Inside The Stanford Prison Experiment That Revealed The Darkest Depths Of Human Psychology

Published October 23, 2021
Updated October 27, 2021

Created with psychology and science in mind, the Stanford Prison Experiment turned regular people into monsters.

In October 2004, U.S. Army Staff Sergeant Ivan “Chip” Frederick was facing some hard time. He had been one of the accused in the notorious torture scandal that erupted in March of that year from Iraq’s Abu Ghraib prison, and his court-martial saw disturbing details aired about prisoner abuse, sleep deprivation, and sexual humiliation.

One of the witnesses that Frederick called to defend him — and arguably one of the reasons he only got eight years for his crimes — was Stanford psychologist Philip Zimbardo, who argued that Frederick’s actions weren’t necessarily a reflection on his character, but were instead a reaction to the environment that the higher-ups had allowed to develop in Abu Ghraib.

Zimbardo explained that, given the right set of circumstances, almost anyone could be induced to do some of the things of which Frederick stood accused: beat naked prisoners, defile their religious items, and force them to masturbate with hoods over their heads.

Stanford Prison Experiment Photos

PrisonExp.orgPrisoners with bags forced over their heads await their “parole hearing,” their release from the Stanford Prison Experiment upon its conclusion.

Frederick’s actions, Zimbardo argued, were the predictable outcome of his assignment, rather than the isolated acts of a “bad apple,” which had been the Army’s approach to shifting blame onto certain individuals.

At the court-martial, Zimbardo was able to speak with a certain expertise on the subject of prisoner abuse because he had once participated in it himself.

For six days, between August 14 and 20, 1971, he had been the “warden” of a mock prison in the basement of Stanford University’s Jordan Hall.

In an effort to better understand what drove the interactions of prisoners and their guards — funded by a grant from the U.S. Navy and Marine Corps — Zimbardo devised a psychological experiment that saw two-dozen otherwise normal young men randomly assigned the role of either prisoner or guard for what was intended to be a two-week role-playing exercise.


PrisonExp.orgA naked prisoner stands behind bars during the Stanford prison experiment.

Under Zimbardo’s watch, the Stanford prison experiment turned into a struggle between suffering prisoners and the manipulative, sadistic guards who enjoyed torturing them.

The results were written up and widely circulated, making Zimbardo famous throughout his profession, and revealing something very disturbing about how little it sometimes takes to turn people into monsters.

How The Stanford Prison Experiment Got Started

A decade before the Stanford prison experiment, in 1961, Yale psychologist Stanley Milgram carried out an experiment to test the willingness of some people to deliver electric shocks to strangers.

The Milgram experiment, as it came to be known, revealed that it’s upsettingly easy to talk some young men into shocking another person to death (which they were led to believe they may have done, although no subjects were actually harmed).

Stanford Prison Experiment Guard

PrisonExp.orgA guard escorts a blindfolded prisoner through the prison.

This experiment pointed the way forward for more research into situational behavior and the premise that we are only as good or as bad as our surroundings will let us be. Philip Zimbardo was not present for the Milgram experiment, but he had been a psychology student at Yale until 1960, and by 1971, he was ready to take Milgram’s work one step further at Stanford.

That’s when the U.S. Office of Naval Research commissioned him to study the psychology of confinement and power as it exists between guards and their inmates. Zimbardo accepted the grant and got to work on the Stanford prison experiment right away.

The site chosen for the experiment was in the basement of Jordan Hall, on the Stanford campus. There, Zimbardo set up four “prison cells” using interior partitions, as well as a “warden’s office” and various common areas for the guards to use for recreation. There was also a small broom closet, which will become relevant later on.

Zimbardo recruited subjects for his test by placing an ad in the Stanford Daily, asking for “male students” who were needed “to participate in a psychological study of prison life.” The ad promised compensation of $15 a day (equivalent to roughly $90 in 2017).

When his subjects applied for the experiment, Zimbardo carefully screened them to weed out potential bad apples. Anyone with a criminal record, however minor, was declined participation, as were applicants with histories of psychological aberrations and behavioral problems.

In the end, Zimbardo was left with 24 healthy college-age men who had no detectable tendencies toward violence or other negative behaviors. Shortly before the Stanford prison experiment began, the subjects were randomly assigned to either the prisoner group or the guard group.

The night before the experiment, Zimbardo held an orientation meeting for his 12 guards. He gave them firm instructions regarding their duties and limitations: Guards would be organized into three eight-hour shifts to provide round-the-clock supervision of the inmates.

They were given military-surplus khakis, mirrored sunglasses, and wooden batons as a symbol of authority. The guards were all told not to hit or otherwise physically abuse the prisoners, though they were told they’d have wide discretion in how they treated the 12 prisoners under their watch.

