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Zimbardo - Stanford Prison Experiment

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Aim: To investigate how readily people would conform to the roles of guard and prisoner in a role-playing exercise that simulated prison life.

Zimbardo (1973) was interested in finding out whether the brutality reported among guards in American prisons was due to the sadistic personalities of the guards (i.e. dispositional) or had more to do with the prison environment (i.e. situational). For example, prisoner and guards may have personalities which make conflict inevitable, with prisoners lacking respect for law and order and guards being domineering and aggressive. Alternatively, prisoners and guards may behave in a hostile manner due to the situation they are placed in.

If the prisoners and guards behaved in a non-aggressive manner this would support the dispositional hypothesis, or if they behave the same way as people do in real prisons this would support the situational explanation.

Procedure: To study the roles people play in prison situations, Zimbardo converted a basement of the Stanford University psychology building into a mock prison. He advertised for students to play the roles of prisoners and guards for a fortnight. 24 male college students (chosen from 75 volunteers) were screened for psychological normality and paid $15 per day to take part in the experiment.

Participants were randomly assigned to either the role of prisoner or guard in a simulated prison environment. There were 2 reserves and one dropped out, finally leaving 10 prisoners and 11 guards. The guards worked in sets of 3 (being replaced after an 8 hour shift), and the prisoners were housed 3 to a room. There was also a solitary confinement cell for prisoners who 'misbehaved'. The prison simulation was kept as “real life” as possible.

stanford prison experiment picture of a prisoner being arrested

zimbardo prison experiment picture of a prisoner

Prisoners were treated like every other criminal, being arrested at their own homes, without warning, and taken to the local police station. They were fingerprinted, photographed and ‘booked’. Then they were blindfolded and driven to the psychology department of Stanford University, where Zimbardo had had the basement set out as a prison, with barred doors and windows, bare walls and small cells. Here the deindividuation process began.

When the prisoners arrived at the prison they were stripped naked, deloused, had all their personal possessions removed and locked away, and were given prison clothes and bedding. They were issued a uniform, and referred to by their number only. Their clothes comprised a smock with their number written on it, but no underclothes. They also had a tight nylon cap, and a chain around one ankle.

zimbardo prison experiment picture of a guard

Guards were issued a khaki uniform, together with whistles, handcuffs and dark glasses, to make eye contact with prisoners impossible. The guards worked shifts of eight hours each (the other guards remained on call). No physical violence was permitted.

Zimbardo observed the behavior of the prisoners and guards (as a researcher), and also acted as prison warden.

Findings: Within a very short time both guards and prisoners were settling into their new roles, with the guards adopting theirs quickly and easily.

Within hours of beginning the experiment some guards began to harass prisoners. They behaved in a brutal and sadistic manner, apparently enjoying it. Other guards joined in, and other prisoners were also tormented.

The prisoners were taunted with insults and petty orders, they were given pointless and boring tasks to accomplish, and they were generally dehumanized.

The prisoners soon adopted prisoner-like behavior too. They talked about prison issues a great deal of the time. They ‘told tales’ on each other to the guards. They started taking the prison rules very seriously, as though they were there for the prisoners’ benefit and infringement would spell disaster for all of them. Some even began siding with the guards against prisoners who did not obey the rules.

Over the next few days the relationships between the guards and the prisoners changed, with a change in one leading to a change in the other. Remember that the guards were firmly in control and the prisoners were totally dependent on them.

As the prisoners became more dependent, the guards became more derisive towards them. They held the prisoners in contempt and let the prisoners know it. As the guards’ contempt for them grew, the prisoners became more submissive.

As the prisoners became more submissive, the guards became more aggressive and assertive. They demanded ever greater obedience from the prisoners. The prisoners were dependent on the guards for everything so tried to find ways to please the guards, such as telling tales on fellow prisoners.

One prisoner had to be released after 36 hours because of uncontrollable bursts of screaming, crying and anger. His thinking became disorganized and he appeared to be entering the early stages of a deep depression. Within the next few days three others also had to leave after showing signs of emotional disorder that could have had lasting consequences. (These were people who had been pronounced stable and normal a short while before).

Zimbardo (1973) had intended that the experiment should run for a fortnight, but on the sixth day it was terminated. Christina Maslach, a recent Stanford Ph.D. brought in to conduct interviews with the guards and prisoners, strongly objected when she saw the prisoners being abused by the guards. 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.

