The Stanford Prison Experiment

By Ayesh Perera, published May 13, 2022 | Fact Checked by Saul Mcleod, PhD


  • The Stanford Prison Experiment was conducted by a research team led by the psychology professor Philip Zimbardo of Stanford University, during the summer of 1971.
  • The study evaluated the effects of situational forces upon participants’ behaviors and reactions in a simulated prison setting over two weeks.
  • Subjects were randomly divided into 2 groups. While half were assigned to play the role of guards, the others were assigned to be prisoners.
  • The prison guards wore uniforms, including sticks and mirrored sunglasses. Worked shifts and went home at the end of their shift. They were told that they had complete power over the prisoners but were not allowed to use physical violence.
  • The experiment terminated after only 6 days. Within two days, the prisoners rebelled against the harsh treatment by the guards. The guards had become so brutal to the prisoners that two prisoners had some form of nervous breakdown, one developed a nervous rash all over his body and one went on hunger strike.
  • While the guards were giving their orders, the prisoners became subdued and apathetic. They did not stand up to the guards and simply did as they were told, even though it caused them distress.
  • The guards had become so brutal to the prisoners that two prisoners had some form of nervous breakdown, one developed a nervous rash all over his body and one went on hunger strike.
  • This study shows the power of the situation to influence people’s behaviour. Both the guards and the prisoners conformed to their roles within the prison.
  • The dispositional explanation for the behaviour of the participants would be that the guards behaved in the way that they did because they were naturally cruel and sadistic people and that the prisoners were naturally subservient and weak.
  • However, the fact that they were all initially screened and found to be similar in terms of mental and physical health and stability argues against this explanation, as does the fact that they were randomly allocated to the roles of prisoner and guard.
  • Hence a more convincing explanation is that they behaved in the way that they did because of the situation they were in.
  • This would support the initial hypothesis proposed by Zimbardo that the social environment created in prisons is what has the negative and destructive effect on its inhabitants.
  • This experiment ended up becoming a famous and controversial study discussed in articles, textbooks, movies, and psychology classes.

Who Were the Participants?

After the university had granted permission to administer the experiment, advertisements ran in The Stanford Daily and the Palo Alto Times calling for applicants.

Out of the 75 men who applied, 24 were chosen following a screening process (Haney, Banks & Zimbardo, 1973).

The selection excluded individuals with psychological impairments, criminal backgrounds or medical issues.

The subjects had consented to partake in the study for up to 14 days for $15 (equivalent to more than $100 today) per day.

These men were randomly divided into 2 groups. While half were assigned to play the role of guards, the others were assigned to be prisoners.

The Setting

A 35ft section of Stanford’s psychology building’s basement was chosen for the setting.

There were fabricated walls at the entrance and the cell wall to impede observation. Small six-by-nine ft prison cells, each capable of holding 3 prisoners, were set up.

The cells were unlit and there was a mattress, pillow and sheet for every prisoner. Moreover, there was a larger room for the warden and the guards (across from the cells), a corridor connecting the yard, and a solitary confinement closet.

While the guards were granted access to areas for relaxation and rest, the prisoners were to remain in the cells and yard throughout the study.

The Procedures

The guards were asked to operate in teams of 3 men for 8-hour shifts (Haney, Banks & Zimbardo, 1973). Following each shift, the guards could return home.

Moreover, they were instructed not to withhold drink or food from, or physically harm the prisoners.

However, they were asked to humiliate the inmates into submission and helplessness, by, for instance, referring to prisoners not by their names, but by their ID numbers in order to diminish their individuality.

Primarily tasked with maintaining law and order, the guards were equipped with wooden batons.

Additionally, they were garbed in khaki shirts and pants, resembling the apparel of actual prison guards, and were given mirrored sunglasses to create anonymity and prevent eye contact.

Zimbardo, who was administering the whole experiment, would act as the superintendent over the guards.

The prisoners, meanwhile, were treated like normal criminals (Haney, Banks & Zimbardo, 1973).

They were arrested without warning in their homes and fetched to the police station where they were subsequently photographed and fingerprinted.

Afterwards, the prisoners were blindfolded and taken to the basement (the prison setting) of Stanford’s psychology building.

Upon their arrival here, they were unclothed and deloused, and were given uniforms and bedding. Each had a locked chain girding an ankle and a tight cap for the head.

Moreover, all their possessions were taken and locked up. The prisoners also had their ID numbers written on their clothing.

They were permitted to refer to themselves, and their fellow prisoners only by ID number.

The procedure was designed to engender anonymity and a process of deindividuation among the prisoners.


Soon both the prisoners and the guards settled into the setting. Within hours, the guards began asserting their authority by harassing the inmates.

At 2.30am, blasting whistles awakened the prisoners for the first of numerous ‘counts,’ which would serve to acquaint the prisoners with their ID numbers.

The prisoners, for their part, soon began behaving like actual inmates, taking the prison regulations seriously, telling tales’ on each other, and extensively discussing prison-related issues.

They were also given boring chores and petty orders, and were harassed with insults. The physical punishments they endured included push-ups.

These penalties yielded a dehumanizing effect upon the prisoners. The relative tranquility of the first day was ensued by an unexpected rebellion on the morrow.

The prisoners ripped off the numbers and blockaded themselves by erecting their beds against the cell doors.

The guards had to call in reinforcements, and eventually shoot chilling CO2 via a fire extinguisher to quell the rebellion. The ringleaders of the mutiny were assigned to solitary confinement, and the harassment of the prisoners by the guards was steadily compounded following this episode.

