The Stanford Prison Experiment was a landmark psychological study of the human response to captivity, in particular, to the real world circumstances of prison life. It was conducted in 1971 by Philip Zimbardo of Stanford University.
Subjects were randomly assigned to play the role of "prisoner" or "guard". Those assigned to play the role of guard were given sticks and sunglasses; those assigned to play the prisoner role were arrested by the Palo Alto police department, deloused, forced to wear chains and prison garments, and transported to the basement of the Stanford psychology department, which had been converted into a makeshift jail.
Several of the guards became progressively more sadistic - particularly at night when they thought the cameras were off, despite being picked by chance out of the same pool as the prisoners.
The experiment very quickly got out of hand. A riot broke out on day two. One prisoner developed a psychosomatic rash all over his body upon finding out that his "parole" had been turned down. After only 6 days (of a planned two weeks), the experiment was shut down, for fear that one of the prisoners would be seriously hurt.
Although the intent of the experiment was to examine captivity, its result has been used to demonstrate the impressionability and obedience of people when provided with a legitimizing ideology and social and institutional support. It also used to illustrate cognitive dissonance theory and the power of seniority/authority.
Read about the study here - this is a remarkable site: