AbstractWe find that graduates from subjects such as science, engineering, and medicine are strongly overrepresented among Islamist movements in the Muslim world, though not among the extremist Islamic groups which have emerged in Western countries more recently. We also find that engineers alone are strongly over-represented among graduates in violent groups in both realms. This is all the more puzzling for engineers are virtually absent from left-wing violent extremists and only present rather than over-represented among right-wing extremists. We consider four hypotheses that could explain this pattern. Is the engineers’ prominence among violent Islamists an accident of history amplified through network links, or do their technical skills make them attractive recruits? Do engineers have a ‘mindset’ that makes them a particularly good match for Islamism, or is their vigorous radicalization explained by the social conditions they endured in Islamic countries? We argue that the interaction between the last two causes is the most plausible explanation of our findings, casting a new light on the sources of Islamic extremism and grounding macro theories of radicalization in a micro-level perspective.