During the summer of 1971, 24 volunteers living near Stanford University were interviewed, selected, and arrested. They’d all responded to a simple newspaper ad calling for male college students whom, it said, would get $US15 a day to participate in a “psychological study of prison life” that summer. At the fake prison built for the study, the volunteers were randomly divided into fake prisoners and fake guards. Designed to last two weeks, the experiment was cut short after just six days. In other words, it went horribly wrong.