The researchers found that the overrepresentation of engineers held true in other contexts. Of the 40 jihadists who studied at universities abroad, 27 were engineers. In another data set, comprising 71 extremists who were born or grew up in Western countries, 32 were engineers.

The relationship extended beyond Islamist movements to other extremist groups. Violent neo-Nazis and neo-Stalinists in Russia and neo-Nazi and white-supremacist groups in the United States also showed disproportionate numbers of engineers…

Gambetta and Hertog turned to another discipline: psychology.

They focused on three traits. One is the need for cognitive closure, or a preference for order and distaste for ambiguity. Scholars like John T. Jost, Arie W. Kruglanski, and Jonathan Haidt have documented high levels of this trait among politically conservative voters. These groups, Gambetta and Hertog write, also have two other tendencies: They accept prevailing hierarchies and, when confronted with the unfamiliar, they experience high levels of disgust.

The authors observe that these traits are also central to radical Islamist ideology. Did engineers have them, too?