Yet the evidence suggests that the concept is flawed and that such anti-jihadist measures are ineffective, even counterproductive. A secret British government memorandum leaked in 2010 dismissed the idea that there was “a linear ‘conveyor belt’ moving from grievance, through radicalization, to violence.” A 2010 American study sponsored by the Department of Homeland Security similarly noted that radicalization “cannot be understood as an invariable set of steps or ‘stages’ from sympathy to radicalism.”

Many studies show, perhaps counterintuitively, that people are not usually led to jihadist groups by religious faith. In 2008, a leaked briefing from Britain’s domestic security service, MI5, found that far from being religious zealots, many involved in terrorism were not particularly observant.

This view is confirmed by Marc Sageman, a former officer with the Central Intelligence Agency who is now a counterterrorism consultant. “At the time they joined, jihad terrorists were not very religious,” he observed. “They only became religious once they joined the jihad.”

The paradox is that the concept has become central to domestic counterterrorism policy even as government agencies discover it’s wrong. There is a gap between the reality of jihadism and a political desire for a simple narrative of radicalization.

In recent years, the official view of the process has become more nuanced. An F.B.I. website aimed at teenagers acknowledges that “no single reason explains why people become violent extremists.” Updated British strategy also accepts that “there is no single cause of radicalization.”

Yet the idea of a conveyor belt and telltale signatures of radicalization continue to be influential.

For many, though, the first steps toward terror are rarely taken for political or religious reasons. As the French sociologist Olivier Roy, the pre-eminent scholar of European jihadism, puts it, few terrorists “had a previous story of militancy,” either political or religious. Rather, they’re searching for something less definable: identity, meaning, respect.