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.