How the Stanford Prison Experiment Worked

By: Ed Grabianowski

The prisoners wore smocks and stocking caps.
The prisoners wore smocks and stocking caps.

Nine men shuffle through the hallway of a drab prison, each wearing nothing but a skimpy smock. Nine guards watch lazily, insulting the prisoners and thinking up bizarre humiliations to inflict on them. Prisoners are subjected to pushups, group chants, isolation in a tiny closet and bags over their heads. With every passing hour, the guards seem to become more depraved, and the prisoners more defeated. Some of the prisoners crack under the dehumanizing treatment.

The twist ending to this bleak prison story is that it did not occur in a prison — it took place in a hallway at Stanford University in California. The men were not guards and prisoners; they were young men who agreed to take part in an experiment. None of them had committed crimes. None of them realized they were part of an experiment that would become infamous, even legendary, in the annals of the social sciences.


Psychology professor Philip Zimbardo had planned to run the Stanford Prison Experiment for two weeks, long enough to explore how humans react to authority and the conditions inside a prison. Instead, he called the experiment off after just six days. The result of his experiment is sometimes shrouded in its own mythology, the truth it revealed about human psychology obscured by its disturbing story of cruelty and submission.

What really happened during those six days in 1971? Let's delve into what the experiment taught us about human nature and about how society analyzes experiments studying behavior.

An Infamous Experiment

Guards walk through the prison "yard," the school hallway. It didn't take long for the experiment to spiral out of control.
Guards walk through the prison "yard," the school hallway. It didn't take long for the experiment to spiral out of control.

The Stanford Prison Experiment is so well-known that even people who've never taken a course in psychology have heard of it, and anyone who does study psychology learns about it in introductory courses. The events of the experiment are told and retold among social scientists and people interested in human behavior like a spooky campfire story.

Philip Zimbardo carried out the experiment, which was funded by the U.S. Office of Naval Research, in August 1971 at Stanford University. The applicants took psychological tests to ensure they were "average," or didn't have psychological disorders or medical conditions. The researchers recruited two dozen male college students using advertisements in newspapers, paying them $15 per day to spend two weeks in a mock prison. They were randomly divided into groups identified as either guards or prisoners, and the prisoners were "arrested" at their homes without warning by real cops and booked at the Palo Alto police station before being taken to the mock prison. The guards weren't instructed how to do their jobs, but they did create a list of restrictive rules the prisoners had to follow, including remaining silent during rest periods and not using each other's names. Zimbardo (acting as prison superintendent) and a team of researchers (one of whom acted as warden) observed the experiment.


The results were horrifying. The guards adopted a rapidly escalating pattern of humiliation and dehumanization against the prisoners. They turned the prisoners against each other and imposed increasingly bizarre punishments, held in check only by the researchers' rule that no physical violence was allowed. Five prisoners were released early because they suffered serious emotional breakdowns or physical problems [source: Zimbardo]. The other prisoners meekly submitted to whatever treatment the guards dished out and eagerly turned against one another at the prospect of a reward, such as being allowed to sleep in a cell with beds and blankets instead of on the concrete floor. The participants became so invested in their roles that the entire experiment was halted after just six days, when Zimbardo realized it was spiraling out of control.

The lesson of the Stanford Prison Experiment seems pretty obvious: There's a cruel streak inside all people, a latent evil waiting to be unleashed should they be given the slightest hint of authority and power. By the same token, the results of the experiment could show that people are driven to obey, conform and respond to authority with submission and compliance. It's a profound and disquieting statement about human nature that 24 "average" young men could be so easily and quickly twisted.

But things aren't really so simple. The lessons to be learned aren't confined to "guards" and "prisoners," but extend to prisons and other powerful institutions, and even to the ways scientists conduct experiments on human behavior. What really happened at the "Stanford County Prison"? Let's find out.

A Deeper Look at the Stanford Prison Experiment

With bags over their heads, the prisoners wait on their parole hearing.
With bags over their heads, the prisoners wait on their parole hearing.

