The Stanford prison experiment (SPE) was a controversial psychological experiment performed in August 1971 at Stanford University. It was designed to be a two-week simulation of a prison environment that examined the effects of situational variables on participants' reactions and behaviors. Stanford psychology professor Philip Zimbardo managed the research team that administered the study.
Participants were recruited from the local community through an advertisement in the newspapers offering US$15 (equivalent to $119.25 in 2025) per day to male college students who agreed to participate in a "psychological study of prison life". Twenty-four participants were chosen after assessments of psychological stability and then assigned randomly to the role of prisoners or prison guards. Critics have questioned the validity of these methods. Those volunteers selected to be "guards" were given uniforms designed specifically to de-individuate them, and they were instructed to prevent prisoners from escaping. The experiment started officially when "prisoners" were arrested by the real police of Palo Alto, California. During the next five days, psychological abuse of the prisoners by the "guards" worsened. After psychologist Christina Maslach visited to evaluate the conditions, she was troubled to see how study participants were behaving and treated and confronted Zimbardo. He ended the experiment on the sixth day, reporting that he was surprised by how much he had internalized his role as superintendent in the prison.
The experiment has been referenced and critiqued as an example of an unethical psychological experiment, and the harm inflicted on the participants in this and other experiments during the post-World War II era prompted American universities to improve their ethical requirements and institutional review for human experiment subjects in order to prevent them from being similarly harmed. Other researchers have found it difficult to reproduce the study, especially given those constraints.
Certain critics have also described the experiment as unscientific and fraudulent. In particular, Thibault Le Texier asserted that Zimbardo asked the guards to behave in certain ways in order to confirm his conclusions, which were largely written in advance of the experiment. Zimbardo claimed that Le Texier's article was mostly an ad hominem attack on him which ignored available data that contradicted his arguments, but original participants who were interviewed for the National Geographic documentary The Stanford Prison Experiment: Unlocking the Truth largely confirmed many of Le Texier's claims.
Funding and methodology
Philip Zimbardo's official website described the experiment goal as follows:
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 prison and then carefully note the effects of this institution on the behavior of all those within its walls.
A 1996 article from the Stanford News Service described the experiment goal in a more detailed way:
Zimbardo's primary reason for conducting the experiment was to focus on the power of roles, rules, symbols, group identity and situational validation of behavior that generally would repulse ordinary individuals. "I had been conducting research for some years on deindividuation, vandalism and dehumanization that illustrated the ease with which ordinary people could be led to engage in anti-social acts by putting them in situations where they felt anonymous, or they could perceive of others in ways that made them less than human, as enemies or objects," Zimbardo told the Toronto symposium in the summer of 1996.
The study was funded by the US Office of Naval Research to understand antisocial behavior. The United States Navy and the United States Marine Corps wanted to investigate conflict between military guards and prisoners.
Publishing
Before publishing in American Psychologist and other peer-reviewed journals, the researchers reported the findings in Naval Research Reviews, International Journal of Criminology and Penology (IJCP), and the New York Times Magazine. David Amodio, psychology instructor at both New York University and the University of Amsterdam, dismissed Zimbardo's study, stating that releasing the article to an "obscure journal" demonstrated that Zimbardo was unable to convince fellow psychologists of the validity and reliability of his study. By publishing in other journals before publishing in a scientific peer-reviewed journal, Zimbardo violated the tradition of scientific dissemination.
Zimbardo has stated that the grant agreement with the Office of Naval Research included a requirement to publish data in their journal, Naval Research Reviews. He states that the International Journal of Criminology and Penology invited Zimbardo to write about his study in their journal, and he then wrote an article with the New York Times Magazine to share the findings with a broad audience. He states that the article still needed to pass through the very strict requirements of the American Psychologist, the official journal of the American Psychological Association, in order to be published. After publishing the article in the American Psychologist, the findings were also reported in other peer-reviewed journals and books.
Preparation
Recruitment and selection
After Zimbardo received approval from the university to perform the experiment, study participants were recruited using an advertisement in the "help wanted" section of the Palo Alto Times and The Stanford Daily newspapers in August 1971:
Male college students needed for psychological study of prison life. $15 per day for 1–2 weeks beginning Aug. 14. For further information and applications, come to Room 248, Jordan Hall, Stanford U.
Seventy five men applied; after screening assessments and interviews, 24 were selected to participate in a two-week prison simulation. The applicants were predominantly white, middle-class, and appeared to be stable psychologically and healthy. The group of subjects was selected intentionally to exclude those with criminal backgrounds, psychological impairments, or medical problems.
On a random basis, half of the subjects were assigned the role of guard (nine plus three potential substitutes), and half were assigned to the role of prisoner (also nine plus three potential substitutes). They agreed to participate for a 7- to 14-day period for $15 per day (roughly equivalent to $126.50 in 2026).
Prison environment
The day before the experiment began, offices in the psychology department were repurposed into small mock prison cells, each arranged to hold three prisoners. There was a small corridor for the prison yard, a closet for solitary confinement, and a bigger room across from the prisoners for the guards and warden.
The experiment was performed in a 35 ft (11 m) section of the basement of Jordan Hall, Stanford's psychology building. The prison had two fabricated walls: one at the entrance and one at the cell wall to block observation. Each cell (7 ft × 10 ft or 2.1 m × 3.0 m) was unlit, was intended to house 3 prisoners and had a cot (with mattress, sheet, and pillow) for each prisoner. Prisoners were confined and were to stay in their cells and the yard all day and night until the study was finished. In contrast, the guards were to stay in a different environment, separate from the prisoners. The guards were given access to special areas for rest and relaxation, were told to work in teams of three for eight-hour shifts, and were not required to stay on-site after their shift.
