Narrator: Stanford University, Northern California. One of America's most prestigious academic institutions, and in 1971, the scene of one of the most notorious experiments in the history of psychology.

Philip Zimbardo: I was interested in what happens if you put good people in an evil place. Does the situation outside of you— the institution— come to control your behavior, or does the things inside of you— your attitude, your values, your morality— allow you to rise above a negative environment?

Narrator: The negative environment Zimbardo chose to test his ideas was a prison. He would convert the basement of the university's psychology department into a subterranean jail.

Philip Zimbardo: We put prison doors on each of three office cells. In the cells, there was nothing but three beds. And there was actually very little room for anything else, because they were very small. And then here, we had solitary confinement, which we call the hole. And the hole was the place where prisoners would be put for punishment. It was a very, very small area. When you closed the door, it was totally dark.

All the guards wore military uniforms. And we had them wear these silver reflecting sunglasses. And what it does is, you can't see someone eyes. And so that loses some of the humanness— the humanity. In general, we wanted to create a sense of power. That is, the guards, as a category, are people who have power over others. In this case, power over the prisoners.

Narrator: Zimbardo was interested in the power of social situations to overwhelm individuals. His experiment would test people's responses to an oppressive regime. Would they accept it or act against it? Zimbardo's experiment was conducted against a backdrop of civil rights activism and protest against the Vietnam War.

Philip Zimbardo: There was a sense of student power, student dominance, and student rebellion against authority in general.

Narrator: It was from the student body that Zimbardo selected his participants. After passing tests to screen out anyone with a psychological abnormality, they were paid $15 a day. Each was randomly assigned the role of guard or prisoner.

Clay Ramsay: I was 20. And that September, I was going to college. And it would be nice to have a summer job, but there sure wasn't a lot of time left. And I looked in the want ads, and I found this thing which was just going to fit. It was just two weeks.

Dave Eshleman: I was on summer break from my first year in college, and I was looking for a job. Had to choose between that and making pizzas. That sounded like a lot more fun.

Narrator: Prisoners were brought to the basement prison— blindfolded, to confuse them about their whereabouts. They were stripped and deloused.

Philip Zimbardo: Of course, the guards started making fun of their genitals and humiliating them. And really, it's the start of what's known as the degradation process which, not only prisons, but lots of military-type outfits used that process.

Dave Eshleman: I recall walking up and down the very short the hallway, which was the prison hall, and looking in on the prisoners. And they were basically lounging around on their beds. I felt it was like a day in summer camp.

Philip Zimbardo: The first day, I said, this might be a very long and very boring experiment, because it's conceivable nothing will happen.

Dave Eshleman: I arrived independently at the conclusion that this experiment must've been put together to prove a point about prisons being a cruel and inhumane place. And therefore, I would do my part to help those results come about.

I was a confrontational and arrogant 18-year-old at the time. And I said, somebody ought to stir things up a bit here.

Philip Zimbardo: I didn't expect a rebellion because not much happened. And it wasn't clear what they were rebelling against. But they were rebelling against the status, rebelling against being anonymous, against having to follow orders from these other students.

The guards felt that they now have to up the ante of being tough. The prisoners made the mistake of beginning to use profanity against the guards in a very personalized way. So not against the guards but, you little punk, you big shit and stuff. And the guards got furious.

Narrator: Prisoners were repeatedly woken in the middle of the night. The guards made them do menial, physical tasks and clean out toilets with their bear hands.

Dave Eshleman: We made it a point to not give them any sense of comfort or what to expect, that anything could happen to them at any time, including being rousted from their sleep at any hour. I think that I was the instigator of this whole schedule of harassment.

Narrator: The harassment of the guards took its toll on rebellion leader 8612. He told Zimbardo he wanted to leave the experiment.

Philip Zimbardo: I said, well, I can see to it the guards don't hassle you personally. And in return, all I would like is some information from time to time about what the prisoners are doing. So essentially, I'm saying, I'd like you to be a snitch, an informer. And I said, think it over, and if you still want to leave, fine.

Narrator: Confused, prisoner 8612 returned to his cell and told the other prisoners that no one could leave.

Philip Zimbardo: That really transformed the experiment into a prison.

Narrator: Soon after returning to his cell, prisoner 8612 started showing signs of severe distress. He came up with a plan that, if he acted crazy, we would have to release him.

Doug Korpi: I feel so fucked up inside. I feel really fucked up inside. You don't know. I've got to go. I— to a doctor, anything. I can't stay in here. I'm fucked up. I don't know how to explain it. I'm all fucked up inside! I want out! Let me out now!

Narrator: It starts with make believe. And then he's doing it and cursing and screaming. And whatever that little boundary is, he moved across. Not that he became really crazy, but he became excessively disturbed. I mean, so much so that we immediately said we have to release him.

Doug Korpi: As an experience, it was unique. I've never screamed so loud in my life. I've never been so upset in my life. And it was an experience of being out of control.

Dave Eshleman: What was one of the surprising things to me is that there was so little that the prisoners did to support one another after we started our campaign of divide and conquer.

Narrator: Isolated and distraught, prisoner 819 told Zimbardo he wanted to leave.

