THE STANFORD PRISON EXPERIMENT
by Philip Zimbardo
A Simulation Study of
the Psychology of Imprisonment
How we went about testing these questions and what we found may astound you. Our planned two-week investigation into the psychology of prison life had to be ended prematurely after only six days because of what the situation was doing to the college students who participated. In only a few days, our guards became sadistic and our prisoners became depressed and showed signs of extreme stress. Please join me on a slide tour of describing this experiment and uncovering what it tells us about the nature of Human Nature.
--Philip G. Zimbardo
A Quiet Sunday Morning...
On a quiet Sunday morning in August, a Palo Alto, California, police car swept through the town picking up college students as part of a mass arrest for violation of Penal Codes 211, Armed Robbery, and Burglary, a 459 PC. The suspect was picked up at his home, charged, warned of his legal rights, spread-eagled against the police car, searched, and handcuffed -- often as surprised and curious neighbors looked on.
The suspect was then put in the rear of the police car and carried off to the police station, the sirens wailing.
The car arrived at the station, the suspect was brought inside, formally booked, again warned of his Miranda rights, finger printed, and a complete identification was made. The suspect was then taken to a holding cell where he was left blindfolded to ponder his fate and wonder what he had done to get himself into this mess.
What suspects had done was
to answer a local newspaper ad calling for volunteers in a study of the
psychological effects of prison life. We wanted to see what the
psychological effects were of becoming a prisoner or prison guard. To do
this, we decided to set up a simulated a prison and then carefully note
the effects of this institution on the behavior of all those within its
Constructing the Experiment
To help us closely simulate
a prison environment, we called upon the services of experienced
consultants. Foremost among them was a former prisoner who had served
nearly seventeen years behind bars. This consultant made us aware of what
it was like to be a prisoner. He also introduced us to a number of other
ex-convicts and correctional personnel during an earlier Stanford summer
school class we co-taught on "The Psychology of Imprisonment."
At one end of the hall was a small opening through which we could videotape and record the events that occurred. On the side of the corridor opposite the cells was a small closet which became "The Hole," or solitary confinement. It was dark and very confining, about two feet wide and two feet deep, but tall enough that a "bad prisoner" could stand up.
An intercom system allowed us to secretly bug the cells to monitor what the prisoners discussed, and also to make public announcements to the prisoners. There were no windows or clocks to judge the passage of time, which later resulted in some time-distorting experiences.
With these features in place, our jail was ready to receive its first prisoners, who were waiting in the detention cells of the Palo Alto Police Department.
A State of Mild Shock...
Blindfolded and in a state of mild shock over their surprise arrest by the city police, our prisoners were put into a car and driven to the "Stanford County Jail" for further processing. The prisoners were then brought into our jail one at a time and greeted by the warden, who conveyed the seriousness of their offense and their new status as prisoners.
Each prisoner was systematically searched and stripped naked. He was then deloused with a spray, to convey our belief that he may have germs or lice -- as can be seen in this series of photos.
A degradation procedure was designed in part to humiliate prisoners and in part to be sure they wasn't bringing in any germs to contaminate our jail. This procedure was similar to the scenes captured by Danny Lyons in these Texas prison photos.
he prisoner was then issued a uniform. The main part of this uniform was a dress, or smock, which each prisoner wore at all times with no underclothes. On the smock, in front and in back, was his prison ID number. On each prisoner's right ankle was a heavy chain, bolted on and worn at all times. Rubber sandals were the footware, and each prisoner covered his hair with a stocking cap made from a woman's nylon stocking.
It should be clear that we were trying to create a functional simulation of a prison -- not a literal prison. Real male prisoners don't wear dresses, but real male prisoners do feel humiliated and do feel emasculated. Our goal was to produce similar effects quickly by putting men in a dress without any underclothes. Indeed, as soon as some of our prisoners were put in these uniforms they began to walk and to sit differently, and to hold themselves differently -- more like a woman than like a man.
