What Happened During the Stanford Prison Experiment?

It started with a classified ad in the “help wanted” section of the Palo Alto Times. Between job postings for Avon reps and security system sales was an ad reading:  

“Male College students needed for psychological study of prison life. $15 per day for 1-2 weeks beginning Aug. 15. For further information & applications, come to Room 248, Jordan Hall, Stanford U.” 

It was 1971, and the ad was placed by Philip Zimbardo, a psychology professor at Stanford University. The previous semester, some of Zimbardo’s undergraduate students had built a small mock prison in their dorm for a class project studying the psychological effects of incarceration on both prisoners and guards. The results were alarming. Over just three days, the role-playing prisoners became despondent and desperate, and the guards turned into bullies.  

Zimbardo was intrigued. If an unscientific dorm room experiment could elicit such strong reactions, what about a full-blown psychological study? Twenty-four young men were recruited from the ad—a mix of Stanford students and others in Palo Alto for the summer—and were randomly divided into two groups: prisoners and guards.  

On Sunday, August 14, real Palo Alto police officers arrived at the dorms and homes of the prisoners with sirens blaring. They cuffed the prisoners, read them their Miranda rights and booked them at the station on charges of burglary and armed robbery. Then, blindfolded, the students were taken to “prison,” three mocked-up cells in the basement of Jordan Hall on the Stanford campus. 

What happened in that fake prison over the next six days is the stuff of legend. The Stanford Prison Experiment was supposed to run for two weeks, but Zimbardo had to call it off early when things spiraled out of control. Guards acted sadistically. Prisoners had full mental breakdowns. Even Zimbardo, acting as the prison’s “superintendent,” later said he got swept up in his role and nearly forgot it was all fake.  

More than 50 years later, the Stanford Prison Experiment remains one of the most famous psychological studies in the history of the field. Its shocking conclusion, that “ordinary college students could do terrible things,” has been used to explain prison riots and abuses at the Abu Ghraib prison in Iraq.  

But the Stanford Prison Experiment is also hugely controversial. Not only would its methods be considered dangerously unethical by today’s standards, but its key findings have also been called into question. Did the guards really transform overnight from normal college kids into callous monsters, or were they playing roles coached by Zimbardo and his staff?  

“The narrative is that [the guards] just spontaneously, organically adopted these behaviors once they’re in the situation [and] in these roles,” says Jay Van Bavel, a psychology professor at New York University. But thanks to new information, including previously unreleased audio recordings, there’s evidence that Zimbardo and his team “pressur[ed] the guards behind the scenes to engage in greater brutality against the prisoners.” 

Seventy-five people responded to Zimbardo’s ad, and the researchers selected 24 individuals who were “the most stable (physically and mentally), most mature and least involved in anti-social behavior,” Zimbardo explained in his 1973 paper about the experiment. With a coin toss, 12 young men were designated “prisoners” and 12 were assigned to be “guards.” (Some were put on “standby,” so the experiment started with nine prisoners and nine guards.) 

Everyone got uniforms. The guards wore khaki shirts and pants, carried nightsticks and whistles and donned reflective sunglasses that hid their eyes. The prisoners, on arrival, were stripped naked and sprayed with disinfectant. Then they were ordered to put on loose smocks (no underwear) that resembled dresses, each bearing a prisoner ID number on the front. Instead of shaving their heads—which is common in some prisons—the prisoners’ hair was covered with a tight cap made from nylon stockings. They also wore a chain around one ankle day and night.   

Once the prisoners were locked in their cells—cramped spaces with three cots each—the “warden” read the rules. Zimbardo’s undergraduate research assistant, David Jaffe, who had previously helped run a mock prison in a Stanford dorm, assumed this role.

Prisoners could only call each other by their numbers, not their names. Guards were to be addressed as “Mr. Correctional Officer.” Prisoners had to remain silent during meals, rest periods and after lights out. They would have five minutes to use the bathroom. Any breach of the rules could result in punishment, including time in “the Hole,” a claustrophobic closet with only enough room to stand.  

At 2:30 a.m. on the first night, the prisoners awoke to shrill whistles for the first “count.” The main purpose of the count was to make sure that no one had escaped, but it also gave the guards an opportunity to assert their authority.  

