It’s arguably one of the most controversial experiments.

It all started in the basement of the psychology building at Stanford University on August 17, 1971 after psychologist Phil Zimbardo and colleagues took an ad out in the paper stating: “Male college students needed for psychological study of prison life. $15 per day for 1-2 weeks.”

Over 70 people volunteered for the Stanford Prison Experiment. Twenty-four healthy, smart college-aged men were picked and randomly assigned either to be a guard or a prisoner. The aim of the study was to explore the psychology of prison life and how specific situations affect people’s behavior.

But the experiment didn’t last very long — six days to be exact. Zimbardo was forced to pull the plug because of the disturbing behavior of the guards and the downright despair and other negative reactions of the prisoners.

According to a piece in Stanford Magazine:

For six days, half the study’s participants endured cruel and dehumanizing abuse at the hands of their peers. At various times, they were taunted, stripped naked, deprived of sleep and forced to use plastic buckets as toilets. Some of them rebelled violently; others became hysterical or withdrew into despair. As the situation descended into chaos, the researchers stood by and watched—until one of their colleagues finally spoke out.

The magazine features interviews with “some of the key players,” including Zimbardo, his wife (the “whistle-blower” who called for the study to stop), a guard (who was “the most abusive”) and a prisoner.

Like the fake guards, Zimbardo got caught up in the study, and started embodying the role of the prison’s warden. He told the magazine:

There was zero time for reflection. We had to feed the prisoners three meals a day, deal with the prisoner breakdowns, deal with their parents, run a parole board. By the third day I was sleeping in my office. I had become the superintendent of the Stanford county jail. That was who I was: I’m not the researcher at all. Even my posture changes—when I walk through the prison yard, I’m walking with my hands behind my back, which I never in my life do, the way generals walk when they’re inspecting troops.

We had arranged for everyone involved—the prisoners, guards and staff—to be interviewed on Friday by other faculty members and graduate students who had not been involved in the study. Christina Maslach, who had just finished her PhD, came down the night before. She’s standing outside the guard quarters and watches the guards line up the prisoners for the 10 o’clock toilet run. The prisoners come out, and the guards put bags over their heads, chain their feet together and make them put their hands on each other’s shoulders, like a chain gang. They’re yelling and cursing at them. Christina starts tearing up. She said, “I can’t look at this.”

I ran after her and we had this argument outside Jordan Hall. She said, “It’s terrible what you’re doing to these boys. How can you see what I saw and not care about the suffering?” But I didn’t see what she saw. And I suddenly began to feel ashamed. This is when I realized I had been transformed by the prison study to become the prison administrator. At that point I said, “You’re right. We’ve got to end the study.”

Soon after the experiment ended, Zimbardo became a sought-after speaker and expert on prison issues. He also stated that the experience helped him become a better person. He retired from Stanford in 2007 after nearly 40 years there as a psychology professor.

Zimbardo’s wife, now a professor of psychology at the University of California at Berkeley, talked about the changes she witnessed in him as the study went on and how she finally persuaded him to end it.

At first Phil didn’t seem different. I didn’t see any change in him until I actually went down to the basement and saw the prison. I met one guard who seemed nice and sweet and charming, and then I saw him in the yard later and I thought, “Oh my God, what happened here?” I saw the prisoners being marched to go down to the men’s room. I was getting sick to my stomach, physically ill. I said, “I can’t watch this.” But no one else was having the same problem.

Phil came after me and said, “What’s the matter with you?” That’s when I had this feeling like, “I don’t know you. How can you not see this?” It felt like we were standing on two different cliffs across a chasm. If we had not been dating before then, if he were just another faculty member and this happened, I might have said, “I’m sorry, I’m out of here” and just left. But because this was someone I was growing to like a lot, I thought that I had to figure this out. So I kept at it. I fought back, and ended up having a huge argument with him. I don’t think we’ve ever had an argument quite like that since then.

I feared that if the study went on, he would become someone I no longer cared for, no longer loved, no longer respected. It’s an interesting question: Suppose he kept going, what would I have done? I honestly don’t know.

The interview with Dave Eshelman, the abusive guard, was one of the most interesting. With little remorse, he recounted how he made a calculated decision to play a role and wanted to give the researchers something to work with.

What came over me was not an accident. It was planned. I set out with a definite plan in mind, to try to force the action, force something to happen, so that the researchers would have something to work with. After all, what could they possibly learn from guys sitting around like it was a country club? So I consciously created this persona. I was in all kinds of drama productions in high school and college. It was something I was very familiar with: to take on another personality before you step out on the stage. I was kind of running my own experiment in there, by saying, “How far can I push these things and how much abuse will these people take before they say, ‘knock it off?’” But the other guards didn’t stop me. They seemed to join in. They were taking my lead. Not a single guard said, “I don’t think we should do this.”

The fact that I ramped up the intimidation and the mental abuse without any real sense as to whether I was hurting anybody— I definitely regret that. But in the long run, no one suffered any lasting damage. When the Abu Ghraib scandal broke, my first reaction was, this is so familiar to me. I knew exactly what was going on. I could picture myself in the middle of that and watching it spin out of control. When you have little or no supervision as to what you’re doing, and no one steps in and says, “Hey, you can’t do this”—things just keep escalating. You think, how can we top what we did yesterday? How do we do something even more outrageous? I felt a deep sense of familiarity with that whole situation.

Another guard, John Mark, felt as though Zimbardo was trying to manipulate the experiment to go out with a bang.

I didn’t think it was ever meant to go the full two weeks. I think Zimbardo wanted to create a dramatic crescendo, and then end it as quickly as possible. I felt that throughout the experiment, he knew what he wanted and then tried to shape the experiment—by how it was constructed, and how it played out—to fit the conclusion that he had already worked out. He wanted to be able to say that college students, people from middle-class backgrounds—people will turn on each other just because they’re given a role and given power.

The only prisoner interviewed, Richard Yacco, helped to instigate a revolt against the guard. He told the magazine:

I don’t recall exactly when the prisoners started rebelling. I do remember resisting what one guard was telling me to do and being willing to go into solitary confinement. As prisoners, we developed solidarity—we realized that we could join together and do passive resistance and cause some problems. It was that era. I had been willing to go on marches against the Vietnam war, I went on marches for civil rights, and was trying to figure out what I would do to resist even going into the service. So in a way I was testing some of my own ways of rebelling or standing up for what I thought was right.

Yacco was paroled a day before the experiment ended, because he was showing signs of depression. He’s now a teacher at an Oakland public high school and wonders if the students who drop out and come unprepared are doing so because they’re also filling a role that society has created for them, just like the Prison Experiment.

I highly suggest learning the ins and outs of the experiment here. You really get an appreciation for the lengths the researchers went to simulate an authentic prison environment. The site even features a slideshow that explains how the experiment officially began: Participants were picked up at their homes by real police officers and then booked! (Here’s a clip.)

Plus, learn more about Zimbardo and his incredibly interesting research. And here’s more than you ever wanted to know about the experiment, Zimbardo’s research, media articles, imprisonment and more.

And, last but not least, check out this short BBC clip that interviews Zimbardo, Eshelman and another prisoner and has clips from the experiment 40 years ago.