MARGARET WARNER: And for more on the psychology behind abusive behavior like this, we turn to Dr. Robert Jay Lifton, a psychiatry professor at Harvard Medical School who's studied Nazi doctors and Vietnam veterans among others.
His latest book on violence is called "Superpower Syndrome." Lieutenant Colonel Dave Grossman, a retired psychology professor at West Point. His new book is "On Killing: The Psychological Cost of Learning to Kill" Jay Winik, a professor and war historian, and author of "April 1865: The Month That Saved America."
And Philip Zimbardo, a psychology professor at Stanford University. In 1971, he conducted a landmark study in which two dozen college students were directed to play the roles of prisoners and guards in a simulated jail with disturbing results. Welcome to you all.
And Professor Lifton, let me begin with you. Why do apparently ordinary people commit brutal acts like this? ...