What really happened in the Stanford prison

The prison was a corridor in the basement of Jordan Hall, Stanford's psychology building, boarded up at both ends. On Sunday 15 August real Palo Alto police arrested the prisoners at their homes and brought them in. They wore smocks and numbers in place of names; the guards had khaki uniforms and mirrored sunglasses.

On the second morning the prisoners staged a rebellion, pushing their beds against the cell doors. The guards broke it and turned to control: endless roll calls, lost privileges, and a special cell where a few cooperative prisoners got better food, to set the rest against each other. The harshness grew through the week, much of it on one evening shift led by the harshest guard. Five prisoners were released early, several in visible distress. Planned for two weeks, the study was stopped after six days, on Friday 20 August.

By Zimbardo's account, the study ended because one visitor said what no one else had. Christina Maslach had just finished her doctorate at Stanford and was about to start teaching at Berkeley. On the evening of Thursday 19 August she came to help interview the participants, and she saw the guards march the prisoners to the toilet with bags over their heads and their legs chained. She was upset, and told Zimbardo that what was being done to these boys was terrible.

Zimbardo later wrote that more than 50 outsiders had seen the prison, among them parents, colleagues and a priest, and that she was the only one who ever questioned its morality. He also admitted that he had stopped acting as a researcher and become the prison's superintendent, more concerned with running the jail than with the students in it. The next morning, he ended the study.

What did it mean? Zimbardo's answer became one of psychology's best-known ideas. The students had been screened as normal and sorted by chance, so, he argued, the cruelty could not come from bad character. It came from the situation: the uniforms, the numbers in place of names, the power of one group over another, and roles that told each person how to behave.

This view, often called situationism, runs against a habit most of us share. When someone behaves badly, we tend to blame the kind of person they are and overlook the pressures around them, a mistake psychologists call the fundamental attribution error. Zimbardo's prison seemed to expose that mistake on a grand scale: put good people in a bad place, and the place wins.

Zimbardo carried the lesson far beyond the campus. On 25 October 1971, two months after the study, he spoke at hearings on prison reform held by a committee of the US House of Representatives. Prisons, he argued, shape the people inside them, guards as much as prisoners, so to stop abuse you must reform the prison system itself: better training for guards and more protection for prisoners' rights.

In 2004, photographs showed American soldiers abusing detainees at Abu Ghraib prison in Iraq. Zimbardo served as an expert witness for one of the guards, and in 2007 he set out his case in a book, The Lucifer Effect. Those guards, he wrote, were not a few bad apples but good apples in a bad barrel. On this view, preventing abuse means changing the system that builds the barrel, not only punishing the people found inside it.

For decades the story was told much as Zimbardo told it. Then a French researcher, Thibault Le Texier, went through the study's recordings and papers in the Stanford archives and interviewed 15 of the participants. His book, Histoire d'un mensonge, "history of a lie", came out in 2018, and a paper in the journal American Psychologist followed in 2019.

The guards, he showed, had been coached by the researchers. At an orientation the day before the arrests, Zimbardo told them "we can create fear in them" and described taking away the prisoners' privacy and freedom. The warden, a student assistant, later urged a gentle guard to be tougher, and several harsh routines came from a consultant who had been held at San Quentin prison. A press release on the study's second day already described what it aimed to show.

People in an experiment rarely behave as if no one is watching. They look for clues to what the researcher hopes to find and, often without meaning to, supply it. The psychologist Martin Orne called these clues demand characteristics, and they threaten any study whose purpose is easy to guess.

The prison study's purpose was very easy to guess. In 1975 two researchers, Ali Banuazizi and Siamak Movahedi, gave 150 students only a description of the setup, with the advert and the arrest procedure. 81 per cent worked out what the experimenters expected, and 90 per cent predicted oppressive, hostile guards. If outsiders reading a page could guess the script, volunteers living inside it could guess it too. The guards may have been acting out the study everyone expected rather than discovering their own nature.

