The Stanford Prison Experiment

The Stanford Prison Experiment
Darrin Klimek / Getty Images

In 1971, psychologist Philip Zimbardo and his colleagues set out to create an experiment that looked at the impact of becoming a prisoner or prison guard. The Stanford Prison Experiment, also known as the Zimbardo Prison Experiment, went on to become one of the best-known (and controversial) in psychology's history.

The study has long been a staple in textbooks, articles, psychology classes, and even movies, but recent criticisms have called the study's scientific merits and value into question.

Purpose of the Stanford Prison Experiment

Zimbardo was a former classmate of the psychologist Stanley Milgram. Milgram is best known for his famous obedience experiment.

Zimbardo was interested in expanding upon Milgram's research. He wanted to further investigate the impact of situational variables on human behavior.

The researchers wanted to know how the participants would react when placed in a simulated prison environment.

The researchers wondered if physically and psychologically healthy people who knew they were participating in an experiment would change their behavior in a prison-like setting.

Participants in the Stanford Prison Experiment

The researchers set up a mock prison in the basement of Stanford University's psychology building. They selected 24 undergraduate students to play the roles of both prisoners and guards.

The participants were chosen from a larger group of 70 volunteers because they had no criminal background, lacked psychological issues, and had no significant medical conditions. The volunteers agreed to participate during a one to two-week period in exchange for $15 a day.

The Setting and Procedures

The simulated prison included three six-by-nine-foot prison cells. Each cell held three prisoners and included three cots.

Other rooms across from the cells were utilized for the jail guards and warden. One tiny space was designated as the solitary confinement room, and yet another small room served as the prison yard.

The 24 volunteers were then randomly assigned to either the prisoner group or the guard group. Prisoners were to remain in the mock prison 24 hours a day during the study.

Guards were assigned to work in three-man teams for eight-hour shifts. After each shift, guards were allowed to return to their homes until their next shift.

Researchers were able to observe the behavior of the prisoners and guards using hidden cameras and microphones.

Results of the Stanford Prison Experiment

So what happened in the Zimbardo experiment? While the experiment was originally slated to last 14 days, it had to be stopped after just six due to what was happening to the student participants. The guards became abusive, and the prisoners began to show signs of extreme stress and anxiety.

Some of these included:

  • While the prisoners and guards were allowed to interact in any way they wanted, the interactions were hostile or even dehumanizing.
  • The guards began to behave in ways that were aggressive and abusive toward the prisoners while the prisoners became passive and depressed.
  • Five of the prisoners began to experience severe negative emotions, including crying and acute anxiety, and had to be released from the study early.

Even the researchers themselves began to lose sight of the reality of the situation. Zimbardo, who acted as the prison warden, overlooked the abusive behavior of the jail guards until graduate student Christina Maslach voiced objections to the conditions in the simulated prison and the morality of continuing the experiment.

Impact of the Zimbardo Prison Experiment

The experiment became famous and was widely cited in textbooks and other publications. According to Zimbardo and his colleagues, the Stanford Prison Experiment demonstrated the powerful role that the situation can play in human behavior.

Because the guards were placed in a position of power, they began to behave in ways they would not usually act in their everyday lives or other situations. The prisoners, placed in a situation where they had no real control, became submissive and depressed.

In 2011, the Stanford Alumni Magazine featured a retrospective of the Stanford Prison Experiment in honor of the experiment’s 40th anniversary. The article contained interviews with several people involved, including Zimbardo and other researchers as well as some of the participants in the study.

Richard Yacco, one of the prisoners in the experiment, suggested that the experiment demonstrated the power that societal roles and expectations can play in a person's behavior.

In 2015, the experiment became the topic of a feature film titled The Stanford Prison Experiment that dramatized the events of the 1971 study.

Criticisms of the Stanford Prison Experiment

In the years since the experiment was conducted, there have been a number of critiques of the study. Some of these include:

Ethical Issues

The Stanford Prison Experiment is frequently cited as an example of unethical research. The experiment could not be replicated by researchers today because it fails to meet the standards established by numerous ethical codes, including the Ethics Code of the American Psychological Association.

Lack of Generalizability

Other critics suggest that the study lacks generalizability due to a variety of factors. The unrepresentative sample of participants (mostly white and middle-class males) makes it difficult to apply the results to a wider population.

Lack of Realism

The Zimbardo Prison Experiment is also criticized for its lack of ecological validity. Ecological validity refers to the degree of realism with which a simulated experimental setup matches the real-world situation it seeks to emulate.

While the researchers did their best to recreate a prison setting, it is simply not possible to perfectly mimic all of the environmental and situational variables of prison life. Because there may have been factors related to the setting and situation that influenced how the participants behaved, it may not really represent what might happen outside of the lab.

Recent Criticisms

More recent examination of the experiment's archives and interviews with participants have revealed major issues with the research's design, methods, and procedures that call the study's validity, value, and even authenticity into question.

These reports, including examinations of the study's records and new interviews with participants, have also cast doubt on some of the key findings and assumptions about the study.

Among the issues described:

  • One participant, for example, has suggested that he faked a breakdown so that he could leave the experiment because he was worried about failing his classes.
  • Other participants also reported altering their behavior in a way designed to "help" the experiment.
  • Evidence also suggests that the experimenters encouraged the behavior of the guards and played a role in fostering the abusive actions of the guards.

In 2019, the journal American Psychologist published an article debunking the famed experiment, detailing its lack of scientific merit, and concluding that the Stanford Prison Experiment was "an incredibly flawed study that should have died an early death."

In a statement posted on the experiment's official website, Zimbardo maintains that these criticisms do not undermine the main conclusion of the study—that situational forces can alter individual actions both in positive and negative ways.

Why was Zimbardo's experiment unethical?

Zimbardo's experiment was unethical due to a lack of fully informed consent, abuse of participants, and lack of appropriate debriefings. More recent findings suggest there were other significant ethical issues that compromise the experiment's scientific standing, including the fact that experimenters may have encouraged abusive behaviors.

A Word From Verywell

The Stanford Prison Experiment is well known both in and out of the field of psychology. While the study has long been criticized for many reasons, more recent criticisms of the study's procedures shine a brighter light on the experiment's scientific shortcomings.

11 Sources
Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Kendra Cherry, MSEd
Kendra Cherry, MS, is a psychosocial rehabilitation specialist, psychology educator, and author of the "Everything Psychology Book."