Prisoners were made to memorize each other’s numbers then recite the numbers backward then sing them. Mistakes were punished with push-ups. At first, Zimbardo thought push-ups were juvenile, but he later learned that Nazi guards had used push-ups as punishment in concentration camps. The first count lasted only 10 minutes, but as the days progressed, counts started to drag on for hours. 

To Zimbardo’s surprise, some of the prisoners started to rebel. They ripped off their numbers, tore off their stocking caps and barricaded themselves inside their cells with their beds. They also taunted and cursed the guards.

To put down the rebellion, the guards sprayed prisoners with a fire extinguisher, broke into the cells, stripped the prisoners naked and removed their beds. They also locked the ringleaders in the Hole. Meanwhile, the “good” prisoners were placed in a cell with full privileges and were fed special meals in front of the “bad” prisoners. According to Zimbardo, the guards used this punishment-reward system to sow distrust among the prisoners and weaken their solidarity.

After the attempted rebellion, the cruelty of some guards escalated. Prisoners were kept in solitary for hours beyond the agreed time limits. They were denied bathroom visits after 10 p.m. and forced to defecate in buckets in their cells.  

One guard in particular stood out for his uniquely sadistic behavior. Nicknamed “John Wayne” by the prisoners, Dave Eshleman later said he deliberately played the role of a cruel guard and modeled his behavior on the fictional warden in the movie Cool Hand Luke.

Zimbardo described Eshleman as “the most creatively evil,” goading his fellow guards to treat the prisoners with increasing cruelty. As Eshleman later explained, “I was kind of running my own experiment in there, by saying, ‘How far can I push these things and how much abuse will these people take before they say, “knock it off?”’ But the other guards didn’t stop me. They seemed to join in. They were taking my lead. Not a single guard said, ‘I don’t think we should do this.’” 

Just days into the experiment, prisoners started to crack. On Tuesday, prisoner No. 819 tore up his pillow and threw feathers all over his cell. He was tossed in the Hole while the other prisoners were instructed to chant, “Prisoner 819 did a bad thing!” They kept chanting until the young man was sobbing uncontrollably. Ultimately, five prisoners had to be “paroled” or released from the experiment because of “extreme emotional depression, crying, rage and acute anxiety,” wrote Zimbardo.  

Even Zimbardo wasn’t immune to the effects of his own experiment. After one of the prisoners (No. 8612) was released, the guards heard a rumor that he and some friends were going to return that night and attempt a prison break. As superintendent, Zimbardo was prepared to take extreme measures to protect his prison.  

Zimbardo asked the Palo Alto Police Department if he could transfer his prisoners to the department’s old jail. When the police refused, Zimbardo was “angry and disgusted at this lack of cooperation between our correctional facilities.” Zimbardo considered chaining the prisoners together, putting bags over their heads and taking them to a fifth-floor storage room until the threat of a prison break had passed. He even considered luring prisoner No. 8612 back to the basement under false pretenses and locking him up again. As it happened, the rumored escape never materialized. 

Zimbardo later said he became so caught up in the day-to-day “reality” of his fake prison that didn’t see how insane the situation in the prison had become. It was Christina Maslach, his girlfriend and a recent Stanford doctorate in psychology, who opened his eyes. She visited Zimbardo in the basement prison and was appalled by the prisoner’s fragile emotional states and the disgusting conditions in which they were living.  

By this point, Eshleman had forced prisoners to simulate sexual behavior with each other. Zimbardo also later wrote that guards escalated their abuse when they believed the cameras were off and researchers were not watching. Who knows how much worse it might have gotten without Maslach’s intervention.  

“Out of 50 or more outsiders who had seen our prison, [Christina] was the only one who ever questioned its morality,” Zimbardo wrote of Maslach, who later became his wife.  

Just six days into the planned two-week Stanford Prison Experiment, Zimbardo shut it down. 

The Stanford Prison Experiment might have become a forgotten footnote in the history of psychology if not for what happened next.

The day after the experiment ended abruptly on August 20, 1971, there was an attempted prison break at San Quentin Penitentiary, just 90 minutes north of Stanford. Three inmates and three guards were killed in the melee. Just weeks later, rioting inmates overpowered guards at the Attica Correctional Facility in New York; 43 people, including 10 hostages, were killed during the ensuing uprising and retaking of the prison.