Two of the study's best-known figures later said they had been performing. The harshest guard, Dave Eshelman, nicknamed "John Wayne" by the prisoners, said he modelled himself on the cruel prison captain in the film Cool Hand Luke. He called it a kind of improvisation and said he believed he was doing what the researchers wanted.

Douglas Korpi, prisoner 8612, gave the study its most-watched scene when he screamed and raged in his cell and was released. In 2018 he told the journalist Ben Blum that the breakdown was faked. He had signed up expecting quiet time to study for a graduate-school exam, the researchers refused to let him quit, and acting mad was the easiest way to get out. Zimbardo rejected his account, but the archives record Korpi telling the same story in 1971.

The coin made guards and prisoners alike, but it could not make the volunteers like everyone else. In 2007 the psychologists Thomas Carnahan and Sam McFarland tested this with two nearly identical adverts. One asked for volunteers for "a psychological study of prison life", as Zimbardo's had; the other for "a psychological study", with the prison left out.

Those who answered the prison advert scored higher on aggressiveness, authoritarianism and the wish to dominate others, and lower on empathy and altruism. This is self-selection: when people choose whether to take part, the kind of person who chooses shapes the sample before any experiment begins. A coin shares out whatever traits a sample holds; it cannot remove them. So the volunteers were already skewed, and the authors concluded that behaviour comes from the person and the situation together.

Zimbardo's study could not ethically be repeated as it was run, but in December 2001 the social psychologists Stephen Reicher and Alex Haslam built a new one with the BBC, overseen by an independent ethics committee. Fifteen men, matched on personality, were split into five guards and ten prisoners for eight days of filming, shown on television in May 2002.

It did not go the Stanford way. The guards, uneasy with power, never came together as a group. The prisoners, once told that no one would be promoted out of their ranks, built a strong shared identity, a sense of being "us", and overturned the guards. Their commune later collapsed, and some then wanted a harsh new regime. The researchers' lesson: roles do not turn people cruel by themselves; groups that hold together can resist, and tyranny tempts when groups fail.

If roles do not turn people cruel by themselves, what happened at Stanford? By Zimbardo's own account, only about a third of the guards were hostile and inventive in their cruelty; others were tough but fair, and some did the prisoners small favours. The role was the same for every guard; each chose how to act in it.

Haslam and Reicher's reading, set out in 2012, is engaged followership. People harm others under authority not because they obey mindlessly, but because they come to identify with a leader, believe the cause is right, and then work at it with energy and invention. Situations still matter, since a leader and a cause can draw people in, but they work through people's choices. Zimbardo had told his guards what he wanted, and the zealous ones followed him. Others in the same uniform chose differently.

The study had been approved by Stanford's committee for research on people, and every student signed a consent form. Yet the form said prisoners would be released only for health reasons or others the researchers judged fit, and the plan submitted for approval said prisoners would be discouraged from quitting. When two prisoners asked to leave on the second day, they were refused.

Today no ethics board would approve that plan. A basic rule of research ethics is the right to withdraw: a participant may stop at any moment, without giving a reason and without penalty. A US government guide published in December 1971 already listed it, and US law required review boards for federally funded research from 1974, after a far graver scandal, the Tuskegee syphilis study. The prison study is now one of the standard cases in teaching research ethics.

Whatever the study proved, its afterlife raised a moral question: if a role or a leader pushes you, who answers for what you do? Islam gives a clear answer. Al-Bukhārī and Muslim report that the Prophet sent out a troop under a commander who lit a fire and ordered his men to walk into it. Some refused. When the Prophet heard, he said that had they entered it, they would have stayed in it until the Day of Resurrection, and: "There is no obedience in disobedience to God; obedience is only in what is good."

The Qur'an adds that "no bearer of burdens will bear the burden of another" (al-Anʿām 6:164): each person carries their own deeds. A uniform, an order or a researcher's hopes may press hard, but in Islamic ethics they do not move accountability away from the one who acts.