Conclusion: People will readily conform to the social roles they are expected to play, especially if the roles are as strongly stereotyped as those of the prison guards. The “prison” environment was an important factor in creating the guards’ brutal behavior (none of the participants who acted as guards showed sadistic tendencies before the study). Therefore, the findings support the situational explanation of behavior rather than the dispositional one.

Zimbardo proposed that two processes can explain the prisoner's 'final submission'. The participants became de-individualized and lost their sense of personal identity because of the uniform they wore. Therefore, the roles that people play can shape their behavior and attitudes. Also, learned helplessness could explain the prisoners submission to the guards. The prisoners learnt that whatever they did had little effect on what happened to them. In the mock prison the unpredictable decisions of the guards led the prisoners to give up responding.

After the prison experiment was terminated Zimbardo interviewed the participants. Here’s an excerpt:

    ‘Most of the participants said they had felt involved and committed. The research had felt "real" to them. One guard said, "I was surprised at myself. I made them call each other names and clean the toilets out with their bare hands. I practically considered the prisoners cattle and I kept thinking I had to watch out for them in case they tried something." Another guard said "Acting authoritatively can be fun. Power can be a great pleasure." And another: "... during the inspection I went to Cell Two to mess up a bed which a prisoner had just made and he grabbed me, screaming that he had just made it and that he was not going to let me mess it up. He grabbed me by the throat and although he was laughing I was pretty scared. I lashed out with my stick and hit him on the chin although not very hard, and when I freed myself I became angry."’

Most of the guards found it difficult to believe that they had behaved in the brutalizing ways that they had. Many said they hadn’t known this side of them existed or that they were capable of such things. The prisoners, too, couldn’t believe that they had responded in the submissive, cowering, dependent way they had. Several claimed to be assertive types normally. When asked about the guards, they described the usual three stereotypes that can be found in any prison: some guards were good, some were tough but fair, and some were cruel.

Critical Evaluation: Demand characteristics could explain the findings of the study. Most of the guards later claimed they were simply acting. Because the guards and prisoners were playing a role their behaviour may not be influenced by the same factors which affect behavior in real life. This means the studies findings cannot be reasonably generalized to real life, such as prison settings. I.e the study has low ecological validity.

The study may also lack population validity as the sample comprised US male students. The studies findings cannot be applied to female prisons or those from other countries. For example, America is an individualist culture (were people are generally less conforming) and the results maybe different in collectivist cultures (such as Asian countries).

A strength of the study is that it has altered the way US prisons are run. For example, juveniles accused of federal crimes are no longer housed before trial with adult prisoners (due to the risk of violence against them).

Another strength of the study is that the harmful treatment of participant led to the formal recognition of ethical guidelines by the American Psychological Association. Studies must now undergo an extensive review by an institutional review board (US) or ethics committee (UK) before they are implemented. A review of research plans by a panel is required by most institutions such as universities, hospitals and government agencies. These boards review whether the potential benefits of the research are justifiable in the light of possible risk of physical or psychological harm. These boards may request researchers make changes to the studies design or procedure, or in extreme cases deny approval of the study altogether.

Ethical Issues: The study has received many ethical criticisms, including lack of fully informed consent by participants as Zimbardo himself did not know what would happen in the experiment (it was unpredictable). Also, the prisoners did not consent to being 'arrested' at home.

Also, participants playing the role of prisoners were not protected from psychological harm, experiencing incidents of humiliation and distress. For example, one prisoner had to be released after 36 hours because of uncontrollable bursts of screaming, crying and anger. However, in Zimbardo's defence the emotional distress experienced by the prisoners could not have been predicted from the outset. In addition Zimbardo did conduct debriefing sessions for several years afterwards and concluded they were no lasting negative effects.


Haney, C., Banks, W. C., & Zimbardo, P. G. (1973). A study of prisoners and guards in a simulated prison. Naval Research Review, 30, 4-17.

Further Information

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The Stanford Prison Experiment Audio Broadcast

How to cite this article:

McLeod, S. A. (2008). Zimbardo - Stanford Prison Experiment. Retrieved from www.simplypsychology.org/zimbardo.html

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