Zimbardo has written extensively about the experiment, filling in major details about what happened. There were nine prisoners and nine active guards. The remaining three participants on each side were on standby in case they were needed. The guards operated in shifts, and the prisoners were always present. The guards were initially reluctant in their roles, the prisoners defiant. But on the second day, the prisoners united: They locked themselves in their cells, insulted the guards and ripped the prisoner numbers off their uniforms, rebelling against the guards' authority. The guards used the incoming shift and the standby guards as reinforcements to put down the rebellion, stripping the prisoners naked and taking away their beds [source: Zimbardo]. This incident also marked the introduction of physical punishments like pushups (often with a guard placing his foot on a prisoner's back).

The guards even instituted a "privilege" cell, one with beds and good meals for the three best-behaved prisoners, and used it to turn the prisoners against each other. At the apex of the guards' brutality, they tried to keep a prisoner in solitary confinement, which was literally a tiny closet barely large enough to fit a person, for an entire night, only relenting when one of the researchers stepped in. And during the night shift, when the guards thought they were not being observed, their torment of the prisoners turned to more intense physical punishment, waking the prisoners throughout the night and forcing them to perform vaguely erotic acts (such as standing very close to each other).


But the guards were not all equally brutal. There was a "ringleader" guard, nicknamed John Wayne, who seemed especially vicious, as well as guards who did the prisoners favors and did not punish them severely. However, the "good" guards never objected to or complained about the behavior of the sadistic guards. The brutal guard, whose real name is Dave Eshelman, has claimed in interviews that he was acting the part he thought the researchers wanted to see. But he has noted that pretending didn't absolve him of his cruelty, since his actions obviously caused misery [source: Ratnesar].

By the end of the experiment, the prisoners exhibited no solidarity, while the guards and even Zimbardo had grown to see the prisoners as a threat that needed to be subdued for the safety of the guards and the integrity of the prison [source: Stanford Prison Experiment]. Everyone involved became so deeply immersed in the experiment's role-playing scenario that at one point the prisoners were offered "parole." That is, they could be released if they would forfeit any money they'd earned for participating. In a parole hearing, most of the prisoners said they would forfeit their money, and the parole board members (secretaries, students and the prison consultant) said they had to consider if they would allow parole for those prisoners who accepted the offer. The board sent the prisoners back to their cells, and the prisoners complied, even though they had the ability to walk away from the experiment (giving up the $15 per day) at any time [source: Haney et al.].

The five prisoners who were released (no guards left the experiment) experienced irrational thinking, unstable emotions and severe anxiety. One even broke out in a psychosomatic rash. In fact, the prisoners either had breakdowns, faked breakdowns so they could leave or simply became "zombies," going along with whatever the guards made them do with little or no emotional reaction.

But on day six, Zimbardo called off the experiment, realizing it had quickly become problematic.

Zimbardo's Mistakes

Some of the prisoners meet with Philip Zimbardo, the prison superintendent and lead researcher. Zimbardo admits that he shouldn't have played both roles.
Some of the prisoners meet with Philip Zimbardo, the prison superintendent and lead researcher. Zimbardo admits that he shouldn't have played both roles.

While the experiment was still happening, Zimbardo realized that he made several serious mistakes in designing and running it. One mistake was his taking on the role of prison superintendent. Instead of simply observing from a neutral location or reviewing the data later, Zimbardo made himself an authority figure, which meant he was part of the experiment. Not only did this affect the behavior of the guards, but it also affected his own behavior. He became enmeshed in the role-playing scenario just as much as the guards and prisoners, making several decisions detrimental to running an experiment. In one instance, he responded to a rumor of a planned breakout by sending in an experiment confederate to act as an informant, contacting local police for help, then relocating the entire prison to another floor temporarily, only to find out the plan was a rumor.

Zimbardo's other major mistake was in not using a control group, so he could study a specific variable or set of variables in the prison. If you want to see what happens when you expose tomatoes to radiation, you also need a group that you expose to no radiation so you can measure the difference. Zimbardo didn't do this. He created an elaborate role-playing scenario, but there was no control prison with different rules or conditions to measure his results against. During the experiment, one of his old roommates visited the prison and asked what the independent variable was (the variable that differed between the control group and the experimental group) [source: Stanford Prison Experiment]. Zimbardo didn't realize until later what an important question this was.


It wasn't until Christina Maslach, a Stanford graduate and Zimbardo's girlfriend at the time, expressed moral outrage at the conditions in the prison and Zimbardo's behavior that he realized that the experiment had spun out of control. He ended it the next day.

