Stanford prison experiment

• 1. Introduction

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  The Stanford prison experiment began with a short newspaper advertisement. It asked for male college students for a "psychological study of prison life" and offered $15 per day. The study took place in August 1971 at Stanford University, run by psychology professor Philip Zimbardo. It was planned as a two-week simulation of a prison. It stopped on the sixth day, when Zimbardo decided the guards' abuse of the prisoners had gone too far. Since then the experiment has been taught, filmed, and attacked as unscientific. Some critics have even called it fraudulent. How did a basement simulation turn harsh so fast? What were the guards actually told? And does the study prove what its creator claimed?

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• 2. The Cast and the Cells

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  Seventy-five men applied after the advertisement ran in the Palo Alto Times and The Stanford Daily. Screening assessments and interviews cut the group to 24. The chosen men were mostly white and middle-class, and they seemed healthy and psychologically stable. People with criminal backgrounds, psychological impairments, or medical problems were deliberately left out. A random draw then gave nine men the guard role and nine the prisoner role, with three alternates on each side. All of them agreed to serve for somewhere between seven and fourteen days.

  The money came from the US Office of Naval Research, which wanted to understand antisocial behavior. The Navy and the Marine Corps were interested in conflict between military guards and prisoners. Zimbardo described his own aim as studying the power of roles, rules, symbols, and group identity. He had spent years researching deindividuation, vandalism, and dehumanization before this.

  On the day before the study began, offices in the psychology department were turned into small mock cells. The prison filled a 35 ft section of the basement of Jordan Hall, Stanford's psychology building. Two fabricated walls sealed it off, one at the entrance and one at the cell wall to block observation. Each unlit cell measured 7 x 10 ft, held three prisoners, and had one cot per man. A closet served as solitary confinement. Prisoners were to stay in the cells and a small corridor yard day and night. Guards worked in teams of three on eight-hour shifts, had rest areas of their own, and were not required to stay on-site after a shift.

  Zimbardo took the title of Superintendent. An undergraduate research assistant named David Jaffe played the Warden.

  At an orientation for the guards, the researchers handed out wooden batons to signal status. The guards also received khaki shirts and pants from a local military surplus store, plus mirrored sunglasses that blocked eye contact and created anonymity. They were told not to harm the prisoners physically or withhold food or drink, but to maintain law and order. They were also told to address prisoners by number instead of by name. Zimbardo said this was meant to wear down each prisoner's sense of individuality.

  Zimbardo later argued that asking a guard to be "firm" was mild compared with the pressure on real wardens, who can face demotion or dismissal for holding back.

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• 3. Six Days in the Basement

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  On Sunday, the 15th of August 1971, the prisoners were arrested at their homes or at assigned sites by the real Palo Alto police. They had not been told this would happen, because the researchers wanted a surprise. That decision contradicted the contract every participant had signed. The charges were armed robbery and burglary, Penal Codes 211 and 459. At police headquarters the men were read their Miranda rights, fingerprinted, and photographed for mug shots. A local San Francisco television reporter filmed the process, using Zimbardo's car. The prisoners then rode to the mock prison with sirens wailing, were strip searched, and received identification numbers. Their uniforms were ill-fitting smocks with no underwear, stocking caps, and a chain around one ankle. The Warden greeted them with a stern account of their offense and their new status.

  At 2:30 am on Monday, guards woke the prisoners with whistles and clanging batons, and the prisoners rebelled. They refused to leave their cells to eat in the yard, ripped off their number tags, removed their caps, and insulted the guards. The guards answered with fire extinguishers. Three backup guards were called in. The guards stripped the prisoners, took away the mattresses, and sent the main instigators to the special detention unit. One guard remarked to another that "these are dangerous prisoners".

  On Tuesday the guards began splitting the prisoners apart. Three men with minor roles in the rebellion were moved to a "good" cell, where they received clothing, beds, and food denied to everyone else. After an estimated 12 hours they went back to cells without beds. Guards made prisoners count off and do pushups at random, limited bathroom access, and forced men to use a bucket in the cell.

  By Wednesday the strategy had worked. Prisoners distanced themselves from one another. Some rioters believed the others were snitches, and others saw the rebels as a threat because they wanted their cots and clothes back.

  Thursday was set aside for visits from family and friends. Zimbardo and the guards made visitors wait a long time. Only two people could see a prisoner, for just ten minutes, with a guard watching. Parents worried about whether their sons were getting enough to eat, and some left planning to contact lawyers. That day a colleague, Gordon H. Bower, asked Zimbardo what the independent variable of the research was. That night psychologist Christina Maslach visited and was distressed to see guards forcing prisoners to wear bags over their heads. She challenged Zimbardo on his lack of caring oversight and the morality of the study. She also told him he had become someone she did not recognize and did not like. Researcher Curtis Banks and others shared her view.

