The Stanford prison experiment (SPE), also referred to as the Zimbardo prison experiment (ZPE), 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:
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