In 1971 researchers at Stanford University created a simulated prison in the basement of the campus psychology building. They randomly assigned 24 students to be either prison guards or prisoners for two weeks. Within days the "guards" had become swaggering and sadistic, to the point of placing bags over the prisoners' heads, forcing them to strip naked and encouraging them to perform sexual acts. The landmark Stanford experiment and studies like it give insight into how ordinary people can, under the right circumstances, do horrible things — including the mistreatment of prisoners at the Abu Ghraib prison in Iraq. What...