Crimes like those in Paris, New York, London, and Madrid – attacks on countless cafes, malls, buses, trains, and nightclubs – affront our most basic human values, because they involve the deliberate murder of innocents and seek to spread fear throughout society. We are wont to declare them the work of lunatics and sociopaths, and we feel repulsed by the very idea that they may have an explanation beyond the insanity of their perpetrators.