What was the Stanford prison experiment? What really happened

The Stanford prison experiment was a 1971 study, led by the psychologist Philip Zimbardo, in which male college students were randomly assigned to be guards or prisoners in a mock jail in a Stanford University basement. Planned for two weeks, it was stopped after six days as guards grew abusive and prisoners broke down. For decades it was taught as proof that ordinary people turn cruel in a bad situation. Later evidence shows the guards were steered, and most psychologists no longer accept the study as evidence for that lesson.

In short

• Zimbardo's study put 24 screened students into roles of guard or prisoner and was halted on day six; he concluded that the situation, not personality, made people cruel.
• Archive recordings and interviews published from 2018 show the guards were encouraged to be tough, some participants were acting, and the study was never a controlled experiment.
• It remains a landmark mostly as a lesson in research ethics, in demand characteristics and in why findings need to be replicated.

What happened in the Stanford prison experiment?

In August 1971, Zimbardo and his team advertised in local newspapers for male students to take part in "a psychological study of prison life" for $15 a day. From more than 70 who replied, they screened out anyone with signs of psychological problems or a criminal record and chose 24 young men, mostly white and middle class. A coin toss made half of them guards and half prisoners; nine of each took part at a time, with the rest as reserves.

The prisoners were "arrested" at their homes by real Palo Alto police officers, booked, and taken to a corridor of the psychology building, Jordan Hall, fitted out as a jail. They were stripped, deloused, dressed in smocks with numbers on them and called only by those numbers. The guards wore khaki uniforms and mirrored sunglasses and carried wooden batons. They were told they could not hit the prisoners but could make them feel frustrated, fearful and powerless. Zimbardo cast himself as the prison's superintendent, and an undergraduate on his team, David Jaffe, played the warden.

On the second day the prisoners rebelled, barricading their cells. The guards broke the revolt with fire extinguishers and then turned harsher: forced push-ups, sleep interrupted for headcounts, toilet privileges withdrawn, solitary confinement in a closet. One guard, nicknamed "John Wayne" by the prisoners, was especially inventive in humiliating them. Within 36 hours a prisoner, Douglas Korpi, appeared to break down, screaming and crying, and was released. Others followed.

On the fifth evening, Christina Maslach, a recent Stanford PhD whom Zimbardo was dating and later married, saw prisoners being marched to the toilet with bags over their heads and objected forcefully. Zimbardo ended the study the next morning, after six days instead of fourteen. His official account describes guards who became sadistic and prisoners who became depressed and showed extreme stress.

What was it supposed to show?

Zimbardo's conclusion was that situations overpower individuals. Put decent young men into a prison, with uniforms, anonymity and unequal power, and the roles take over: guards abuse, prisoners submit. Because they were assigned by coin toss, he argued, the difference could not be in their personalities. The study was first written up in 1973, in the US Navy's Naval Research Reviews and a criminology journal, not in a major peer-reviewed psychology journal.

The message spread far beyond psychology. It appeared in almost every introductory textbook, in documentaries and a 2015 film. When pictures of American soldiers abusing Iraqi prisoners at Abu Ghraib emerged in 2004, Zimbardo testified for the defense of one of the soldiers, and in his 2007 book The Lucifer Effect he argued that "bad barrels", not "bad apples", explain such abuse.

What did later evidence reveal?

The study drew criticism from the start, but the strongest evidence arrived in 2018. The French researcher Thibault Le Texier went through the Stanford archives and interviewed 15 participants for his book Histoire d'un mensonge, or History of a Lie, and later a 2019 article in American Psychologist. The journalist Ben Blum found archive recordings and interviewed participants for a piece on Medium, as Live Science reported. The table compares the story as it was taught with what the later evidence shows.

The textbook story	What the later evidence shows
Guards became cruel on their own, as the roles took over	Guards received instructions on how to treat prisoners; a recording captures the warden urging a reluctant guard to be "tough"
The guards were subjects being studied	Le Texier found guards were not told they were subjects; many saw themselves as research assistants helping produce a result
Prisoners suffered real breakdowns	Korpi told Blum he faked his breakdown to get out and study for a graduate school exam: "Anybody who is a clinician would know that I was faking"
The worst guard was transformed by the situation	The "John Wayne" guard, Dave Eshleman, has said he was deliberately acting a part, modeled on a sadistic prison guard from a film
A neutral scientist observed the results	Zimbardo ran the prison himself, and data collection was patchy and selective
Anyone would have done the same	A 2007 study by Thomas Carnahan and Sam McFarland found that an ad mentioning "prison life" drew volunteers higher in aggressiveness and authoritarianism than an ad without those words

These problems add up to what psychologists call demand characteristics: participants work out what the researcher hopes to see and give it to them. The guards knew the study was about prison brutality, were told prisoners should feel powerless, and were watched by the man running it. Their behavior may say more about wanting to please Zimbardo than about what prisons do to people.

Attempts to repeat the study have not produced the same result. In the BBC Prison Study of 2002, the psychologists Alex Haslam and Stephen Reicher set up a similar mock prison without telling guards how to act. The guards were reluctant to impose their authority, and the prisoners organized to challenge them.

Still argued

Zimbardo, who died in 2024, defended the study to the end. In his 2018 response he called it an "exploratory investigation" and a cautionary tale, not a claim that a simulation equals a real prison. He argued that the guards' instructions were mild compared with the pressures of real prisons, that the cruelest guards chose their own methods, and that critics had offered no evidence undermining its core point about situational forces. He conceded it lacked an independent scientific observer. Most psychologists now treat the study as a vivid demonstration, not as evidence, though many still accept the broader idea that roles and situations shape behavior, through processes such as shared identity and leadership that the study itself didn't test.

Why does it still matter?

Mostly as a warning about how research should be done. By modern standards the study would not be approved. Participants were humiliated and distressed. The lead researcher was also the jailer, with no one independent to stop it. And the participants' right to leave was unclear: prisoners who asked to quit were sometimes told they couldn't.

In the years after, US research ethics were tightened. The Belmont Report of 1979 set out the principles of respect for persons, beneficence and justice, and studies with human participants must now be approved by an institutional review board, a committee that weighs risks, consent and the right to withdraw at any time.

The second lesson is about replication. A single dramatic study, published outside the main journals and never repeated under the same conditions, became one of the most famous findings in psychology. It is now one of the standard examples in discussions of the field's replication crisis, and a useful case for critical thinking: how a story that confirms what people already suspect about human nature can travel for decades on very thin evidence. The reluctance of everyone involved, including textbook writers, to revise a belief they had invested in is a small case of cognitive dissonance.

The question it set out to answer is still a real one: how much of behavior comes from the person and how much from the setting. The biology of that interplay runs through how the brain works.

Check yourself

• Without looking back: what were demand characteristics in this study, and why do they undermine its conclusion?
• Name two pieces of evidence published from 2018 on that changed how the study is seen.
• Which features of the study would an ethics review board refuse today, and why?

Sources

• Stanford Prison Experiment — Philip G. Zimbardo
• Response to recent criticisms of the Stanford Prison Experiment — Philip G. Zimbardo, 2018
• Debunking the Stanford Prison Experiment — Thibault Le Texier, American Psychologist, 2019
• One of Psychology's Most Famous Experiments Was Deeply Flawed — Live Science, 2018