Prisoner In Stanford Experiment

PrisonExp.orgStanford Police handcuff prisoner #8612 before transporting him to the prison.

The next day, members of the Palo Alto Police Department arrived at the designated prisoners’ homes and took them into custody. The 12 men were booked into the county jail and searched, fingerprinted, and had their mugshots taken.

At length, they were transported to the Stanford campus and escorted into the basement, where the guards were waiting for them. Prisoners were given ill-fitting smocks and told to wear large stocking caps. Each had a short length of chain looped around his ankle to drive home their status as prisoners. They were assigned three to a cell and given a lecture about the rules.

Every angle had been worked out to make the prisoners feel subordinate to the guards, including the large numbers sewn onto their smocks; guards had been told to address inmates only by these numbers, rather than allowing them the dignity of names.

By the end of the Stanford prison experiment’s first day, both sides had fully internalized the rules and begun acting toward each other as if their extreme power dynamic had existed all along.

Revolt And Uproar

The Stanford Experiment

PrisonExp.orgPrisoners are made to face the wall as a guard watches over them.

Though both sides had internalized their roles and some inmates seemed to chafe at the boredom and the arbitrary nature of their guards’ commands, the first day of the Stanford prison experiment had passed by more or less uneventfully.

Prisoners would sometimes be pulled out of their cells and searched, even though they couldn’t possibly have had contraband that early in the experiment. The guards were generally rude and condescending. They often demanded that prisoners repeat their numbers to drive home their lowly status. Menial tasks were assigned, and penalties, such as being ordered to hold stress positions for extended periods, were being imposed.

By that first night, the guards had decided to punish less-compliant prisoners by taking away their mattresses, forcing them to sleep on the cold floor. They also disrupted the inmates’ sleep by being noisy in their common area, which was adjacent to the cells.

By noon on day two, prisoner #8612 had started showing the signs of a breakdown. He started screaming and raging, and Zimbardo himself had to come in to control the situation. The prisoner refused to calm down, and so the decision was made to release him from the study for his own sake.

This took the form of a “parole hearing,” accompanied by an extended stint in the broom closet, which was doing duty as a solitary confinement area. The release process was intended to be long and arduous, in order to further the impression that the prison was an all-powerful institution where inmates were helpless.

Bear in mind that this was all a voluntary exercise, and that — in principle, at least — everybody was free to go whenever they wanted to.

Cleaning A Toilet

PrisonExp.orgAs a form of punishment during the Stanford experiment, a prisoner is made to scrub a toilet.

While prisoner #8612 was being processed out, the other 11 prisoners were in an uproar. Arbitrary and cruel treatment by the guards had already provoked inmates to refuse to obey orders or to leave their cells. They refused to answer to their numbers when called.

Inmates in one cell barricaded themselves in by propping a mattress against the door. By that evening, things were bad enough that some guards, who were free to go home after their shift, volunteered to stay overtime and suppress the revolt.

After the clinical staff observing the experiment went home, the guards left on duty hit prisoners with blasts from a fire extinguisher and transferred them to the other cells to increase crowding. The empty cell was reserved for “good” inmates who hadn’t participated in the uprising. Perceived ringleaders, on the other hand, were shut up in solitary for hours.

Prisoners in the regular cells were refused leave to use the bathroom and were instead given buckets to relieve themselves in. The buckets were then left unemptied in the cell all night long. The next day, guards forced the prisoners to stand in stress positions without their clothes on for hours at a time.

Too Dangerous To Continue

Shirtless Stanford Prisoner

PrisonExp.orgGuards begin a strip search of a prisoner.

By day three of the Stanford prison experiment, things were rapidly coming unglued. According to Zimbardo, roughly one-third of the guards spontaneously developed signs of genuine sadism, consistently inventing new forms of punishment and egging the other guards on as these punishments were inflicted on the helpless inmates.

Guards and inmates both — who had, remember, been randomly assigned their roles only a few days prior — began identifying with their side and acting collectively. After a few days, most of the inmates had joined together in a hunger strike to protest their conditions, while the guards were pulling extra shifts for free and becoming increasingly paranoid.

When a rumor got started about prisoner #8612 coming back with a small army of supporters to stage a jailbreak, none other than Zimbardo ordered the basement prison to be disassembled and moved upstairs while he waited alone in the basement for the attackers. He later said his plan, if the man had actually shown up, was to tell him the experiment had been terminated and to send him home.

Philip Zimbardo

PrisonExp.orgPhilip Zimbardo waits alone in the basement of Jordan Hall for the rumored attack on the experiment led by a prisoner who became unhinged and was released early.

