Philip Zimbardo is fascinated by why people can behave in awful ways. Some psychologists believe those who commit cruelty are innately evil. Zimbardo disagrees. In The Lucifer Effect, he argues that sometimes good people do evil things simply because of the situations they find themselves in, citing many historical examples to illustrate his point. Zimbardo details his 1971 Stanford Prison experiment, where ordinary volunteers playing guards in a mock prison rapidly became abusive. But he also describes the tortures committed by US army personnel in Iraq's Abu Ghraib prison in 2003—and how he himself testified in defence of one of those guards.