Stanford Prison Experiment

The Stanford Prison Experiment (SPE) is a highly influential and controversial study run by Philip Zimbardo and his colleagues at Stanford University in 1971. The researchers originally set out to support the notion that situational forces are just as powerful and perhaps more powerful than dispositional forces in influencing prison behavior. In addition to providing support for their hypothesis, the study was heavily covered in the mainstream media and had far-reaching ethical implications. Regardless, and perhaps because of its controversial nature, the SPE remains one of the most well-known experiments in social psychology.

The Purpose of the Stanford Prison Experiment

Stanford Prison ExperimentThe SPE was conceived as a reaction to the popular belief that the violent and oppressive nature of U.S. prisons and subsequent reports of humanitarian violations were due to the unique personality characteristics of the prisoners and guards. Because of self-selection, prison guards were believed to possess characteristics such as sadism and a lack of sensitivity. Prisoners, of course, are usually incarcerated because at some point in time they exhibited illegal behavior. Zimbardo and colleagues argued that this view discounts the powerful influence of the social situation in which guards are pitted against prisoners under a variety of social and political influences.

Stanford Prison Experiment Methodology

To test their hypothesis, Zimbardo and colleagues created a realistic mock prison in the basement of Stanford University. The participants included 21 male college students, specifically chosen for their normal responses on a battery of background questionnaires. The participants were randomly assigned to be either a guard or a prisoner, with an undergraduate research assistant acting as warden and Zimbardo himself taking on the role of superintendent. The prisoners stayed in the prison 24 hours per day, while the guards worked 8-hour shifts. Aside from a restriction on physical violence, guards were given great latitude in how they could deal with prisoners, including the rules they could establish and punishments they could dole out.

The experimenters went to great lengths to establish realism. Prisoners were unexpectedly “arrested” at their houses by the local police department, were taken to the police station to be charged their “crime” and brought to the prison at Stanford. Prisoners were assigned a number and wore only a smock, which was designed to deindividuate the prisoners. Guards were fitted with a uniform, nightstick, and reflective sunglasses to establish power. The prison cells consisted of a 6- by 9-foot space furnished with only a cot. To further increase realism, a catholic priest and attorney were brought in and a parole board was established.

Once the participants had arrived at the prison, the situation escalated at a surprising rate. On the second day, a prisoner rebellion was quickly quelled by the guards, who punished the prisoners through means conceived without guidance from the experimenters. For example, prisoners were stripped naked, forced to do menial tasks, and in many cases were deprived of their cots, meals, and bathroom privileges. After the attempted revolt and subsequent punishment, five prisoners began to experience extreme emotional reactions and were eventually released. As the obedience tactics became more brutal and humiliating and prisoners displayed increasingly negative affectivity, Zimbardo eventually decided to end the study on the sixth day of what had been planned as a 2-week study.

Findings of the Stanford Prison Experiment

Zimbardo and colleagues construed the increasingly hostile behavior of the guards and increasingly passive behavior of the prisoners, each of which had started out as groups of normal young men, as evidence that the extreme nature of the prison situation breeds such volatile and desperate behavior. Indeed, the SPE is often cited as evidence for the strong role of the situation over individuals in ways in which they often do not predict. The researchers also compared their work to Stanley Milgram’s research on obedience in that both provide support for the notion that given an extreme situation, good people can be coerced into doing evil things. Despite these exciting findings, the SPE has been criticized from both a methodological and ethical standpoint.

Methodological Criticisms of the Stanford Prison Experiment

Methodologically, critics have argued that participants of the SPE never fully accepted the situation as real and were merely playing the stereotypic roles of prisoners and guards. In essence, it was argued that the results were driven by demand characteristics of the experimental situation. It did not help the original authors’ argument that Zimbardo himself played a prominent role in the experiment, sometimes guiding the way in which the study played out. Regardless, evidence suggests that participants did internalize the situation, as well as their roles in the situation. For instance, only one-tenth of the conversations between prisoners contained speech about life outside of the experiment.

Ethical Criticisms of the Stanford Prison Experiment

The SPE was likely more controversial from an ethical point of view. The ethical implications of the study, as well as Zimbardo’s dual role as investigator and superintendent of the Stanford prison were highly criticized at the time. Zimbardo himself admitted that his own acceptance of the prison situation and his desire to run a good prison clouded his judgment, suggesting that even he had internalized his role in the situation. Although the experiment did conform to the guidelines set forth by the ethics review board at the time, few would argue that the sadistic and humiliating acts performed during the study were ethical by today’s standards. Even Zimbardo admits that it was unethical for the study to continue after the first prisoner showed an extreme negative reaction.

Stanford Prison Experiment Replications

Stephen Reicher and S. Alexander Haslam attempted to replicate the SPE, skirting ethical guidelines through allowing the experiment to take place in the context of a British reality television show. The results of the SPE were not replicated; in Reicher and Haslam’s version, the guards never organized themselves and were eventually overthrown.


  1. Adam, D. (2002). Reality TV show recreates famed social study. Nature, 417, 213.
  2. Haney, C., Banks, C., & Zimbardo, P. (1973). Interpersonal dynamics in a simulated prison. International Journal of Criminology and Penology, 1, 69-97.
  3. Zimbardo, P. G., Maslach, C., & Haney, C. (1999). Reflections on the Stanford Prison Experiment: Genesis, transformations, consequences. In T. Blass (Ed.), Obedience to authority: Current perspectives on the Milgram paradigm (pp. 193-237). Mahwah, NJ: Erlbaum.