By this point, Zimbardo had become fully immersed in the experiment himself. As he later admitted, it was never going to be possible for him to maintain objectivity in his role as prison administrator, and so he found himself bound up in the fantasy world he had created for his test subjects. Zimbardo found himself becoming morbidly curious about where the experiment was going and what new developments each day would bring.

By day four, when certain inmates were becoming suicidal and apparently losing their grip on reality, Zimbardo thought the situation interesting enough to bring in his girlfriend — herself a psychology graduate student — to have a look at what was going on. The woman, 26-year-old Christina Maslach, was appalled by what she saw and said so.

In the past, whenever a new person was brought in from the outside — such as prisoner #416, who replaced #8612 — they went through a period of normalization.

But #416’s objections to his treatment got him locked up in solitary, where the guards would torment him by pounding on the door with their hands in shifts. By the time he got out of the solitary confinement closet, prisoner #416 had been sufficiently broken as to accept the routine of prison life as normal.

Maslach, on the other hand, couldn’t be locked up or broken in that way, and her fresh perspective on what was going on shocked her boyfriend into seeing his nightmare through her eyes. So it was that on day six of the Stanford prison experiment, Dr. Zimbardo announced its termination — much to the dismay of his guards, who had grown to quite like the power they had been abusing all week.

Afterward, everybody was still unhinged enough that it took a full day to “parole” the remaining inmates, though — again — the experiment was over and they weren’t being paid anymore; they could have just left.

The Legacy Of The Stanford Prison Experiment

The Stanford Prison Experiment

PrisonExp.orgA guard wears his issued uniform while holding his baton during the Stanford prison experiment.

The Stanford prison experiment instantly became a classic of human psychology and power dynamics. Perhaps the most stunning findings were that the people who took part in the study almost instantly internalized their roles so completely that they seem to have forgotten that they even had lives outside of the prison.

Guards acted with exceptional brutality, as if they would never have to answer for their actions, while prisoners put up with appalling abuses of their human rights without, for the most part, demanding to be let go.

Perhaps even more disturbing, many researchers and graduate students had passed through the basement during the Stanford prison experiment, observed the conditions of the men’s confinement, and said nothing about it. Later on, Zimbardo estimated that perhaps 50 people had seen what was going on in his basement prison, and his girlfriend was the only one who objected.

Zimbardo’s findings immediately found relevance when, just two weeks after the Stanford prison experiment ended, inmates in the notorious San Quentin and Attica prisons rose up in violent revolts that were strikingly similar to what had happened on day two of the experiment at Stanford.

Zimbardo was called to testify before the House Judiciary Committee about prison conditions and their effect on human behavior. Zimbardo’s contention had always been that external conditions, rather than an individual’s personality, determine how people react under stress.

Partly as a result of Zimbardo’s research, the decision was made in the United States to separate juvenile and adult offenders, as well as to impose stricter controls and protections for prison inmates who, for instance, wish to file a lawsuit challenging their conditions.

But the Stanford prison experiment, like the earlier Milgram experiment, had implications that go far beyond prison management.

Blindfolded Stanford Prisoner

PrisonExp.orgA prisoner sits blindfolded inside the prison.

In both experiments, seemingly normal, healthy human beings were induced — with virtually no coercion and only a little encouragement — to commit horrific crimes against other people in their group. In both cases, the decisions made by individuals would have been unthinkable had they been acting on their own, and strongly suggest that reactions can be conditioned by the local environment when the decisions are made.

This casts a gloomy light on the distinctions that society makes between criminals and law-abiding citizens, as well as suggesting some disturbing possibilities in regards to the perpetrators of humanity’s greatest crimes.

Nazi death squad members, for example, famously argued that they bore no personal malice, but were only following orders; if they had been ordered to do something other than shooting thousands of civilians, they would have.

The postwar trials of these men did not accept this as a defense, but Zimbardo’s research suggests it might have been an excellent excuse; worse, it could be the excuse that any normal person uses when some dictator or other autocrat issues them with khakis and eyeshades, gives them a baton, and tells them to control the prisoners in their cells — as seems to have happened at Abu Ghraib, and probably many other places that didn’t make the news.

After this look at the Stanford Prison Experiment, read up on the most evil science experiments ever conducted. Then, discover what some of the cruelest Nazi research and experimentation contributed to medical science.

Richard Stockton
Richard Stockton is a freelance science and technology writer from Sacramento, California.
John Kuroski
John Kuroski is the editorial director of All That's Interesting. He graduated from New York University with a degree in history, earning a place in the Phi Alpha Theta honor society for history students. An editor at All That's Interesting since 2015, his areas of interest include modern history and true crime.
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Stockton, Richard. "Inside The Stanford Prison Experiment That Revealed The Darkest Depths Of Human Psychology.", October 23, 2021, Accessed May 21, 2024.