The chain on their foot, which also is uncommon in most prisons, was used in order to remind prisoners of the oppressiveness of their environment. Even when prisoners were asleep, they could not escape the atmosphere of oppression. When a prisoner turned over, the chain would hit his other foot, waking him up and reminding him that he was still in prison, unable to escape even in his dreams.
The use of ID numbers was a way to make prisoner feel anonymous. Each prisoner had to be called only by his ID number and could only refer to himself and the other prisoners by number.
The stocking cap on his head was a substitute for having the prisoner's hair shaved off. The process of having one's head shaved, which takes place in most prisons as well as in the military, is designed in part to minimize each person's individuality, since some people express their individuality through hair style or length. It is also a way of getting people to begin complying with the arbitrary, coercive rules of the institution. The dramatic change in appearance of having one's head shaved can be seen on this page.
The guards were given no
specific training on how to be guards. Instead they were free, within
limits, to do whatever they thought was necessary to maintain law and
order in the prison and to command the respect of the prisoners. The
guards made up their own set of rules, which they then carried into effect
under the supervision of Warden David Jaffe, an undergraduate from
Stanford University. They were warned, however, of the potential
seriousness of their mission and of the possible dangers in the situation
they were about to enter, as, of course, are real guards who voluntarily
take such a dangerous job.
This is what one of our guards looked like. All guards were dressed in identical uniforms of khaki, and they carried a whistle around their neck and a billy club borrowed from the police. Guards also wore special sun-glasses, an idea I borrowed from the movie "Cool Hand Luke." Mirror sunglasses prevented anyone from seeing their eyes or reading their emotions, and thus helped to further promote their anonymity. We were, of course, studying not only the prisoners but also the guards, who found themselves in a new power-laden role.
We began with nine guards and nine prisoners in our jail. Three guards worked each of three eight-hour shifts, while three prisoners occupied each of the three barren cells around the clock. The remaining guards and prisoners from our sample of 24 were on call in case they were needed. The cells were so small that there was room for only three cots on which the prisoners slept or sat, with room for little else.
At 2:30 A.M. the prisoners were rudely awakened from sleep by blasting whistles for the first of many "counts." The counts served the purpose of familiarizing the prisoners with their numbers (counts took place several times each shift and often at night). But more importantly, these events provided a regular occasion for the guards to exercise control over the prisoners. At first, the prisoners were not completely into their roles and did not take the counts too seriously. They were still trying to assert their independence. The guards, too, were feeling out their new roles and were not yet sure how to assert authority over their prisoners. This was the beginning of a series of direct confrontations between the guards and prisoners.
Push-ups were a common form of physical punishment imposed by the guards to punish infractions of the rules or displays of improper attitudes toward the guards or institution. When we saw the guards demand push-ups from the prisoners, we initially thought this was an inappropriate kind of punishment for a prison -- a rather juvenile and minimal form of punishment. However, we later learned that push-ups were often used as a form of punishment in Nazi concentration camps, as can be seen in this drawing by a former concentration camp inmate, Alfred Kantor. It's noteworthy that one of our guards also stepped on the prisoners' backs while they did push-ups, or made other prisoners sit or step on the backs of fellow prisoners doing their push-ups.
Because the first day passed without incident, we were surprised and totally unprepared for the rebellion which broke out on the morning of the second day. The prisoners removed their stocking caps, ripped off their numbers, and barricaded themselves inside the cells by putting their beds against the door. And now the problem was, what were we going to do about this rebellion? The guards were very much angered and frustrated because the prisoners also began to taunt and curse them. When the morning shift of guards came on, they became upset at the night shift who, they felt, must have been too lenient. The guards had to handle the rebellion themselves, and what they did was fascinating for the staff to behold.
At first they insisted that reinforcements be called in. The three guards who were waiting on stand-by call at home came in and the night shift of guards voluntarily remained on duty to bolster the morning shift. The guards met and decided to treat force with force.
They got a fire extinguisher which shot a stream of skin-chilling carbon dioxide, and they forced the prisoners away from the doors. (The fire extinguishers were present in compliance with the requirement by the Stanford Human Subjects Research Panel, which was concerned about potential fire threats.)