But Zimbardo had made another serious error: He wanted to create a neutral prison with so-called average participants. He failed to some extent, and the reasons have serious implications in social science experiments.

Not-so-average Prison

The researchers attempted to recruit an "average" group of participants. But the study was problematic from the beginning, as evidenced by the wording of the newspaper ad for the experiment.
The researchers attempted to recruit an "average" group of participants. But the study was problematic from the beginning, as evidenced by the wording of the newspaper ad for the experiment.

Zimbardo sought to eliminate as many variables as possible in his mock prison. To do so, he had the more than 75 men who answered the newspaper ad take psychological tests so he could use "a homogenous, 'normal' sample" [source: Haney et al]. All but one of the participants were white, and they were all middle-class.

But the experiment introduced bias right from the start. For one thing, the newspaper ad explicitly mentioned that it was a prison experiment, which suggests that anyone who responded had pre-existing attitudes, either positive or negative, about prisons. In fact, a 2007 study found that people who responded to an ad about a prison experiment had higher levels of aggressiveness, authoritarianism, Machiavellianism, narcissism and social dominance than people who responded to an ad that did not mention prison [source: Carnahan]. Zimbardo reported that all the participants expressed a preference for being a prisoner rather than a guard prior to the experiment (although no guards left the study), and one of the prisoners was even a left-wing activist who suspected the experiment was a government attempt to find ways to control protesters [sources: Sedacca, Stanford Prison Experiment].


The participants also knew they were being watched, which could have encouraged them to be more performative in their roles. And the experiment involved mostly young white men to help control against race or age issues affecting behavior, so it studied a very specific subset of the American population. This kind of selection bias doesn't invalidate psychological studies, but it's vital to realize they're present when interpreting experimental results.

In trying to create an ideal simulated prison, Zimbardo accelerated some of the processes that happen to guards and prisoners in real prisons over time: deindividuation and dehumanization. The guards were given military-style uniforms, batons and mirror glasses modeled on those worn by TV and movie cops. These uniforms gave them a sense of power and authority, but also anonymity and a sense that they were part of a unified group of guards. The prisoners wore sandals, chains on their feet and smocks with no underwear, which Zimbardo did to emasculate and humiliate them [source: Haney et al]. They were also assigned numbers, and were only allowed to refer to themselves and each other by these numbers, not their real names. They even wore stocking caps to simulate having their heads shaved.

Finally, Zimbardo and his fellow researchers (plus a consultant who'd served 17 years as a prisoner) rarely intervened in the guards' actions, particularly early in the experiment. Rather than seeing this lack of oversight as the action of a neutral party, the guards likely saw Zimbardo and his team as authority figures who, by not intervening, tacitly approved of the guards' behavior [source: Zimbardo et al.].

Now let's look at what Zimbardo learned from his experiment.

The Takeaways

Zimbardo poses in front of posters for the 2015 movie  Taylor Hill/FilmMagic/Getty Images
Zimbardo poses in front of posters for the 2015 movie Taylor Hill/FilmMagic/Getty Images

Zimbardo realized that rather than a neutral scenario, he created a prison much like real prisons, where corrupt and cruel behavior didn't occur in a vacuum, but flowed from the rules and principles of the institution to the people who carried out those principles. The behavior of the guards and prisoners wasn't dictated by some inherent internal trait, but by the situation they were in. The theory that external circumstances are the primary drivers of human behavior is known as situationist theory. Zimbardo didn't form the theory, but his experiment and later writings helped popularize it.

Experimental ethics are also an issue to consider. The Stanford Human Subjects Review Committee and Zimbardo's superiors approved the experiment — another layer of authority complicit in the experiment's outcome (it's like situationist "Inception") — but experimental ethics are more rigorous today. A modern institutional review board would likely never approve such an experiment without major modifications. Zimbardo has said that he feels the initial experiment was ethical (all the participants understood what they'd signed up for and consented), but that he suffered an ethical lapse when he allowed it to continue beyond the first emotional breakdown of a prisoner [source: Stanford Prison Experiment].