Roles
Zimbardo assumed the role of Superintendent, and an undergraduate research assistant, David Jaffe, assumed the role of Warden.
Digitized recordings available on the official SPE website were widely discussed in 2017, particularly one in which warden David Jaffe tried to influence the behavior of one of the guards by encouraging him to participate more and be more "tough" for the benefit of the experiment.
Orientation
The researchers had an orientation session for the guards the day before the experiment began, during which the guards were instructed not to harm the prisoners physically or withhold food or drink, but to maintain law and order. The researchers provided the guards with wooden batons to establish their status, deindividuating clothing similar to that of an actual prison guard (khaki shirt and pants from a local military surplus store), and mirrored sunglasses to prevent eye contact and create anonymity.
Based on recordings from the experiment, guards were instructed by the researchers to refer to prisoners by number rather than by name. This, according to Zimbardo, was intended to diminish the prisoners' individuality. With no control, prisoners learned they had little effect on what happened to them, ultimately causing them to stop responding and give up.
Zimbardo has explained that guard orientations in the prison system instructed the guards to exert power over the prisoners. Further, Zimbardo asserts that his fellow researcher explicitly instructed the guards not to inflict physical harm on the prisoners, but at the same time make the prisoners feel that they were in an actual prison.
Asking a person role-playing a guard in a prison simulation to be "firm" and "in the action" is mild compared to the pressure exerted by actual wardens and superior officers in real-life prison and military settings, where guards failing to participate fully can face disciplinary hearings, demotion, or dismissal.
Critiques of scientific validity
In 1975, Ali Banuazizi and Siamak Movahedi argued that the behavior of the participants in the SPE was a result of demand characteristics and not the prison environment, that there is no single definition of prisoner behavior, and that participants were simply acting in the role in which they had been cast. Participants' behavior may have also been influenced by knowing that they were watched (Hawthorne effect). Instead of being restrained by fear of an observer, guards may have behaved more aggressively when supervisors observing them did not restrain them. The study was also criticized for demand characteristics in 2012 by psychologist Peter Gray, who argued that participants in psychological experiments are more likely to do what they believe the researchers want them to do, and specifically in the case of the SPE, "to act out their stereotyped views of what prisoners and guards do."
In 2007, Thomas Carnahan and Sam McFarland argued that the experiment suffers from selection bias, as the recruiting posters for SPE specifically advertised participation in a study about prison life. Carnahan and McFarland argued that those who applied to participate with the SPE thus likely already had traits associated with abusiveness, aggression, authoritarianism, Machiavellianism, social dominance orientation, and narcissism. Further, low dispositional empathy and altruism would also be indicators of someone who would volunteer.
In 2018, Thibault Le Texier, a French researcher, questioned the scientific validity and merit of the experiment in his book Histoire d'un mensonge (The History of a Lie). He further discussed his critiques in an article published by the American Psychological Association in 2019. Le Texier asserts his arguments using testimonies of those participants who were assigned as guards. In his opinion, the sadism and submission displayed in the SPE was directly caused by Zimbardo's instructions to the guards and the guards' desire to please the researchers. In particular, he has established that the guards were asked directly to behave in certain ways in order to confirm Zimbardo's conclusions, which were largely written in advance of the experiment. Zimbardo and colleagues contend that Le Texier's critiques were mostly ad hominem and ignored available data that contradicts his counterarguments.
In 2020, Dutch historian Rutger Bregman claimed that the experiment was suspect. In his book Humankind: A Hopeful History he suggests that the guards were urged to act aggressively towards the prisoners. He argues that researchers in similar experiments intentionally created hostility between groups and then interpreted the findings to suit their needs.
Participant guard David Eshelman acknowledged that his theater background lent itself well to his role as guard, that he purposely thought of new ways to demean the prisoners – on one shift, Eshelman instructed the prisoners to simulate sodomy. Zimbardo has responded to this argument by stating that other guards acted similarly or engaged with Eshelman in the treatment of prisoners. While it is possible that one guard adopted his behavior from a movie (Eshelman identified with the warden in Cool Hand Luke), others did not. Also, guards on a different shift than Eshelman exacted similar acts of emotional and mental brutality. Zimbardo further argues that the behaviors of the participant guards were not unlike those of real-world prison atrocities or the torture and abuse of prisoners by American soldiers at Abu Ghraib. Most of the guards have stated since the SPE that they were intentionally acting.
The BBC prison study has indicated the importance of direction, of the form displayed by Zimbardo when briefing guards in the Stanford experiment, in the emergence of tyranny.
Carlo Prescott as a prison consultant
In 2005, an article was published by Carlo Prescott in The Stanford Daily, explaining that the antagonistic tactics used by the guards were ones that he experienced during his time spent in San Quentin. He shared each one in detail with the researchers prior to the experiment. In Prescott's opinion, the participants with the experiment, having no experience as prison guards, could not have acted in the ways they did unless they had been told of the explicit details of the actions they took.
Zimbardo has stated that he believed that the article was not written by Prescott, but rather by the screenwriter and producer Michael Lazarou, who had attempted unsuccessfully to get the movie rights to the story of the SPE. In Zimbardo's opinion, Prescott would not have written in such a legalistic way, and Zimbardo claims that, in telephone records and emails obtained by Brett Emory, the SPE movie's producer asserted Prescott was not the author.
Events
Saturday, August 14: Set up
The small mock prison cells were set up, and the participants who had been assigned a guard role attended an orientation where they were briefed and given uniforms.