WHY would an apparently normal young adult drop out of college and turn up some time later in a video performing a cold-blooded execution in the name of jihad? It’s a conundrum we have been forced to ponder ever since a group calling itself ISIS declared war on infidels. But 70 years ago we were asking something similar of guards in Nazi concentration camps – and, sadly, there have been plenty of opportunities to ponder the matter in between…

This article first appeared in New Scientist on 14 November 2015. To continue reading, click here (paywall).