  On Friday, the 20th of August, Zimbardo ended the study. He gathered guards, prisoners, and researchers and told them it was over, and he arranged to pay them for the six days. He then led several hours of debriefing, first with the prisoners, then the guards, and finally everyone together. Participants were asked to mail in personal retrospectives and invited back a week later. The cells soon went back to being graduate student offices. Zimbardo, Craig Haney, and Curtis Banks started compiling data for several articles.

  Zimbardo was later quoted in a 1997 interview as saying that more than 50 people had observed the study in progress and none had objected.

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• 4. Two Prisoners Who Left

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  Douglas Korpi, prisoner 8612, was the first to go, after 36 hours. He yelled, "Jesus Christ, I'm burning up inside" and "I can't stand another night! I just can't take it anymore!" Research assistant Craig Haney, seeing his distress, let him leave.

  Korpi's account changed over the decades. In the 1992 documentary Quiet Rage, he said the experience had deeply affected him and led him to become a prison psychologist. In a 2017 interview he said the breakdown had been fake. He had hoped to study for his Graduate Record Exam while "imprisoned", and the staff would not let him. He added that he regretted not filing a false imprisonment charge at the time. Zimbardo answered in 2018 that he had to treat the breakdown as real and release the man. He also said he believed the 2017 interview was a lie.

  On the fourth day, prisoner 819 began crying in his cell. A priest was brought in, but the young man refused to talk to him and asked for a medical doctor. Zimbardo reassured him of his real identity and took him out of the cell. While the two spoke, the guards had the remaining prisoners chant loudly and repeatedly that "819 did a bad thing". 819 broke down again as he heard himself called bad. Zimbardo released him and gave him his pay.

  The pay, though, was only part of what the prisoners were owed under the rules they had been given. Researchers had told every participant that he could leave at any time.

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• 5. What the Guards Were Told

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  Warden David Jaffe appears on digitized recordings posted on the official experiment website, and one drew wide discussion in 2017. In it, Jaffe pushes a guard to take part more and to be "tough" for the benefit of the experiment.

  French researcher Thibault Le Texier turned this kind of evidence into a full case. In his 2018 book Histoire d'un mensonge, The History of a Lie, and an APA article in 2019, he used testimony from the guards. He argued that the sadism and submission came from Zimbardo's instructions and from the guards' wish to please the researchers. He also concluded that the guards were asked directly to behave in certain ways. In his view, those requests served conclusions Zimbardo had largely written in advance. Zimbardo and his colleagues called the critique mostly ad hominem and said it ignored data that contradicts it. Yet participants interviewed for a later documentary have largely confirmed many of Le Texier's claims.

  In 2005, Carlo Prescott wrote in The Stanford Daily that the guards' tactics matched ones he had endured in San Quentin. He said he described each in detail to the researchers before the experiment. Men with no experience as guards, he argued, could not have behaved that way unless told. Zimbardo doubted Prescott wrote the piece. He suspected screenwriter and producer Michael Lazarou, who had failed to win the movie rights, and he cited phone records and emails obtained by Brett Emory, the producer of the experiment's film.

  Dutch historian Rutger Bregman reached a similar view in 2020. In Humankind: A Hopeful History, he wrote that the guards were urged to act aggressively and that researchers in similar experiments created hostility on purpose and then interpreted the results to suit themselves.

  Guard David Eshelman said his theater background suited the role. He deliberately thought up new ways to demean the prisoners, and on one shift he told them to simulate sodomy. He identified with the warden in the film Cool Hand Luke. Zimbardo replied that other guards acted similarly, including guards on a different shift, and that their behavior resembled real prison atrocities and the conduct of American soldiers at Abu Ghraib.

  Most of the guards have said since that they were intentionally acting.

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• 6. Doubts About the Evidence

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  Zimbardo's own reading was that the simulated prison, not individual personality, caused the behavior. That fits with the Milgram experiment, in which participants obeyed orders to give seemingly lethal electric shocks to a shill. Others suggest the guards turned aggressive because researchers encouraged them.

  In 1975, Ali Banuazizi and Siamak Movahedi argued that demand characteristics explained the behavior, meaning participants played the roles they had been cast in. Knowing they were watched could also have mattered, an idea known as the Hawthorne effect. Guards may have grown more aggressive because the supervisors watching did not rein them in. In 2012, psychologist Peter Gray made a related point. Volunteers tend to do what they think researchers want, and here that meant "to act out their stereotyped views of what prisoners and guards do."

  Erich Fromm had raised a different objection in 1973. Only a third of the guards showed sadistic behavior, so he saw the study as showing that personality does shape behavior in prison. He also argued the screening could not measure how much sadism the men carried.

