The Stanford Prison Experiment: What Actually Happened

In August 1971, Philip Zimbardo converted the basement of Stanford University's Jordan Hall into a mock prison, recruited 24 male undergraduates, and assigned them randomly to be "guards" or "prisoners." The study was designed to run two weeks. He shut it down after six days. The images and footage became some of the most reproduced in all of psychology — and in 2018, newly unearthed recordings revealed that much of what the world believed about those six days was wrong.

What the Experiment Was Supposed to Test

Philip Zimbardo, then 38 years old and a professor at Stanford, wanted to study how institutional environments shape individual behavior. The specific question: do prisons become brutal because brutal people end up inside them, or does the prison environment itself produce brutality — regardless of who's in it?

Situationism — the idea that external context, not internal character, drives human behavior — was the operating theory. If ordinary, psychologically healthy young men would become cruel guards and submissive prisoners simply because of role assignment, that would be powerful evidence for the situationist view.

The experiment had a comparison: the dispositional hypothesis, which held that "bad apples" caused bad outcomes. Zimbardo explicitly positioned the Stanford Prison Experiment as a refutation of that view.

Source: Zimbardo, P. G. (1971). The Stanford Prison Experiment: A Simulation Study of the Psychology of Imprisonment. Stanford University, Office of Naval Research. [zimbardo.com]

The experiment was funded by the U.S. Office of Naval Research, which was interested in understanding conflict between naval officers and their subordinates — a fact that shaped the study's design and its initial dissemination.

The Setup

Recruiting the Participants

Zimbardo and his team placed a newspaper advertisement in August 1971 seeking male volunteers for "a psychological study of prison life" paying $15 per day (roughly $110 in 2026 dollars). From 75 respondents, 24 were selected after psychological screening to exclude those with any history of mental illness, medical conditions, or prior criminal records. All were university-age men, predominantly white and middle class.

The 24 participants were randomly assigned by coin flip: 12 to the role of prisoner, 12 to the role of guard. This randomization was the backbone of the study's scientific claim — neither group had any dispositional reason to behave differently from the other.

Confederate — in psychology experiments, a person who appears to be a regular participant but is actually working with the researcher, following a script. There were no confederates among the 24 participants in the SPE. All were genuine volunteers.

Turning a Corridor into a Prison

Three small rooms in Jordan Hall's basement were converted into cells. A closet became solitary confinement, designated "the Hole." The corridor outside served as the prison yard. Real Palo Alto police officers cooperated with Zimbardo by arresting the "prisoner" participants at their homes, transporting them in squad cars, booking them at an actual police station, and then delivering them to the mock prison — a theatrical opener designed to induce psychological entry into the role.

Prisoners wore smocks (to undermine masculinity, per the study design), nylon stocking caps (to simulate shaved heads), and ankle chains. Guards wore khaki uniforms, mirrored sunglasses (designed to prevent eye contact and create psychological distance), and carried wooden batons.

Zimbardo himself took on the role of prison superintendent — a decision that would later be identified as a critical methodological flaw.

Source: Haney, C., Banks, W. C., & Zimbardo, P. G. (1973). Interpersonal dynamics in a simulated prison. International Journal of Criminology and Penology, 1, 69–97.

The Protocol

The stated protocol was observational. Guards were told to maintain order but not to use physical violence. They were to enforce a set of rules — lights out at 10 p.m., prisoners must remain silent after dark, eating only at mealtimes — and to report any infractions.

What guards were told, in Zimbardo's own words (from an orientation meeting the night before the study started): "We cannot physically abuse or torture prisoners. We can create boredom. We can create a notion of frustration. We can create fear in them, to some degree. We can create a notion of arbitrariness that their life is totally controlled by us, by the system, you, me."

This instruction is now at the center of the experiment's contested legacy. It was not a neutral observation of what untrained people do with power. It was a specification of what the guards should do.

Source: Le Texier, T. (2019). Debunking the Stanford Prison Experiment. American Psychologist, 74(7), 823–839. [APA PsycNet]

The Six Days

The first night passed without incident. On the second day, prisoners staged a rebellion, barricading their cells and taunting the guards. Guards from the overnight shift called in their daytime colleagues and began a counter-offensive: they stripped the prisoners, removed their beds, and placed the ringleaders in solitary confinement.

