As you read this, remember that these were college students who had no other obvious reasons to behave as they did: no 9.11, no war, no military training, they weren't in another country, etc. What is also fascinating is that the researchers themselves took on their role, or at least stepped out of the role of researcher and supported the "guards". I am tempted to quote some material from Sagan's _Freud, Women and Morality_ about the psychological process of 'identifying with the aggressor'.
"In August 1971, an advertisement appeared in the Palo Alto Times: "Male college students needed for psychological study of prison life. $15 per day for 1-2 weeks..." Seventy men responded. Among them, two dozen were chosen to participate in the experiment because based on interviews and a battery of psychological tests they were judged to be the most normal, average and healthy. They were then assigned randomly, by a flip of coin, either to be guards or prisoners.
Those assigned to be prisoners were subsequently "arrested" in their homes by Palo Alto police. They were booked at a real jail, then blindfolded and led to the basement of Jordan Hall, which had been remodeled to resemble a real prison. Each prisoner was searched, stripped naked and deloused. They were then made to wear smocks with their prison ID numbers both in front and in back. Those assigned to be guards were given uniforms and instructed to maintain control of the prison but not to use violence. What happened next was so shocking that the experimenters (i.e., the psychologists) had to call off the planned two-week experiment after only six days.
Two days into the experiments, the prisoners started to exhibit rebellious behavior. They began to taunt and curse the guards and even stage a revolt. The guards were enraged and retaliated, initially by using a fire extinguisher. They broke into each cell, stripped the prisoners naked, took the beds out, forced the ringleaders of the prisoner rebellion into solitary confinement, and generally began to harass and intimidate the prisoners. They also applied psychological tactics such as setting up a "privilege cell" for model prisoners to break the solidarity among prisoners. Alternately, they also put "bad prisoners" in the "privilege cell" in order to create confusion, suspicion, and aggression among prisoners. By then, these research participants had really taken on the roles that they were randomly assigned to play.
Guards applied total control on each prisoner's life, including going to the toilet. Prisoners were often not allowed to use the toilet and forced to urinate or defecate in a bucket in their cell, but not allowed to empty the buckets. Repeatedly, guards also punished prisoners by forcing them to do push-ups, jumping jacks, cleaning out toilet bowls with their bare hands, and acting out other degrading scenarios. Often, they also coerced prisoners to become snitches in exchange for reduced abuse. Especially when they were bored or thought that the experimenters were not watching, their treatment to the prisoners would escalate and became more pornographic. The humiliation and dehumanization got so severe, that the experimenters had to frequently remind the guards to refrain from such tactics.
The prisoners, on the other hand, started to experience acute emotional disturbance and rage. They exhibited disorganized thinking, uncontrollable crying, withdrawing, and behaving in pathological ways. As a result, researchers had to release five prisoners from the experiment prematurely. Other people connected to the experiment were also sucked in by the situation. The experimenters forgot that they were there to observe and collect data. Instead, they started to assume the role of prison staff and supervisor. A priest who visited the prison started to contact parents of the prisoners about arranging lawyers to bail them out. The parents, who had visited the prison themselves, seem to also have forgot that theirs sons had the right to withdraw from the experiment. They actually started to arrange lawyers. And a lawyer actually came...
http://psychology.about.com/library/weekly/aa060100b.htm