The guards broke into each cell, stripped the prisoners naked, took the beds out, forced the ringleaders of the prisoner rebellion into solitary confinement, and generally began to harass and intimidate the prisoners.
The rebellion had been
temporarily crushed, but now a new problem faced the guards. Sure, nine
guards with clubs could put down a rebellion by nine prisoners, but you
couldn't have nine guards on duty at all times. It's obvious that our
prison budget could not support such a ratio of staff to inmates. So what
were they going to do? One of the guards came up a solution. "Let's use
psychological tactics instead of physical ones." Psychological tactics
amounted to setting up a privilege cell.
After half a day of this treatment, the guards then took some of these "good" prisoners and put them into the "bad" cells, and took some of the "bad" prisoners and put them into the "good" cell, thoroughly confusing all the prisoners. Some of the prisoners who were the ringleaders now thought that the prisoners from the privileged cell must be informers, and suddenly, the prisoners became distrustful of each other. Our ex-convict consultants later informed us that a similar tactic is used by real guards in real prisons to break prisoner alliances. For example, racism is used to pit Blacks, Chicanos, and Anglos against each other. In fact, in a real prison the greatest threat to any prisoner's life comes from fellow prisoners. By dividing and conquering in this way, guards promote aggression among inmates, thereby deflecting it from themselves.
The prisoners' rebellion also played an important role in producing greater solidarity among the guards. Now, suddenly, it was no longer just an experiment, no longer a simple simulation. Instead, the guards saw the prisoners as troublemakers who were out to get them, who might really cause them some harm. In response to this threat, the guards began stepping up their control, surveillance, and aggression.
Every aspect of the prisoners' behavior fell under the total and arbitrary control of the guards. Even going to the toilet became a privilege which a guard could grant or deny at his whim. Indeed, after the nightly 10:00 P.M. lights out "lock-up," prisoners were often forced to urinate or defecate in a bucket that was left in their cell. On occasion the guards would not allow prisoners to empty these buckets, and soon the prison began to smell of urine and feces -- further adding to the degrading quality of the environment.
The guards were especially tough on the ringleader of the rebellion, Prisoner #5401. He was a heavy smoker, and they controlled him by regulating his opportunity to smoke. We later learned, while censoring the prisoners' mail, that he was a self-styled radical activist. He had volunteered in order to "expose" our study, which he mistakenly thought was an establishment tool to find ways to control student radicals. In fact, he had planned to sell the story to an underground newspaper when the experiment was over! However, even he fell so completely into the role of prisoner that he was proud to be elected leader of the Stanford County Jail Grievance Committee, as revealed in a letter to his girlfriend.
The First Prisoner Released
Less than 36 hours into the
experiment, Prisoner #8612 began suffering from acute emotional
disturbance, disorganized thinking, uncontrollable crying, and rage. In
spite of all of this, we had already come to think so much like prison
authorities that we thought he was trying to "con" us -- to fool us into
Parents and Friends
The next day, we held a
visiting hour for parents and friends. We were worried that when the
parents saw the state of our jail, they might insist on taking their sons
home. To counter this, we manipulated both the situation and the visitors
by making the prison environment seem pleasant and benign. We washed,
shaved, and groomed the prisoners, had them clean and polish their cells,
fed them a big dinner, played music on the intercom, and even had an
attractive former Stanford cheerleader, Susie Phillips, greet the visitors
at our registration desk.
Some of the parents got upset when they saw how fatigued and distressed their son was. But their reaction was to work within the system to appeal privately to the Superintendent to make conditions better for their boy. When one mother told me she had never seen her son looking so bad, I responded by shifting the blame from the situation to her son. "What's the matter with your boy? Doesn't he sleep well?" Then I asked the father, "Don't you think your boy can handle this?"
He bristled, "Of course he can -- he's a real tough kid, a leader." Turning to the mother, he said, "Come on Honey, we've wasted enough time already." And to me, "See you again at the next visiting time."