The behavior of one guard in particular — a participant who gave himself the nickname "John Wayne" — became central to the experiment's narrative. He harassed prisoners through the night, forcing them to do push-ups, conducting counts at 2 a.m., and subjecting prisoners to degrading exercises. His behavior was later cited by Zimbardo as evidence of the situational hypothesis.

Five "prisoner" participants were released early due to acute emotional distress. On the sixth day, Christina Maslach, a social psychologist who had begun dating Zimbardo (and later married him), visited the experiment and told him it was morally wrong. He ended the study that evening.

Source: Zimbardo, P. G. (2007). The Lucifer Effect: Understanding How Good People Turn Evil. Random House.

The Results — As Originally Reported

Zimbardo's 1971 findings, as reported in academic publications and later in his 2007 book The Lucifer Effect, painted a picture of rapid, spontaneous role absorption:

• Guards became increasingly authoritarian and creative in their methods of degradation without any external instruction.
• Prisoners became passive, resigned, and institutionalized.
• The situation, not individual character, produced pathological behavior in normal men within 36 hours.

The study entered the textbook canon. By 2011, it appeared in roughly 60% of introductory psychology textbooks in the United States, typically cited as definitive evidence of the power of situations over individuals.

Source: Griggs, R. A. (2014). Coverage of the Stanford prison experiment in introductory psychology textbooks. Teaching of Psychology, 41(3), 195–203.

The Controversy

Le Texier's 2018 Investigation

The story Zimbardo told began to unravel in 2018, when French researcher Thibault Le Texier published Histoire d'un mensonge ("History of a Lie"), based on previously unseen archives at Stanford's special collections library. His findings, expanded in a 2019 American Psychologist article, were damaging.

Le Texier found audio recordings of David Jaffe, Zimbardo's research assistant, explicitly coaching guards before and during the study. In one recording, Jaffe tells a guard who is behaving too passively: "We really want to get you active and involved because the guards have to know that every guard is going to be what we call a 'tough guard.'"

This instruction directly contradicts the experiment's central claim — that guard behavior emerged spontaneously from the situation. Guards were told to be tough. At least some of them were following directions.

Le Texier also found that the "John Wayne" guard, whose name is Douglas Korpi, gave an interview to a reporter in 2017 in which he stated that he had been consciously performing a character based on films he had seen, and that Zimbardo's team had encouraged him. Korpi later clarified his statements, but the recordings Le Texier found supported the claim of active coaching.

Source: Le Texier, T. (2019). Debunking the Stanford Prison Experiment. American Psychologist, 74(7), 823–839. [APA PsycNet]

The Emotional Breakdown Question

One of the experiment's most striking episodes was prisoner #8612 — a man named Douglas Korpi — appearing to have an emotional breakdown and demanding release. This breakdown was cited as evidence of how rapidly the prison environment had affected psychologically healthy participants.

Le Texier's archival research, however, uncovered a recording in which Korpi describes his breakdown as a deliberate performance — he wanted to leave to study for an exam, realized the only way out was to appear mentally unwell, and acted accordingly. Zimbardo later disputed this characterization, but could not produce evidence to refute it.

Source: ScienceAlert (2018). Famous 1971 Prison Experiment Wasn't What Most of Us Thought. [sciencealert.com]

Zimbardo's Dual Role

Perhaps the most fundamental methodological problem Le Texier identified was Zimbardo's role conflict. As both the principal investigator and the "prison superintendent," Zimbardo was simultaneously the designer of the study and a character within it. He made decisions as superintendent — ordering punishments, structuring guard schedules — that shaped the behavior the study was supposed to be observing neutrally. This violates basic principles of experimental design.

A neutral experimenter observes behavior. Zimbardo was creating it.

Source: The Conversation (2023). The infamous Stanford prison experiment was flawed — so why is it still so influential today? [theconversation.com]

The BBC Prison Study: A Genuine Replication Attempt

In 2002, psychologists Alex Haslam (University of Exeter) and Steve Reicher (University of St Andrews) conducted a controlled replication for the BBC, published in the British Journal of Social Psychology in 2006. Their version made one critical change: guards received no coaching, and no one was told how to behave.

The results inverted Zimbardo's findings. Guards did not become authoritarian. Prisoners organized, built social identity, and at one point effectively took over the prison. The system collapsed not into tyranny but into negotiation.

Haslam and Reicher concluded that tyranny is not an automatic product of the situation. It requires active institutional support, ideology, and leadership that encourages cruel behavior. Without that scaffolding, ordinary people do not spontaneously become oppressors.