What was causing all the violence in America’s prisons? The prevailing theories were “personality-based,” Van Bavel says. Psychologists and sociologists often attributed violence to the personalities of incarcerated people or guards.

When Zimbardo and his colleagues published their 1973 paper on the Stanford Prison Experiment, they offered a different theory: Individuals aren’t to blame for rising prison violence; it’s the prison environment itself that turns people into monsters.  

In a “surprisingly short period of time, we witnessed a sample of normal, healthy American college students fractionate into a group of prison guards who seemed to derive pleasure from insulting, threatening, humiliating and dehumanizing their peers,” Zimbardo wrote. “[As] the prisoners became resigned to their treatment over time, many acted in ways to justify their fate at the hands of the guards, adopting attitudes and behavior which helped to sanction their victimization.”  

The Stanford Prison Experiment became one of psychology’s best-known studies and a fixture in introductory psychology courses and textbooks, often presented as evidence of the power of situations and social roles to shape behavior. Zimbardo also used it as a rallying cry for prison reform.   

“Zimbardo [who died in 2024] was extremely charismatic and a salesman for the study and for the message,” Van Bavel says. “He wrote bestselling books. He was on TV all the time. He was in front of Congress testifying about this. He was an influencer before we even had a language for that.”  

While the Stanford Prison Experiment is one of the best-known and most widely-cited psychological studies, it has also come under sharp criticism for being unethical and unscientific.  

Van Bavel says that no university ethics board today would approve a study like the Stanford Prison Experiment, so it’s impossible to replicate. The study was also very small and very short—just nine prisoners and nine guards over six days—making it difficult to draw broad generalizations from its results.  

“How do we know it’s not some fluke or because a couple weird participants got in that one version of the study?” asks Van Bavel. “How generalizable is this beyond one week in the 1970s in Palo Alto in the basement of the Stanford psychology department?” 

More troubling was information that came to light in 2018, when Zimbardo uploaded all the records from the 1971 experiment to the Stanford library archives, including previously unreleased audio recordings of conversations between the guards and Zimbardo’s research team. The tapes showed how Stanford researchers coached the guards to act with toughness, even cruelty, if necessary, in order to achieve the goals of the experiment.  

“If we do not do anything, then we’re just playing a game, and nothing happens—we do not learn anything,” said David Jaffe, the “warden,” in an initial briefing with the guards. “So your inventiveness and whatever else—stick-to-it-ness as far as devising things, working out a schedule, and then rewards and punishments and all of this kind of thing are very important part of making the thing run.” 

The guards were told not to use violence but also informed that the university medical center was nearby and “we can get them over here really fast.” When some of the guards voiced apprehensions about “harassment” and “sadistic” behavior, Zimbardo’s team explained that the experiment wouldn’t work—and real prisons wouldn’t be reformed—if the guards didn’t act sufficiently “tough.” Critics argue that such instructions influenced the guards’ behavior, while Zimbardo maintained that researchers never instructed them to use brutality.

“We’re not trying to do this just because we’re sadists,” Jaffe told the guards. “If you need an excuse, and I think most of us do really, it is so we can learn what happens in a total institution... So that we can, we can get on the media and, um, and, and into press with it. And, and, and say ‘Now look at what, what this really about.’” 

Van Bavel says that conversations like this between a researcher and his research subjects are “a massive departure from scientific and research norms. I’ve run hundreds of studies in my lab. I would never even tell the participants the hypothesis, let alone pressure them to act in a certain way that fits it.” 

Part of the reason why the Stanford Prison Experiment attained such viral popularity was that it appeared to show how normal people could adopt cruel behavior after simply putting on a uniform and wielding a night stick. But the archival evidence challenged the idea that the guards’ behavior emerged spontaneously, raising new questions about what the study actually demonstrated.

“There’s a camp that says, ‘This is a sham, this is a fraud, that we were misled about this, that there was too much manipulation by the experimenters going on, and therefore it’s just really untrustworthy,’ but I don’t fully agree that it’s a sham,” says Van Bavel, who co-authored a paper reevaluating the Stanford Prison Experiment in light of the new recordings.  

When the experiment came under fire from critics in 2019, Zimbardo argued that it was never really an experiment, but more of an “exploratory investigation.”

“That’s just baloney,” Van Bavel says. “I mean, it’s literally called the Stanford Prison Experiment.”