We often hear that in order to wreak evil, we have to dehumanize the enemy--hence the political propaganda that painted the Japanese and Germans as inherently degraded races, fundamentally different from and less moral than ourselves. Zimbardo, however, makes an interesting point: in order to do evil, we also have to dehumanize ourselves. He points to research showing that warriors in tribes that kill, mutilate, and torture their opponents almost all change their appearance substantially before they go into battle. Tribes where the warriors go into battle in their day clothes, so to speak, are considerably gentler.

There are a lot of ways to depersonalize the relationship between attacker and victim. In experiments, people who are anonymous are more willing to administer aggressive treatment, such as electric shocks, to "victims". Another way is to disappear into the group. That's why there are firing squads, rather than a single bullet to the head. When you collectively commit atrocities, as the Germans did under the Nazi rule, it is easy not to think about what you are authorizing. The camps, after all, are very far away. And you are only one one-millionth of the decision to send Jews and Gypsies there.

Update One of the interesting things that I meant to mention, but didn't, is that there is one way of predicting which groups will succeed in Vernon Smith's cooperation experiments: the groups that talk to each other cooperate. The experiments are done on computers with a chat system. People who talk a lot, especially random chit chat, are much more likely to cooperate with each other.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.