What took place on a peaceful Californian university campus nearly four decades ago still has the power to disturb. Eager to explore the way that "situation" can impact on behaviour, the young psychologist enrolled students to spend two weeks in a simulated jail environment, where they would randomly be assigned roles as either prisoners or guards.

Zimbardo's volunteers were bright, liberal young men of good character, brimming with opposition to the Vietnam war and authority in general. All expressed a preference to be prisoners, a role they could relate to better. Yet within days the strong, rebellious "prisoners" had become depressed and hopeless. Two broke down emotionally, crushed by the behaviour of the "guards", who had embraced their authoritarian roles in full, some becoming ever-more sadistic, others passively accepting the abuses taking place in front of them.

Transcripts of the experiment, published in Zimbardo's book The Lucifer Effect: Understanding How Good People Turn Evil, record in terrifying detail the way reality slipped away from the participants. On the first day ­ Sunday ­ it is all self-conscious play-acting between college buddies. On Monday the prisoners start a rebellion, and the guards clamp down, using solitary confinement, sleep deprivation and intimidation. One refers to "these dangerous prisoners". They have to be prevented from using physical force.

Control techniques become more creative and sadistic. The prisoners are forced to repeat their numbers over and over at roll call, and to sing them. They are woken repeatedly in the night. Their blankets are rolled in dirt and they are ordered painstakingly to pick them clean of burrs. They are harangued and pitted against one another, forced to humiliate each other, pulled in and out of solitary confinement.

On day four, a priest visits. Prisoner 819 is in tears, his hands shaking. Rather than question the experiment, the priest tells him, "You're going to have to get less emotional." Later, a guard leads the inmates in chanting "Prisoner 819 did a bad thing!" and blaming him for their poor conditions.

Zimbardo finds 819 covering his ears, "a quivering mess, hysterical", and says it is time to go home. But 819 refuses to leave until he has proved to his fellow prisoners that he isn't "bad". "Listen carefully to me, you're not 819," says Zimbardo. "You are Stewart and my name is Dr Zimbardo. I am a psychologist not a prison superintendent, and this is not a real prison."819 stops sobbing "and looks like a small child awakening from a nightmare", according to Zimbardo. But it doesn't seem to occur to him that things are going too far.

Guard Hellmann, leader of the night shift, plumbs new depths. He wakes up the prisoners to shout abuse in their faces. He forces them to play leapfrog dressed only in smocks, their genitals exposed. A new prisoner, 416, replaces 819, and brings fresh perspective. "I was terrified by each new shift of guards," he says. "I knew by the first evening that I had done something foolish to volunteer for this study."

The study is scheduled to run for two weeks. On the evening of Thursday, the fifth day, Zimbardo's girlfriend, Christina Maslach, also a psychologist, comes to meet him for dinner. She is confronted by a line of prisoners en route to the lavatory, bags over their heads, chained together by the ankles. "What you're doing to these boys is a terrible thing," she tells Zimbardo. "Don't you understand this is a crucible of human behaviour?" he asks. "We are seeing things no one has witnessed before in such a situation." She tells him this has made her question their relationship, and the person he is.

Downstairs, Guard Hellmann is yelling at the prisoners. "See that hole in the ground? Now do 25 push-ups, fucking that hole. You hear me?" Three prisoners are forced to be "female camels", bent over, their naked bottoms exposed. Others are told to "hump" them and they simulate sodomy. Zimbardo ends the experiment the following morning.

To read the transcripts or watch the footage is to follow a rapid and dramatic collapse of human decency, resilience and perspective. And so it should be, says Zimbardo. "Evil is a slippery slope," he says. "Each day is a platform for the abuses of the next day. Each day is only slightly worse than the previous day. Once you don't object to those first steps it is easy to say, 'Well, it's only a little worse then yesterday.' And you become morally acclimatised to this kind of evil."