The spate of cases involving “homegrown” terrorists seeking to strike in the United States and Europe, or volunteer for conflict zones abroad, have intensified intelligence efforts to understand the radicalization process that transforms ordinary individuals into violent extremists. Specifically, governments and security services are under intense pressure to detect and stop budding terrorists early in the radicalization process, as well as implement counterradicalization measures that promote disengagement from terrorism. Consequently, they are pushing their analysts to uncover hidden, often unconscious, personality forces that propel perpetrators toward extremist behavior, or identify the social, economic, and political contexts that encourage and facilitate violent radicalization.

John Horgan cautions against these two approaches and insists that becoming a terrorist is a product of a complex radicalization process that is not uniform, linear, or easily predictable. It is more fruitful to map radicalization pathways taken by extremists than to seek after demographic or psychological profiles of terrorists (p. 1). Radicalization is the gradual progression from radical attitudes to more concrete and manifest forms of violent extremism produced by socialization in communities of practice. Rather than profiles and root causes that produce terrorists, Horgan proposes a list of “predisposing risk factors” (p. 11) that make some individuals prone to joining radical milieus. These include temporary emotional states driven by rage or grievances, dissatisfaction with conventional political contestation, desire to act, identification with victims, inclination to justify violence, expectations of rewards, and social ties to other radicals. Separately, none of these risk factors can predict who becomes a terrorist, argues Horgan, but their combined effect can help us identify persons vulnerable to recruitment.

There are three problems with this approach. First, the risk factors strategy—similar to the profiling and root-causes approaches—can simultaneously overpredict and underpredict who joins terrorism. They can overpredict because the posited risk factors are sufficiently widespread and susceptible to subjective interpretations by overly vigilant security services. By casting the net too wide, there is a danger of conflating aggrieved individuals with potential terrorists that intend to do us harm. The risk-factors approach can also underpredict because in contexts of protracted war, say Afghanistan or Gaza, the posited factors are very pervasive but the majority of the population chooses to remain neutral or free ride. Second, Horgan is ambiguous about the necessary and sufficient cumulative threshold of these risk factors. Are all of them necessary to produce the predisposition to radicalization, or just a few? Are certain combinations more important than others? For example, is the desire to act in combination with social ties to extremists sufficient to produce recruits, even if the other risk factors are absent? Third, from a social scientific viewpoint, it is not clear how one can test the putative validity of these risk factors in nontautological ways. Looking at individual terrorists, one can certainly find these attitudes and relationships ex post facto. But how do we know if they are there before the leap to violence, especially since many of the risk factors identified by the author involve mental attitudes, not observable behaviors?

Horgan's other purpose in this book is to explain why people walk away from terrorism. To that end, he interviewed 29 former terrorists from a number of countries and organizations. Most of the interviews lasted for about one hour, but some were longer. He highlights only six cases with varying degrees of depth—one from Norway, three from Northern Ireland, and two Islamists based in London and Beirut. At first look, one may question the reliability of conclusions drawn from such a small number of cases and relatively short interviews. Those who study terrorism, however, know well the difficulties with such field interviews and would undoubtedly appreciate the author's valuable contribution in this regard. To his credit, Horgan frames these cases as illustrative of the complexity of radicalization and disengagement from terrorism, not definitive tests of his views on the subject.

The six interviews point to two conclusions. First, disengagement from terrorism is not the same as deradicalization. Individuals can cease their terrorist activities without having to abandon their radical beliefs or ideologies. Five of the six cases involved individuals who remain more or less committed to their cause, but in ways that do not necessarily involve violence. This finding is good news for security services faced with the daunting challenge of winning the hearts and minds of people—it may not be necessary to do so. Second, disillusionment with the group and its activities often precedes disengagement. This finding implies that the taboo-breaking activities of terrorists are just as important in encouraging people to walk away from violence as the measures taken by the counterterrorists.

Beyond these two broad conclusions, one is hard-pressed to find a pattern of disengagement across the six cases. Indeed, Horgan does not offer claims of patterns in his concluding chapter, which suggests that there are none to be found. Two of his cases disengaged after being arrested; one deradicalized before ever taking the leap to violence in Afghanistan, deterred by the image of an injured Pakistani boy returning home from the conflict zone; and another deradicalized after his comrades reveled in the killing of a policewoman. The two remaining cases are of persons who are still considered a security threat by the authorities and are essentially in exile. Four of the narratives, therefore, cast a pall on the author's assertion that disillusionment precedes disengagement. Arrests and imposed exile seem to have done the trick in four of the six cases.

The value of this book lies not in its ability to advance a coherent theory of radicalization and disengagement, but rather in its ability to highlight the complexity of these issues and, thus, serve as a corrective to simplistic, linear, and presumably predictive models of radicalization and deradicalization. It should be required reading for government and security analysts besieged by theory mongers that peddle tidy charts, precise lists, and prophetic schemas.