Stanford Prison Experiment

The Stanford Prison Experiment (SPE) has become a classic in the social sciences for its dramatic demonstration of the power of situational processes over individual dispositions of its participants. It pitted a powerful set of situational variables, which together comprised what is worse in the psychological experience of imprisonment, against the will to resist by a group of normal, healthy young men playing roles of prisoners or guards.

The SPE was conducted in 1971 by a group of Stanford research psychologists, led by Professor Phillip Zimbardo, and two of Zimbardo’s graduate students, Curtis Banks and Craig Haney. The experiment was designed to control for the individual personality variables that were often used at that time to explain behavior in prison and other institutional settings. That is, the researchers in the SPE neutralized the explanatory argument that pathological traits alone could account for extreme and abusive behavior by (a) selecting a group of participants who were psychologically healthy and who had scored in the normal range of the numerous personality variables that they measured and selected for, and (b) assigning participants to either the role of prisoner or guard on a completely random basis. The behavior that resulted when these otherwise healthy, normal participants were placed in the extreme environment of a simulated prison would have to be explained largely if not entirely on the basis of the characteristics of the social setting or situation in which they had been placed.

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The setting itself was designed to be as similar to an actual prison as possible. Constructed in the basement of the Psychology Department at Stanford University, the “Stanford County Prison” had barred doors on the small rooms that served as cells, cots on which the prisoners slept, a hallway area that was converted to a prison “yard” where group activities were conducted, and a small closet that served as a short-term “solitary confinement” cell that could be used for disciplining unruly prisoners. The prisoners wore uniforms that were designed to deemphasize their individuality and underscore their powerlessness. Guards, on the other hand, donned military-like garb, complete with reflecting sunglasses and nightsticks. These guards generated a set of rules and regulations that in many ways resembled those in operation in actual prisons, and prisoners were expected to comply with their orders. However, guards were instructed not to resort to physical force to gain prisoner compliance.

Despite the lack of any legal mandate for the “incarceration” of the prisoners and despite the fact that both groups were told that they had been randomly assigned to their roles (so that, e.g., guards knew that prisoners had done nothing to “deserve” their degraded prisoner status), the behavior that ensued was remarkably similar to behavior that takes place inside actual prisons and surprisingly extreme in intensity and effect. Thus, initial prisoner resistance and rebellion was met forcibly by guards, who quickly struggled to regain their power and then proceeded to escalate their mistreatment of prisoners throughout the study at the slightest sign of affront or disobedience. In some instances, the guards conspired to physically mistreat prisoners outside the presence of the experimenters and to leave prisoners in the solitary confinement cell beyond the 1-hour limit that the researchers had set.

Conversely, prisoners resisted the guards’ orders at first but then succumbed to their superior power and control. Some prisoners had serious emotional breakdowns in the course of the study and had to be released; others became compliant and conforming, rarely if ever challenging the “authority” of the guards. Despite the fact that the researchers could not keep the prisoners in the study against their will (and they had been informed at the outset of the study of their legal right to leave), as the study proceeded, they “petitioned” the prison “administrators” for permission to be “paroled” or returned passively to their cells when their requests were denied. By the end of the study, they had disintegrated as a group. The guards, on the other hand, solidified and intensified their control. Although some of the guards were more extreme and inventive in the degradation they inflicted on the prisoners, and others were more passive and less involved, none of the guards intervened to restrain the behavior of their more abusive colleagues. Although the study was designed to last for two full weeks, the extreme nature of the behavior that occurred led the researchers to terminate it after only 6 days.

Controversial from the outset, and widely discussed and cited since it was conducted, the study has come to stand in psychology and related disciplines as a demonstration of the power of situations—especially extreme institutional settings such as prisons—to shape and control the behavior of the persons placed inside them. Its results give lie to the notion that extreme social behavior can only—or even mostly—be explained by the extreme characteristics of persons who engage in it. The SPE counsels us to look instead to the characteristics of the settings or situations in which the behavior occurs. It also stands as a challenge to what might be termed the “presumption of institutional rationality”—that is, the tendency to assume that institutions operate on the basis of an inherent rationality that should be accepted rather than questioned. Instead, the SPE (itself the most “irrational” of prisons, in the sense that the guards had no legal authority over the prisoners, who in turn, had committed no crimes that warranted their punishment) suggests that a kind of “psycho-logic” may operate in these settings, which controls role-bound behavior, whether or not that behavior actually furthers legitimate goals.

A recent detailed chronology of the events that transpired in the Stanford prison experiment from initial city police arrests through final termination is provided in Zimbardo’s book, The Lucifer Effect: Understanding How Good People Turn Bad. The story unfolds in the present tense, first-person narrative for 8 chapters, with subsequent chapters discussing the ethics of such research, presenting its various data sources, and then setting that study in a broader context of other social science research that also demonstrates the power of social situations to influence or dominate individual behavior (additional information is available at All original data and forms have been stored at the archives of the History of American Psychology in Akron, Ohio.


  1. Haney, C. (1999). Reflections on the Stanford Prison Experiment: Genesis, Transformations, Consequences (“The SPE and the Analysis of Institutions”). In T. Blass (Ed.), Obedience to authority: Current perspectives on the Milgramparadigm (pp. 221-237). Hillsdale, NJ: Lawrence Erlbaum.
  2. Haney, C. (2006). Reforming punishment: Psychological limits to the pains of imprisonment. Washington, DC: American Psychological Association Books.
  3. Haney, C., Banks, C., & Zimbardo, P. (1973). Interpersonal dynamics in a simulated prison. International Journal of Criminology and Penology, 1, 69-97.
  4. Haney, C., & Zimbardo, P. (1977). The socialization into criminality: On becoming a prisoner and a guard. In J. Tapp & F. Levine (Eds.), Law, justice, and the individual in society: Psychological and legal issues (pp. 198-223). New York: Holt, Rinehart, and Winston.
  5. Haney, C., & Zimbardo, P. (1998). The past and future of U.S. prison policy: Twenty-five years after the Stanford prison experiment. American Psychologist, 53, 709-727.
  6. Zimbardo, P. (2007). The Lucifer effect: Understanding how good people turn bad. New York: Random House.

Return to the overview of Sentencing and Incarceration in Forensic Psychology.