A Mass Escape Plot
The next major event we had
to contend with was a rumored mass escape plot. One of the guards
overheard the prisoners talking about an escape that would take place
immediately after visiting hours. The rumor went as follows: Prisoner
#8612, whom we had released the night before, was going to round up a
bunch of his friends and break in to free the prisoners.
After our meeting, we
decided to put an informant (an experimental confederate) in the cell that
#8612 had occupied. The job of our informant would be to give us
information about the escape plot. Then I went back to the Palo Alto
Police Department and asked the sergeant if we could have our prisoners
transferred to their old jail.
Then we formulated a second plan. The plan was to dismantle our jail after the visitors left, call in more guards, chain the prisoners together, put bags over their heads, and transport them to a fifth floor storage room until after the anticipated break in. When the conspirators came, I would be sitting there alone. I would tell them that the experiment was over and we had sent all of their friends home, that there was nothing left to liberate. After they left, we'd bring our prisoners back and redouble the security of our prison. We even thought of luring #8612 back on some pretext and then imprisoning him again because he was released on false pretenses.
I was sitting there all
alone, waiting anxiously for the intruders to break in, when who should
happen along but a colleague and former Yale graduate student roommate,
Gordon Bower. Gordon had heard we were doing an experiment, and he came to
see what was going on. I briefly described what we were up to, and Gordon
asked me a very simple question: "Say, what's the independent variable in
Paying Them Back
The rumor of the prison
break turned out to be just a rumor. It never materialized. Imagine our
reaction! We had spent an entire day planning to foil the escape, we
begged the police department for help, moved our prisoners, dismantled
most of the prison -- we didn't even collect any data that day. How did we
react to this mess? With considerable frustration and feelings of
dissonance over the effort we had put in to no avail. Someone was going to
pay for this.
A Kafkaesque Element
At this point in the study,
I invited a Catholic priest who had been a prison chaplain to evaluate how
realistic our prison situation was, and the result was truly Kafkaesque.
The chaplain interviewed each prisoner individually, and I watched in
amazement as half the prisoners introduced themselves by number rather
than name. After some small talk, he popped the key question: "Son, what
are you doing to get out of here?" When the prisoners responded with
puzzlement, he explained that the only way to get out of prison was with
the help of a lawyer. He then volunteered to contact their parents to get
legal aid if they wanted him to, and some of the prisoners accepted his
The only prisoner who did
not want to speak to the priest was Prisoner #819, who was feeling sick,
had refused to eat, and wanted to see a doctor rather than a priest.
Eventually he was persuaded to come out of his cell and talk to the priest
and superintendent so we could see what kind of a doctor he needed. While
talking to us, he broke down and began to cry hysterically, just as had
the other two boys we released earlier. I took the chain off his foot, the
cap off his head, and told him to go and rest in a room that was adjacent
to the prison yard. I said that I would get him some food and then take
him to see a doctor.
As soon as I realized that #819 could hear the chanting, I raced back to the room where I had left him, and what I found was a boy sobbing uncontrollably while in the background his fellow prisoners were yelling that he was a bad prisoner. No longer was the chanting disorganized and full of fun, as it had been on the first day. Now it was marked by utter confomity and compliance, as if a single voice was saying, "#819 is bad."
I suggested we leave, but he refused. Through his tears, he said he could not leave because the others had labeled him a bad prisoner. Even though he was feeling sick, he wanted to go back and prove he was not a bad prisoner.
At that point I said,
"Listen, you are not #819. You are [his name], and my name is Dr. Zimbardo.
I am a psychologist, not a prison superintendent, and this is not a real
prison. This is just an experiment, and those are students, not prisoners,
just like you. Let's go."
