Terrorism scholar John Horgan has made a similar point. “The most valuable interviews I’ve conducted [with former terrorists] have been ones in which the interviewees conceded, ‘To be honest, I don’t really know,’” he writes. “Motivation is a very complicated issue. To explain why any of us does anything is a challenge.” It’s a challenge further compounded by the fact that some actions are informed by multiple motives, and even if these can be reliably identified it is often difficult to disentangle them and calculate their respective causal weight.

As Horgan suggests, a more manageable and useful question to ask about terrorism is not why, but how—and when and where? How did this specific person come to join this specific organization? What networks helped facilitate the act of joining, and where and how were these networks accessed or sought out?

Because these questions are about the circumstances of terrorism, and not the interior world of terrorists, they are not only more intellectually tractable for scholars, but also more directly relevant to efforts to prevent or stop terrorist recruitment. Law-enforcement agents can’t disrupt a motive, but with the right intelligence and skills they may be able to disrupt a network of terrorist recruiters. Marc Sageman’s work on Western “leaderless” jihadists demonstrates the promise of this kind of approach. Although Sageman has some interesting things to say about the why question, the strength of his research lies in showing just how decisive social and kinship networks are in the radicalization process.