“After all this funding and this flurry of publications, with each new terrorist incident we realize that we are no closer to answering our original question about what leads people to turn to political violence,” Marc Sageman, a psychologist and a longtime government consultant, wrote in the journal Terrorism and Political Violence in 2014. “The same worn-out questions are raised over and over again, and we still have no compelling answers.”

When researchers do come up with possible answers, the government often disregards them. Not long after the attacks of Sept. 11, 2001, for instance, Alan B. Krueger, the Princeton economist, tested the widespread assumption that poverty was a key factor in the making of a terrorist. Mr. Krueger’s analysis of economic figures, polls, and data on suicide bombers and hate groups found no link between economic distress and terrorism…

Rather, the murky science seems to imply that nearly anyone is a potential terrorist. Some studies suggest that terrorists are likely to be educated or extroverted; others say uneducated recluses are at risk. Many studies seem to warn of the adolescent condition, singling out young, impatient men with a sense of adventure who are “struggling to achieve a sense of selfhood.”