The next day, all prisoners
who thought they had grounds for being paroled were chained together and
individually brought before the Parole Board. The Board was composed
mainly of people who were strangers to the prisoners (departmental
secretaries and graduate students) and was headed by our top prison
Types of Guards
By the fifth day, a new
relationship had emerged between prisoners and guards. The guards now fell
into their job more easily -- a job which at times was boring and at times
The prisoners even nicknamed
the most macho and brutal guard in our study "John Wayne." Later, we
learned that the most notorious guard in a Nazi prison near Buchenwald was
named "Tom Mix" -- the John Wayne of an earlier generation -- because of
his "Wild West" cowboy macho image in abusing camp inmates.
Prisoners' Coping Styles
Prisoners coped with their
feelings of frustration and powerlessness in a variety of ways. At first,
some prisoners rebelled or fought with the guards. Four prisoners reacted
by breaking down emotionally as a way to escape the situation. One
prisoner developed a psychosomatic rash over his entire body when he
learned that his parole request had been turned down. Others tried to cope
by being good prisoners, doing everything the guards wanted them to do.
One of them was even nicknamed "Sarge," because he was so military-like in
executing all commands.
One Final Act of Rebellion
We did see one final act of
rebellion. Prisoner #416 was newly admitted as one of our stand-by
prisoners. Unlike the other prisoners, who had experienced a gradual
escalation of harassment, this prisoner's horror was full-blown when he
arrived. The "old timer" prisoners told him that quitting was impossible,
that it was a real prison.
An End to the Experiment
On the fifth night, some
visiting parents asked me to contact a lawyer in order to get their son
out of prison. They said a Catholic priest had called to tell them they
should get a lawyer or public defender if they wanted to bail their son
out! I called the lawyer as requested, and he came the next day to
interview the prisoners with a standard set of legal questions, even
though he, too, knew it was just an experiment.
I ended the study prematurely for two reasons. First, we had learned through videotapes that the guards were escalating their abuse of prisoners in the middle of the night when they thought no researchers were watching and the experiment was "off." Their boredom had driven them to ever more pornographic and degrading abuse of the prisoners.
Second, Christina Maslach, a recent Stanford Ph.D. brought in to conduct interviews with the guards and prisoners, strongly objected when she saw our prisoners being marched on a toilet run, bags over their heads, legs chained together, hands on each other's shoulders. Filled with outrage, she said, "It's terrible what you are doing to these boys!" Out of 50 or more outsiders who had seen our prison, she was the only one who ever questioned its morality. Once she countered the power of the situation, however, it became clear that the study should be ended.
And so, after only six days, our planned two-week prison simulation was called off.
On the last day, we held a series of encounter sessions, first with all the guards, then with all the prisoners (including those who had been released earlier), and finally with the guards, prisoners, and staff together. We did this in order to get everyone's feelings out in the open, to recount what we had observed in each other and ourselves, and to share our experiences, which to each of us had been quite profound.
We also tried to make this a time for moral reeducation by discussing the conflicts posed by this simulation and our behavior. For example, we reviewed the moral alternatives that had been available to us, so that we would be better equipped to behave morally in future real-life situations, avoiding or opposing situations that might transform ordinary individuals into willing perpetrators or victims of evil.
Two months after the study, here is the reaction of prisoner #416, our would-be hero who was placed in solitary confinement for several hours:
"I began to feel that I was losing my identity, that the person that I called "Clay," the person who put me in this place, the person who volunteered to go into this prison -- because it was a prison to me; it still is a prison to me. I don't regard it as an experiment or a simulation because it was a prison run by psychologists instead of run by the state. I began to feel that that identity, the person that I was that had decided to go to prison was distant from me -- was remote until finally I wasn't that, I was 416. I was really my number."
Compare his reaction to that of the following prisoner who wrote to me from an Ohio penitentiary after being in solitary confinement for an inhumane length of time:
"I was recently released from solitary confinement after being held therein for thirty-seven months. The silence system was imposed upon me and if I even whispered to the man in the next cell resulted in being beaten by guards, sprayed with chemical mace, black jacked, stomped, and thrown into a strip cell naked to sleep on a concrete floor without bedding, covering, wash basin, or even a toilet....I know that thieves must be punished, and I don't justify stealing even though I am a thief myself. But now I don't think I will be a thief when I am released. No, I am not rehabilitated either. It is just that I no longer think of becoming wealthy or stealing. I now only think of killing -- killing those who have beaten me and treated me as if I were a dog. I hope and pray for the sake of my own soul and future life of freedom that I am able to overcome the bitterness and hatred which eats daily at my soul. But I know to overcome it will not be easy."
