“Why did they do it?” That’s one of the first questions on the lips of every reporter and pundit after a tragedy like the Boston Marathon bombing, and often there is no satisfying answer. In cases of domestic terrorism, the motives of the perpetrator leave us with other, equally difficult questions: what separates angry young men, most of whom will never commit acts of mass violence, from those who do?
Former C.I.A. operations officer Marc Sageman is a forensic psychiatrist who’s spent the last eight years working with the intelligence community. He says that the attacks on 9/11 triggered a ground swell of research into terrorism – but produced little in the way of results.