Philip Zimbardo (1933 - ) is known for leading the Stanford Prison Study, a controversial experiment which investigated the psychological effects of being a prisoner or prison guard. In the experiment, college students were randomly assigned to become prisoners or guards.
During the course of the experiment, both the prisoners and guards rapidly internalized their roles (they started to become their roles), with the guards becoming sadistic, and the prisoners becoming passive and depressed. Originally designed as a two-week experiment, the duration was cut short after 6 days due to the emotional trauma experienced by the participants.
The study demonstrated what Zimbardo calls the Lucifer Effect - how good people can turn evil in response to the situation they are in, and not as a result of their real personalities.