Over the remainder of the experiment, special privileges were given to the more docile inmates (e.g., eating special food in front of their recalcitrant counterparts), as the guards grew increasingly aggressive toward the unruly prisoners.

Additionally, prisoners #8612 and #819 had emotional breakdowns. Furthermore, the guards permitted a visiting hour for family and friends, and a Catholic priest (a former prison chaplain) was invited in to assess how realistic the prison setting was.

The priest interviewed each prisoner, and informed the inmates that only the help of a lawyer could procure their release.


Although the experiment was supposed to last for 14 days, it ended following just 6 days.

Christina Maslach, a graduate student of Stanford, who was brought in for interviews with prisoners and the guards objected strongly to what she saw as the abuse of the prisoners at the hands of the guards.

Out of the nearly 50 outsiders who had seen the prison setting, she was the only one who seemed to be disturbed.

Zimbardo gave into her protest which was filled with outrage, and terminated the experiment.

Zimbardo and his team concluded that their experiment had unveiled how individuals would, with little resistance, conform to social roles others expect them to play. This would be especially true if such roles were strongly stereotyped, as in the case of the guards. Deindividuation and reinforcement, moreover, seemed to render the most potent explanation for the conduct of the experiment’s subjects.

Deindividuation: The subjects’ immersion in group norms seemed to lessen their sense of individual identity and responsibility.

The sadism of the guards for instance, seemed to stem from their group norms which had been further intensified by their uniforms.

In fact, most of the guards, following the experiment were surprised to realize that they had treated the prisoners with such brutality. The prisoners, for their part, were astounded that they had acted so submissively, despite having been assertive individuals in real life.

Reinforcement: It is possible that the inmates, via mostly negative and sometimes positive reinforcements, had learned that their submission to the guards could avert unpleasant experiences.

For instance, the punishments that resulted from insubordination would discourage them from rebelling whereas the special privileges they were granted, on account of docility, could encourage further submission.


Following this research, Zimbardo proposed changes to prisons and to guard training but his suggestions were not taken up and, in fact, prisons in the USA have been radically reformed in the last 25 years to make them less humane!

However, testimony about the research influenced Congress to change one law so that juveniles accused of federal crimes cannot be housed before trail with adult prisoners because of the likelihood of violence against them.

The study also gives a valuable insight into the power of situations and roles on behaviour.


Ethical Concerns

Evidence implies that the experimenters played a contributory role in fostering the guards’ abusive conduct toward the prisoners.

The study is often cited as an example of an unethical experiment. Zimbardo’s project also engendered regulations to preclude the ill-treatment of human subjects in future experiments.

Any replication of the Stanford Prison Experiment would be prohibited today by the American Psychological Association’s code of ethics. Ecological Validity (Bartels, 2015):

Ecological Validity

Movahedi and Banuazizi have noted, “the phenomenological significance of the loss of freedom in the mock prison and the real prison is vastly different” (Banuazizi & Movahedi, 1975).

Ex-convict Carlo Prescott who had helped Zimbardo create the simulated prison environment, acknowledged years later that the results and the simulation had been contrived as the guards’ sadistic conduct had been a reproduction of Prescott’s own subjective experiences (Prescott, 2005).

Zimbardo too, admitted in 2012 that the simulation had been a “minimally adequate representation” of what he had purportedly known about prison-life (Drury, Hutchens, Shuttlesworth & White, 2012).


The experiment’s want of generalizability barely escapes rigorous scrutiny. Zimbardo sought to simulate an American prison setting which hardly resembles prison environments in Asia, Africa or Europe.

Given the more individualistic propensities of American culture, the conduct of the prisoners in the experiment would have been substantially dissimilar to the behavior one could expect in an Asian society that is inclined more toward collectivistic norms.

Moreover, the inmates were mostly middle-class and Caucasian males. The sample consisted of 24 volunteers who were predominantly white, middle class, male students.

This is clearly a biased sample as all the participants are the same gender, age, ethnic group and of similar educational and social backgrounds. Hence it would be difficult to generalise the results of this study to other, different groups in society.

The conclusions of the study, thus, may not be as applicable to African American inmates raised in poverty, or upper-class white-collar criminals with unusually high levels of education.

About the Author

Ayesh Perera recently graduated from Harvard University, where he studied politics, ethics and religion. He is presently conducting research in neuroscience and peak performance as an intern for the Cambridge Center for Behavioral Studies, while also working on a book of his own on constitutional law and legal interpretation.

Fact Checking

Content is rigorously reviewed by a team of qualified and experienced fact checkers. Fact checkers review articles for factual accuracy, relevance, and timeliness. We rely on the most current and reputable sources, which are cited in the text and listed at the bottom of each article. Content is fact checked after it has been edited and before publication.

This article has been fact checked by Saul Mcleod, a qualified psychology teacher with over 17 years' experience of working in further and higher education. He has been published in psychology journals including Clinical Psychology, Social and Personal Relationships, and Social Psychology.

Cite this Article (APA Style)
Perera, A. (2022, May 13). The Stanford Prison Experiment. Simply Sociology.

APA Style References

Banuazizi, A. & Movahedi, S. (1975) Interpersonal dynamics in a simulated prison. American Psychologist, 30, 152–160.

Bartels, JM (2015). The Stanford Prison Experiment in introductory psychology textbooks: A content analysis. Psychology Learning & Teaching,14(1),36-50.

Drury, S., Hutchens, S. A., Shuttlesworth, D. E., White, C. L. (2012) Philip G. Zimbardo on his career and the Stanford prison experiment’s 40th anniversary. History of Psychology 15,161–170.

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.