The idea that humans have an inherent tendency toward abuse of authority and submission to authority is not ruled out by the experiment, however. The Stanford Prison Experiment is closely related to another psychological experiment that's as infamous: Stanley Milgram's obedience to authority experiment. In fact, Zimbardo and Milgram were high school classmates and colleagues at Yale University. In the obedience experiment, volunteers were directed to press buttons delivering increasingly powerful, and eventually fake lethal shocks to another person at the direction of a researcher. A large percentage of volunteers went along with the researcher's demands. However, like the Stanford Prison Experiment, the ethics, methodology and conclusions of Milgram's experiment have been called into question recently. And both experiments influenced changes in the regulation and ethical guidelines of studies with human subjects [sources: Zimbardo et al., Defiesta].

Zimbardo's conclusion was that we are not so much inherently "evil," but that we will commit heinous acts if encouraged to do so by systems that enable or encourage them. He took his results to the U.S. House of Representatives shortly after the experiment ended, testifying before a subcommittee on prison reform. His primary argument was that given the power institutions have to dictate the behavior of the people within them, it's necessary to reform those institutions to avoid those abuses. He suggested better training and pay for guards, better protection for prisoners' human rights, and specific training programs that could include role-playing scenarios to help guards learn to deal humanely with prisoners (and weed out the most sadistic among guards) [source: House of Representatives]. The 2003 scandal surrounding prisoner treatment at the Iraqi prison Abu Ghraib, which mirrored the actions of Zimbardo's guards in disturbing ways, suggests that the experiment is not a relic of the past but still relevant to the way people are treated in modern civilian and military prisons.

Zimbardo's testimony also reflected his belief that researchers should not remain impartial observers, but should engage in social and political ways to act on the discoveries they make and seek ways to improve the world [source: Zimbardo et al.].

Lots More Information

Author's Note: How the Stanford Prison Experiment Worked

Much of the appeal of the Stanford Prison Experiment, aside from the dark, voyeuristic thrill of learning how the subjects acted, is how easily it lets you insert yourself into the narrative Zimbardo created. What kind of guard would you be? How would you react when another guard did something sadistic? How would you react as a prisoner? Would you organize your fellow prisoners, or work against them to gain favor with the guards? Or maybe you envision yourself in Zimbardo's position, pulling the strings in your scenario. How would you have changed the conditions to alter the subjects' behavior? Could you redesign the experiment to be more ethical?

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More Great Links

  • Carnahan, Thomas et al. "Revisiting the Stanford Prison Experiment: Could Participant Self-Selection Have Led to the Cruelty?" Personality and Social Psychology Bulletin. Vol. 33, issue 5. May 1, 2007. (June 22, 2017)
  • Defiesta, Nick. "When Psychologists 'Go Wrong.'" Yale Daily News. Sept. 28, 2011. (July 14, 2017)
  • Haney, Craig et al. "Interpersonal Dynamics in a Simulated Prison." International Journal of Criminology and Penology. 1973. (June 10, 2017)
  • House of Representatives. "Hearings Before Subcommittee No. 3 of the Committee on the Judiciary, House of Representatives, Ninety-Second Congress, First Session on Corrections Part 2, Prisons, Prison Reform, and Prisoner Rights: California." Oct. 25, 1971. (June 12, 2017)
  • Konnikova, Maria. "The Real Lesson of the Stanford Prison Experiment." The New Yorker. June 12, 2015. (June 12, 2017)
  • Ratnesar, Romesh. "The Menace Within." Stanford Magazine. July/August 2011. (July 14, 2017)
  • Reicher, Stephen & S. Alexander Haslam. "Rethinking the psychology of tyranny: The BBC prison study." British Journal of Social Psychology. 2006. (June 12, 2017)
  • Sedacca, Matthew. "The Man Who Played With Absolute Power." Feb. 16, 2017. (July 6, 2017)
  • Stanford Prison Experiment. "The Story." (June 12, 2017)
  • Zimbardo, Philip G. "A Situationist Perspective on the Psychlogy of Evil." "The Social Psychology of Good and Evil," Gilford Press. 2004. (June 12, 2017)
  • Zimbardo, Philip G. et al. "Reflections on the Stanford Prison Experiment: Genesis, Transformations, Consequences." In T. Blass (Ed.),"Obedience to Authority: Current Perspectives on the Milgram Paradigm" (pp. 193-237). Lawrence Erlbaum Assoc. 2000. (June 12, 2017)