Zimbardo initially criticized the BBC study harshly. In 2018, in a notable reversal, he and Haslam issued a joint statement acknowledging that both experiments offered valid, if different, insights into group dynamics under institutional pressure.

Source: Haslam, S. A., & Reicher, S. D. (2006). Rethinking the psychology of tyranny: The BBC Prison Study. British Journal of Social Psychology, 45(1), 1–40.

The Film and Cultural Distortion

The SPE has been adapted into at least two feature films: the German Das Experiment (2001) and the American The Stanford Prison Experiment (2015), directed by Kyle Patrick Alvarez. Both dramatize guard cruelty in ways that go significantly beyond what the archival evidence supports. The 2015 film, in particular, compressed and amplified events, presenting Zimbardo as a passive observer when the record shows he was an active participant.

A Netflix documentary, Zimbardo's Prison (2021), gave Zimbardo editorial access that critics found compromising. It does not present Le Texier's critique with equal weight.

The cultural persistence of the SPE's original narrative — ordinary people become monsters when given power — has made it politically useful for arguments ranging from prison reform advocacy to analysis of Abu Ghraib, which Zimbardo himself invoked in his testimony to the Congressional investigation of the scandal in 2004.

Source: Prison Legal News (2018). Landmark Stanford Prison Experiment Criticized as a Sham. [prisonlegalnews.org]

The Timeline

Date	Event
Aug 14, 1971	Experiment begins; prisoners "arrested" at home by Palo Alto police
Aug 15, 1971	Day 2: prisoner rebellion; guards escalate
Aug 16–19, 1971	Progressive deterioration; five prisoners released early with distress
Aug 20, 1971	Christina Maslach visits; Zimbardo ends the experiment
1973	First academic publication (Haney, Banks, Zimbardo)
1999	The Lucifer Effect research begins
2001	Das Experiment (German film) released
2002	BBC Prison Study conducted by Haslam and Reicher
2006	BBC Prison Study published, challenging SPE conclusions
2007	The Lucifer Effect published by Zimbardo
2015	American film The Stanford Prison Experiment released
2018	Le Texier publishes Histoire d'un mensonge in French
2019	Le Texier's critique published in American Psychologist
2019	Zimbardo and Haslam issue joint statement

Where the Science Stands Today

The Stanford Prison Experiment is no longer treated as valid empirical evidence for situationism by most research psychologists. That does not mean the situationist hypothesis is wrong — there is substantial evidence from other, better-designed experiments that context shapes behavior profoundly. But the SPE itself cannot bear that evidential weight.

What Le Texier's work established is not that situationism is false. It is that this particular experiment cannot confirm it. The guards were coached. The lead researcher was embedded in the situation he was supposed to be studying. At least one breakdown was performed rather than genuine. The study's controls were inadequate, and its data were selectively reported.

Psychologists Ben Blum, who wrote an influential 2018 Medium piece summarizing Le Texier's findings, put it plainly: "The SPE is a story science tells about itself, but it's not actually what the data showed."

The situational vs. dispositional debate continues — it is one of social psychology's most productive tensions. It just needs better evidence than the Stanford basement provided.

Source: K12 Academics (n.d.). Criticism and Response to the Stanford Prison Experiment. [k12academics.com]

Source: Griggs, R. A. (2014). Coverage of the Stanford prison experiment in introductory psychology textbooks. Teaching of Psychology, 41(3), 195–203.

Ethical Legacy

The SPE's ethical violations were real and extensive, regardless of what the data showed:

• Participants were not told that their "right to withdraw" would be actively resisted. When prisoner #8612 tried to leave on day two, Zimbardo initially refused.
• Prisoners were deceived about the nature of the arrest.
• Psychological distress was induced deliberately and monitored insufficiently.
• Debriefing was inadequate; some participants reported lasting distress.

The experiment is now a standard case study in research ethics courses, cited alongside the Milgram obedience experiment as an example of what institutional review boards (IRBs) were created to prevent. The American Psychological Association adopted its current ethical principles for research with human participants in 1973, partly in response to studies like the SPE.

Source: APA (2017). Ethical Principles of Psychologists and Code of Conduct. apa.org

Why the Experiment Still Gets Taught Despite Being Discredited

Open almost any introductory psychology textbook and the Stanford Prison Experiment is there. Richard Griggs's 2014 analysis of 13 introductory psychology textbooks found the SPE appeared in all 13 — every single one — making it among the most consistently included studies in the undergraduate canon, more present than many findings that have actually held up to replication.