Terminated on August 20, 1971
Our study was terminated on
August 20, 1971. The next day, there was an alleged escape attempt at San
Quentin. Prisoners in the Maximum Adjustment Center were released from
their cells by Soledad brother George Jackson, who had smuggled a gun into
the prison. Several guards and some informant prisoners were tortured and
murdered during the attempt, but the escape was prevented after the leader
was allegedly gunned down while trying to scale the 30-foot high prison
The question now is how to change our institutions so that they promote human values rather than destroy them. Sadly, in the decades since this experiment took place, prison conditions and correctional policies in the United States have become even more punitive and destructive. The worsening of conditions has been a result of the politicization of corrections, with politicians vying for who is toughest on crime, along with the racialization of arrests and sentencing, with African-Americans and Hispanics overrepresented. The media has also contributed to the problem by generating heightened fear of violent crimes even as statistics show that violent crimes have decreased.
There are more Americans in
jails and prisons -- both men and women -- than ever before in history.
According to a recent Justice Department survey, the number of jailed
Americans more than doubled during the past 12 years, with over 1.8
million people in jail or prison as of 1998. To learn more about this
issue or the Stanford Prison Experiment, please consult the biobliography
below or visit this site's Related Links.
Zimbardo, P. G., Maslach, C., & Haney, C. (2000). Reflections on the Stanford Prison Experiment: Genesis, transformations, consquences. In T. Blass (Ed.). Obedience to authority: Current Perspectives on the Milgram paradigm (pp.193-237). Mahwah, N.J.: Erlbaum.
Haney, C., & Zimbardo, P. G. (1998). The past and future of U.S. prison policy: Twenty-five years after the Stanford Prison Experiment. American Psychologist, 53, 709-727.
Zimbardo, P. G. (1994). Transforming California's prisons into expensive old age homes for felons: Enormous hidden costs and consequences for California's taxpayers. The Center on Juvenile and Criminal Justice, San Francisco, CA.
Zimbardo, P. G. (1979). (Testimony of Dr. Philip Zimbardo to U.S. House of Representatives Committee on the Judiciary.) In J. J. Bonsignore, et al. (Eds.), Before the law: An introduction to the legal process (pp. 396-399) (2nd ed.). Boston: Houghton Mifflin.
Haney, C., & Zimbardo, P. G. (1976). Social roles and role-playing: Observations from the Stanford prison study. In E. P. Hollander & R. G. Hunt (Eds.), Current perspectives in social psychology (4th ed.) (pp. 266-274). New York: Oxford University Press.
Zimbardo, P. G. (1974). The detention and jailing of juveniles (Hearings before U.S. Senate Committee on the Judiciary Subcommittee to Investigate Juvenile Delinquency, 10, 11, 17, September, 1973) (pp. 141-161). Washington, DC: U.S. Government Printing Office.
Zimbardo, P. G., Haney, C., Banks, W. C., & Jaffe, D. (1973, April 8). The mind is a formidable jailer: A Pirandellian prison. The New York Times Magazine, Section 6, 36, ff.
Haney, C., Banks, W. C., & Zimbardo, P. G. (1973). Interpersonal dynamics in a simulated prison. International Journal of Criminology and Penology, 1, 69-97.
Zimbardo, P. G. (1971). The power and pathology of imprisonment. Congressional Record. (Serial No. 15, October 25, 1971). Hearings before Subcommittee No. 3, of the Committee on the Judiciary, House of Representatives, Ninety-Second Congress, First Session on Corrections, Part II, Prisons, Prison Reform and Prisonerís Rights: California. Washington, DC: U.S. Government Printing Office.