Today the good people at TED posted a video of Philip Zimbardo's talk--brimming with humanity and good will--from the conference earlier this year. Zimbardo is, of course, the psychologist who designed the Stanford Prison Experiment in 1971. More recently, he was called upon to be an expert witness at Abu Ghraib trials, an experience that led him to write "The Lucifer Effect: Understanding How Good People Turn Evil."

You can buy his book, or better yet, watch the video. (And then buy his book.)

Zimbardo's central theory won't be too surprising to anyone even remotely familiar with his work: Many people who do horrible things are not necessarily born "evil." Determining why good people turn evil, or do evil things, has been his life's work. A lot of it, it turns out, has to do with circumstance. Evil, as Zimbardo sees it, is when power is abused in such a way that it hurts people physically, psychically or emotionally. And "if you give people power without oversight, it's a prescription for abuse," he says.

To illustrate his point he uses the Abu Ghraib prison scandal to examine how ordinary soldiers--who would be called "bad apples" by the government that asked them to oversee prisoners with inadequate training and oversight--did things that were extraordinary for their brutality. (In a somewhat related note, this week the 2nd U.S. Circuit Court of Appeals upheld a 2006 ruling by Judge Alvin K. Hellerstein ordering the release of the Abu Ghraib pictures to the ACLU.)

With the caveat that "understanding is not excusing" evil, Zimbardo ends on a positive note: sure, the power to commit evil resides dormant in us all. But so does the potential for great heroism.

Context is key. Just ask Wesley Autrey, the New York City subway hero.