Source: Griggs, R. A. (2014). Coverage of the Stanford prison experiment in introductory psychology textbooks. Teaching of Psychology, 41(3), 195–203.

Textbook inclusion has a logic of its own that is only loosely connected to scientific validity. Authors inherit a structure from earlier editions; editors are reluctant to remove material that instructors have built lectures around for decades; and the SPE is genuinely pedagogically useful — just not for the reason textbooks claim. It works as a story. The arc of normal people corrupted by power, resolved by a single moment of moral courage, maps onto a narrative template that students remember. Truth value and memorability are not the same thing.

Zimbardo himself has never fully stepped back. He delivered a TED talk in 2008 — viewed over five million times — that presented the SPE's original conclusions without substantive caveats, using it as the foundation for his "Lucifer Effect" framework and drawing a direct line from the Stanford basement to Abu Ghraib. His continued presence on the lecture circuit and in Stanford course materials has extended the study's institutional life well past its scientific one.

The BBC Prison Study (Reicher and Haslam, 2002) offers the sharpest contrast and the clearest reason the original SPE's conclusions should not stand as textbook canon. When Haslam and Reicher ran a controlled prison simulation without coaching guards, the results ran in the opposite direction: prisoners developed group solidarity and resisted, while guards felt uncertain and powerless. Tyranny, their 2006 British Journal of Social Psychology paper concluded, is not an automatic product of the situation — it requires active ideological scaffolding and institutional permission.

Source: Haslam, S. A., & Reicher, S. D. (2006). Rethinking the psychology of tyranny: The BBC Prison Study. British Journal of Social Psychology, 45(1), 1–40.

The BBC study, run more carefully and published in a peer-reviewed journal with full data transparency, rarely gets its own textbook entry. It is usually mentioned, if at all, as a footnote beneath the SPE it overturned. This is institutional inertia operating at scale: the first study defines the category, and later, better work fills in the margins.

The Milgram experiments face the same persistence problem. Gina Perry's archival investigation raised nearly identical concerns about coaching, selective reporting, and participant belief — yet Milgram's 65% figure retains the same textbook ubiquity as Zimbardo's prison guards. What these studies share is not methodological strength. It is narrative power, a compelling number, and a decades-long head start in the curriculum.

Textbook authors who do update their coverage typically do so by adding a paragraph of caveats after the original account rather than restructuring the argument. This produces an unusual pedagogical object: a study presented first as landmark evidence, then walked back by three sentences, leaving students to wonder which version to believe. The more honest approach — treating the SPE as a case study in experimental methodology and researcher role contamination rather than as evidence for situationism — is still the minority presentation, though it is gaining ground.

FAQs

Q: Was the Stanford Prison Experiment a real scientific experiment? A: It was conducted by a credentialed researcher at a major university, but it lacked many features of rigorous experimental design: it had no control group, no blinded observation, the investigator played an active role in the scenario, and participants were coached. Most methodologists now classify it as a poorly controlled simulation study rather than a controlled experiment.

Q: Did the guards actually torture prisoners? A: Guards used psychological harassment — sleep deprivation, arbitrary rules, humiliation, push-ups as punishment — but not physical violence. The recordings Le Texier analyzed show that some of the most extreme behavior was performed for the cameras or in response to coaching, but genuine psychological distress among prisoners was real.

Q: Why did Zimbardo end the experiment after six days? A: His future wife, Christina Maslach, visited on day six and told him the situation was morally unacceptable. She was reportedly the only one of 50 people who entered the prison during that period to object. Zimbardo later described this as a personal awakening, though critics note he continued to minimize the study's flaws for decades afterward.

Q: Has the experiment been successfully replicated? A: The BBC Prison Study (2002) is the most systematic attempt, and its results directly contradicted Zimbardo's findings. When guards received no coaching, they did not spontaneously become authoritarian. Full replication under modern ethical standards is not possible.

Q: Is the SPE still taught in psychology courses? A: Yes, though increasingly as a case study in methodology and ethics rather than as evidence for situationism. Some textbooks have updated their coverage to include Le Texier's critique; many have not.

Q: What happened to the participants afterward? A: Most went on to normal lives. Several have spoken publicly, including Douglas Korpi (prisoner #8612), who disputed aspects of Zimbardo's account. Guard "John Wayne" later became a clinical psychologist specializing in prison reform — an irony noted by several commentators.

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