When Virginia Tech student Seung-Hui Cho walked into a classroom building and opened fire, completing a killing spree that left 32 students and professors dead, survivors and family members were left to ask why? Why this student? Why this school? And how can we keep it from happening again?

The answers lie not with any one individual, but in society, said social psychologist Philip G. Zimbardo, PhD, professor emeritus at Stanford University, during a conversation hour about evil, hate and horror with Aaron T. Beck, MD, and Frank Farley, PhD.

Evil is seductive and contagious-it spreads through society like a disease, said Zimbardo, citing his years of research on group violence in studies such as the Stanford Prison Experiment, and in cases such as the prisoner abuse at Abu Ghraib.

"Who creates those situations that lead good people to become perpetrators of evil?" he asked. "The answer is simple-it's a system."

And while all violence is composed of individual acts, psychology errs by not examining outside forces. Political, historical, cultural, legal and economic forces embed people in situations like Abu Ghraib, yet our justice, religious, health and psychiatric systems focus solely on the individual, he continued.

But if we really want to understand violence and evil and prevent their spread, he said, we need to analyze three factors: what does the individual bring into the situation, what are the environmental forces that influence the individual and who or what created those forces.

Evil and violence are also rooted in distorted cognitions, said cognitive-therapy pioneer Beck, a psychiatry professor at the University of Pennsylvania. Through his work with individuals in marital counseling, Beck has seen how hate evolves. "Never did I see such degrees of hate as between two people, both of whom loved each other at one point," he said.

People locked in emotional disagreements start to have distorted views of each other, he explained, describing a classic scenario in which a wife questioned a husband's manliness and he attacked her. Once the wife stopped seeing the husband as the strong and supportive macho man she married but as unreliable and insensitive, the ensuing dynamic left the couple regarding each other as enemies-she saw him as evil, he envisioned her as a witch on a broomstick, Beck realized.

"They're not attacking each other-the real person-they're attacking the image," he explained.

The same process happens in mass violence, he said. An individual or individuals in one group demonize members of another group. Once that happens, the people in this group become homogenized into a single category: the enemy that must be destroyed, said Beck.

Another cause of evil is the thrill of breaking social rules and exercising power, said the presenters.

"What gets a [robber] through the door of a 7-Eleven is the thrill of breaking social rules," posited Farley, who is the Laura H. Carnell Professor of Educational Psychology at Temple University, adding that such exhilaration may also be a motivating factor for suicide bombers. Even watching a crime being committed provides a vicarious thrill, said Farley and Zimbardo. People are drawn in because the perpetrators appear powerful, and violence or destructive acts seem like easier ways to gain power than through grand, heroic acts, explained Zimbardo.

However, society can work against the influence of violence and hate by lionizing everyday heroes-from a teen who stands up for a bullied peer to the man who stood in the way of the tanks at Tiananmen Square, he asserted.

"We have made the mistake of mythologizing our heroes," he concluded. "Most heroic acts are acts by ordinary people. The task before us is to cultivate the heroic imagination of our youth."