The Stanford prison experiment was a study of the psychological effects of becoming a prisoner or prison guard. The experiment was conducted in 1971 by a team of researchers led by psychologist Philip Zimbardo at Stanford University. Twenty-four undergraduates were selected out of 70 to play the roles of both guards and prisoners and live in a mock prison in the basement of the Stanford psychology building.

Prisoners and guards rapidly adapted to their roles, stepping beyond the boundaries of what had been predicted and leading to dangerous and psychologically damaging situations. One-third of the guards were judged to have exhibited "genuine" sadistic tendencies, while many prisoners were emotionally traumatized and two had to be removed from the experiment early. After sensing that everyone had been too absorbed in their roles, including himself, Zimbardo terminated the experiment after six days.

Ethical concerns surrounding the famous experiment often draw comparisons to the Milgram experiment, which was conducted in 1961 at Yale University by Stanley Milgram, Zimbardo's former college friend.

Tom Peters and Robert H. Waterman Jr wrote in 1981 that the Milgram Experiment in the 1960s and the later Zimbardo Experiment were frightening in their implications about the danger which lurks in the darker side of human nature.