  In 2007, Thomas Carnahan and Sam McFarland charged the study with selection bias. The poster itself advertised a study of prison life. They argued that applicants were therefore likely to score high on traits such as aggression, authoritarianism, Machiavellianism, social dominance, and narcissism, and low on empathy and altruism. A 2007 study of prison-life advertisements supported this. Respondents to the prison-life ad scored significantly higher on those measures than respondents to a parallel ad without those words.

  Publication drew fire too. The findings appeared first in Naval Research Reviews, the International Journal of Criminology and Penology, and the New York Times Magazine. Peer-reviewed publication in American Psychologist came afterward. David Amodio, a psychology instructor at New York University and the University of Amsterdam, called that path a violation of scientific tradition. Zimbardo replied that his Navy grant required publishing in its journal. He said the criminology journal invited him, and the American Psychologist article still had to survive strict review.

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• 7. The BBC Rerun

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  In 2002, psychologists Alex Haslam and Steve Reicher ran the BBC Prison Study to test Zimbardo's themes of tyranny and resistance. They published results in 2006. It was a partial replication, made with the BBC, which broadcast a documentary series called The Experiment.

  There was again a makeshift prison and an all-male cast. But the invitation differed: Haslam and Reicher advertised a university-backed social science experiment to be shown on television. Guards received no instructions beyond working out how to run a prison. Prisoners filled out a questionnaire daily. Everyone wore a microphone on the shirt, and cameras followed their actions.

  The results diverged from Zimbardo's and fed publications on tyranny, stress, and leadership. They appeared in journals including the British Journal of Social Psychology and Journal of Applied Psychology. The study is now taught as a core study on the UK A-level Psychology OCR syllabus. It cast doubt on the idea that people slip mindlessly into roles. It also pointed to leadership, of the kind Zimbardo showed when briefing his guards, as the source of tyranny.

  Zimbardo first called the BBC study a reality show. Both sides knew the cameras were on, and he thought they probably over-acted. He noted that prisoners had a confessional and competed in contests. Still, he believed it replicated his finding that the situation affected participants. In 2018, Zimbardo, Reicher, and Haslam issued a joint statement calling both experiments valid. They agreed that the behavior in all participants could have had more than one cause.

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• 8. After the Cells Closed

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  Stanford's basement left a mark on the rules for human research. Since 1971, ethical guidelines have tightened. In the United States an institutional review board, and in the UK an ethics committee, must approve a study before it starts. Approval depends on guidelines from the American Psychological Association or the British Psychological Society. Reviewers weigh scientific benefit against the risk of physical and psychological harm, and consent procedures must be detailed. Prompt debriefing is now seen as essential, and the APA says it should happen as soon as possible. Zimbardo's follow-up debriefing came years later, after many details had faded. He concluded from his follow-up research that participants suffered no lasting harm.

  The central ethical charge was that the study continued even after participants wanted out, despite being told they could leave at any time. The oversight of the day was limited, and parts of the study contradicted the signed contract. Comparisons to the Milgram obedience study at Yale in 1961 followed. A 1967 classroom exercise by Palo Alto high school teacher Ron Jones, the Third Wave, imitated Nazi-style control. It became the 1981 film The Wave and the 2008 German film Die Welle.

  The study has also been credited with changing how US prisons are run. For example, juveniles accused of federal crimes are no longer held before trial alongside adults. Zimbardo gave a statement on the findings to the US House Committee on the Judiciary.

  In March 2004, news of prisoner torture at Abu Ghraib in Iraq broke, and Zimbardo saw echoes of his own work. He was dismayed when officials blamed "a few bad apples" instead of the military incarceration system. His view was that "It's the situation that brings that out." He joined the defense team for guard Staff Sergeant Ivan "Chip" Frederick and testified as an expert witness at the court martial. It ended with an eight-year sentence for Frederick in 2004. Zimbardo then wrote The Lucifer Effect: Understanding How Good People Turn Evil in 2007.

  The story kept reaching audiences. Italian director Carlo Tuzii made La gabbia for Rai 1 in 1977, with Miguel Bose as a prisoner. The band Pooh scored it and had an Italian hit with the theme. Das Experiment came out in Germany in 2001, directed by Oliver Hirschbiegel, and was remade in English in 2010. Kyle Patrick Alvarez directed The Stanford Prison Experiment in 2015, with Billy Crudup. The YouTube series Mind Field covered it, and Richard Powers gave a prisoner role to a character in his novel The Overstory.

  In 2024, Disney released a three-episode National Geographic series, The Stanford Prison Experiment: Unlocking the Truth, built on first-hand accounts from the original participants.

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