The Stanford Prison Experiment was an extremely controversial psychology experiment involving authority and conformity, conducted by Philip Zimbardo in 1971. The experiment had to be terminated early due to extreme brutality on the part of the "guard" subjects.
During the 1960s and 1970s, the anti-establishment movement was in its heyday and academic experimental ethics standards were more lax than they are today. Together these factors gave rise to a number of important if controversial experiments involving authority, conformity, and the nature of evil. Essentially, these may be seen as the radical counterculture's response to the evils of Nazi Germany, centered on the question: why, given that most Germans did not start out as fanatical Nazis, would so many of them have participated in the worst crimes of the century, in the Holocaust? A decade before the Stanford Prison Experiment, Stanley Milgram held an equally controversial experiment (the so-called Milgram experiment) at Yale in which he seemingly established that random Americans would willingly shock fellow Americans, even to the point of death, simply because a man in a labcoat told them to do so.
In addition to the above, Zimbardo had another immediate motive for his research: he wanted something relevant to prison studies, another key institutional area of apparently widespread abuse. He and his research team selected 24 subjects for the experiment, most of them middle-class white and all of them male. They converted the basement of a Stanford building into a simulated prison, over which Zimbardo was designated the superintendent. The subjects were then randomly divided into guards, who were given wooden batons and military uniforms, and prisoners, who were given poor-quality, uncomfortable clothing with numbers stitched on, to be used instead of their names. The guards were explicitly told that they could use any psychological tactic they thought necessary against the prisoners, but they could never actually physically harm them.
The result was chaotic and horrifying. The first day went calmly, but on the second, the prisoners "revolted," demanding better treatment from the guards. The guards suppressed the revolt, but one prisoner, designated only as 8612, appeared to become irrational and enraged. He was released from the experiment, but a rumour then began to circulate amongst the other prisoners that 8612 was on the outside plotting a "prison break." The guards decided to prevent the escape by moving the prisoners to a new location, and then began to harass them increasingly severely. Prisoner counts were called constantly. Physical punishment was introduced. The guards began restricting access to washroom facilities, and removing prisoners' mattresses.
The experiment ultimately had to be halted early, after just six days, before the increasing brutality could lead to genuine physical harm. The results, according to Zimbardo's subsequent analysis, were frightening: humans were capable of cruelty not because of their nature, but simply because, once put in a position of authority, they began to exercise that authority in progressively more uncompassionate and then sadistic ways. Even more interestingly, the prisoner subjects had become more and more docile in accepting the commands given them by those in authority - when one would have expected a group of well-adjusted young men to become more irate about such conditions, instead they became more subservient.
Like the Milgram experiment before it, the Stanford Prison Experiment said something pessimistic, even depressing, about the human being's tendency to accept and inflict abuse so long as the system around them approved of and encouraged that abuse. Also like the Milgram experiment, the results have been challenged and debated to the present, even as ethics requirements have expanded to the point that a similar trial almost certainly could never be conducted today.