After appearing on the Daily Show last night, today, retired Stanford professor Phil Zimbardo appeared on the radio show "Democracy Now!" to publicize his new book, The Lucifer Effect. Click here to download the show's MP3 or here to watch the segment in Real Player.

As the United States enters the fifth year of its occupation of Iraq, some of the most enduring images of the war remain the vivid photographs of US soldiers torturing Iraqi prisoners at Abu Ghraib. The pictures were leaked to the press and first revealed to the world in May 2004. Images showed Iraqis with bags over their heads, beaten, set upon by dogs and forced into sexually humiliating acts. The Bush administration tried to paint the scandal as an isolated incident committed by rogue soldiers. But who is really to blame for the abuses at Abu Ghraib? The answer may lie in a landmark study conducted more than three decades ago.

In 1971, psychology professor Philip Zimbardo created an experiment at Stanford University in which 24 male college students were randomly assigned the roles of prison guards and prisoners at a makeshift jail on campus. The experiment was scheduled to run for two weeks. By Day Two, the guards were going far beyond keeping the prisoners behind bars. In scenes eerily similar to Abu Ghraib, prisoners were stripped naked, bags put on their heads and sexually humiliated. The guards had become dangerously sadistic and the prisoners were breaking down emotionally. The two-week experiment had to be canceled after just six days.

Professor Philip Zimbardo has just written a new book that, for the first time, tells the full story of the famed Stanford Prison Experiment. It's called "The Lucifer Effect: Understanding How Good People Turn Evil." Professor Zimbardo joins me today from our firehouse studio in New York.