorists do not tend to come from the poorest or least literate societies, but neither do they come from richer countries in Europe or elsewhere. (Citizens of richer countries, however, are the main targets of international terrorism.) International terrorists tend to come from countries with limited civil liberties (i.e., authoritarian societies), and they are slightly more likely than not to come from militarily occupied countries. People from rich, liberal countries that are occupying other countries are especially likely to be attacked by terrorists of a different nationality. In short, middle-income, authoritarian, and/or occupied countries are the source of terrorists who attack people from rich, democratic, and occupying countries. So Krueger's “international terrorists” come from very different societies than the most recent wave of Sageman's “global Islamist terrorists”—but they are targeting the same people.

With these books, Krueger and Sageman have made important contributions to the growing field of terrorism studies. We especially know more about who these terrorists are and where they are coming from (geographically, socially, even psychologically), thanks to their work. Unlike many who write in this field, these authors take data very seriously. At the same time, it must be said, they hew closely to certain conventions in this field. Both acknowledge that state terrorism is more lethal than oppositional terrorism, and yet both say almost nothing about state terrorism or state violence more generally. In fact, neither author situates decisions by oppositional groups to employ terrorism within a larger field of strategic interaction involving states, other oppositional groups, and various publics. Finally, and ironically, neither says much about why their “terrorists” (at least some of them) have targeted civilians as opposed to armed actors. Since the targeting of civilians is a defining characteristic of terrorism—perhaps the defining characteristic—this silence is odd and disconcerting. But it is actually a quite common silence in the larger literature on terrorism.