Philip Zimbardo: While I'm interviewing 819, and saying, OK, it's all over. Thank you for your participation. We'll give you money for the whole two weeks even though you're leaving early. He hears the prisoners shouting 819 did a bad thing. And he said, I can't leave. And he's crying. And he says, I can't leave. I said, what do you mean you can't leave? He said, no, I have to go back, because I don't want them to think that I'm a bad prisoner.

And that's when I really flipped out, that in such a short time, a college student's thinking could become so distorted. I said, you're not a bad prisoner. You're not a prisoner, and this is not a prison. And it was this thing where he opened his eyes, and it was seriously really like a cloud being lifted.

Narrator: Seeing things clearly, prisoner 819 reverted to his original request and was released. To replace him, the experimenters called in one of their reserves from the standby list.

Clay Ramsay: I got a phone call saying, are you still available as an alternate, a kind of cheery female secretary voice. I said, yes, sure. And so she said, could you start this afternoon? And I said, yes, sure. And my role in the experiment really began. I was blindfolded and then stripped and supposedly deloused.

Philip Zimbardo: He came into a mad house, full blown. All of us had gradually acclimated to the increasing level of aggression, the increasing powerlessness of the prisoners, the increasing dominance of the guards. And he comes and says, what's happening here? Is it a prison? And they said, yeah, you better not make trouble. It's really terrible. It's a real prison. And he says, I'm out of here. I don't want— and they said, no, you can't leave. Once you're here, you start. This is a real prison.

Narrator: Prisonor 416 was soon subjected to the harassment of Dave Eshleman, nicknamed John Wayne because of his macho attitude.

Dave Eshleman: I had just watched a movie called Cool Hand Luke, and the mean, intimidating Southern prison warden character in that film really was my inspiration for the role that I created for myself.

Narrator: He was creative in his evil. He would think of very ingenious ways to degrade, to demean the prisoners. One of the best guards was also on that shift. And instead of confronting this bad guard, this sadistic guard, essentially— because he didn't want to see what was happening— he became the gopher. He would go out to get the food and things of this kind. And that left the John Wayne guard and another guard on that shift to be dominant.

Narrator: Prisoner 416 decided to go on a hunger strike.

Clay Ramsay: They were pushing my limits. But here was the thing that I could do that could push their limits.

Dave Eshleman: I thought, how dare this newcomer come in and try to change everything that we had worked for the first three days to set up. And by God, he's going to suffer for that. We would use our night sticks to bang on the door and we would kick the door so hard that it must've shaken him very seriously inside, scared the life out of him.

Clay Ramsay: He yelled at me and threatened me, and actually smashed a sausage into my face to try to get me to open up.

Philip Zimbardo: In fact, the prisoners accepted the guards' definition of him as a troublemaker.

Dave Eshleman: I remember some of them saying, would you eat, goddamn it? We're sick and tired of this. And that was proof that there was no solid area. There was no support between the prisoners.

The study showed that power corrupts, and how difficult it is for people who are the victims of abuse to stand up and defend themselves. Why doesn't anybody who is being abused by a spouse or something like that just say, stop it? And we realize now that that's not as easy as it sounds.

Narrator: By the end of the fifth day, four prisoners had broken down and been released. 416 was on the second day of his hunger strike, and the experiment still had another nine days to run.

At this point, a fellow psychologist visited Zimbardo's basement prison, and would witness the brutality of the experiment firsthand.

Philip Zimbardo: The guards had lined up the prisoners to go to the toilet. Had bags over their head, chains on their feet, and were marching by. And I looked up and I saw this circus, this parade. And I said, hey, Chris. Look at that.

Christina Maslach: I looked up, and I just began to feel sick to my stomach. I had this just chilling, sickening feeling of watching this. And I just turned away. And I just let loose in this emotional tirade. I just lost it. I was angry, scared. I was in tears.

Philip Zimbardo: And I'm furious. I'm saying, you're supposed to— and then we had a big argument. You're supposed to be a psychologist. This is this interesting, dynamic behavior in such a few days. But I'm going through this whole thing, the power of the situation. She says, no, no, it's that young boys are suffering and you are responsible. You're letting it happen. I said, oh my god, of course. You're right.

Narrator: The next day, Zimbardo ended the experiment. Studies like his stimulated heated debate about the ethics of using human subjects.

Philip Zimbardo: Clearly young men suffered verbally, physically. Prisoners felt shame in their role. Guards felt guilt. So in that sense, it's unethical. That is, nobody has the right, the power, the privilege to do that to other people.

Dave Eshleman: When I look back on it now, I behaved appallingly. It was just a horror to look at.

Clay Ramsay: Everyone was acting out a part and playing a role. Prisoners, guards, staff. Everyone was acting out a part. It's when you start contributing to the script.

Philip Zimbardo: We like to think there is this core of human nature that good people can't do bad things and that good people will dominate over bad situations. In fact, one way to look at the Stanford prison study is that we put good people in an evil place and we saw who won. Well, the sad message is, in this case, the evil place won over the good people.

Dave Eshleman: It did show some very interesting and maybe some unpleasant things about human behavior. It seems like every century, every decade that we go through, we're suffering the same kind of atrocities. And you need to understand why these things happen. You need to understand why people behave like this.