Stanford Prison Experiment

For example, prisoners and guards may have personalities that make conflict inevitable, with prisoners lacking respect for law and order and guards being domineering and aggressive.

Alternatively, prisoners and guards may behave in a hostile manner due to the rigid power structure environment in prisons.

Zimbardo predicted the situation made people act the way they do rather than their disposition (personality).

Procedure

Recruitment and Screening

• Call for Volunteers: Philip Zimbardo placed an advertisement asking for volunteers to participate in a study of the psychological effects of prison life.
  

• Applicant Pool: Seventy-five men responded to the ad.

• Diagnostic Interviews: The volunteers were interviewed and took personality tests designed to screen out any with psychological issues, medical conditions, or histories of crime/drug abuse.

• Final Selection: Only twenty-four men deemed physically and mentally stable, mature, and least involved in antisocial behaviors were chosen. Two were held in reserve; one participant dropped out, leaving ten prisoners and eleven guards.

Random Assignment and Compensation

• Random Roles: Participants were randomly assigned to be either prisoners or guards. Neither group knew each other beforehand.

• Payment: Each participant was paid $15 per day for the duration of the experiment.

Day 1: Setting the Stage and Initial Arrests

Surprised Arrests and Initial Processing

• Surprise Arrests: Prisoners were unexpectedly arrested at their homes without prior warning. Local police officers assisted in taking them into custody to add realism.

• Police Station Booking: At the station, the prisoners were fingerprinted, photographed, and formally “booked.”

• Transportation to Stanford: After booking, each prisoner was blindfolded and driven to the mock prison, located in the basement of Stanford University’s psychology department.

Entering the Mock Prison

• Prison Layout: The basement had barred doors, windows, small cells, and bare walls—carefully arranged to resemble an actual detention facility.

• Deindividuation Process: Upon arrival, the prisoners were:

  • Stripped naked and deloused

  • Relieved of personal possessions

  • Issued smock-like clothing labeled with ID numbers only

  • Given a tight nylon cap to cover their hair

  • Fitted with a lockable chain around one ankle

• Loss of Identity: Prisoners were referred to solely by their assigned numbers, a tactic intended to induce anonymity and submission.

Guard Uniforms and Instructions

• Guard Attire: The guards wore identical khaki uniforms and sported reflective sunglasses that concealed eye contact. They also carried a whistle and police-style billy club.

• Shifts: Three guards worked in eight-hour rotations, with additional guards on call if needed.

• Role Expectations: Though informed that no physical violence was permitted, guards were told to do whatever was necessary to maintain order and command respect.

Observing and Role-Playing

• Zimbardo’s Dual Role: Zimbardo oversaw the experiment as a psychologist but also assumed the role of the “warden,” monitoring both guards and prisoners.

• First Impressions: At the end of Day 1, conditions were tense but largely uneventful. The stage was set for the accelerated role adoption and power struggles that would soon follow.

Findings

Within a very short time, both guards and prisoners were settling into their new roles, with the guards adopting theirs quickly and easily.

The guards exhibited increasingly authoritarian and abusive behaviors, while the prisoners displayed signs of psychological distress, passivity, and even emotional breakdowns.

The experiment, initially planned for two weeks, was halted after only six days due to the unexpectedly rapid and intense negative psychological effects on the participants.

Escalation of Punishments

The prisoners were taunted with insults and petty orders, they were given pointless and boring tasks to accomplish, and they were generally dehumanized.

• Push-Ups & Insults: Guards regularly forced prisoners to do push-ups (at times with other prisoners sitting on them), subjected them to verbal taunts, and assigned menial tasks like cleaning toilets by hand.

• Further Humiliation: The failure of the Day 2 rebellion emboldened the guards. They used group punishments to discourage solidarity, punishing the entire cell for one prisoner’s perceived infraction.

Growing Submission

• Fear and Control: After witnessing the guards’ forceful response on Day 2, many prisoners became increasingly anxious. They meticulously followed prison rules, ranging from standing a certain way during counts to fulfilling extra chores, hoping to avoid punishment.
• Desire to Stay Under the Radar: Some prisoners attempted to stay as inconspicuous as possible, limiting eye contact with guards and speaking only when spoken to. They feared that any defiance, however minor, would invite harsher punishment.
• Peer Pressure & Informing: The cumulative fear led certain prisoners to monitor each other. They scolded those who disobeyed rules and, in some cases, tattled on each other to the guards. This dynamic further broke down prisoner unity.
• Psychological Toll: Trapped in an environment of uncertainty, the men felt there was no way out, turning compliance into a survival strategy. The psychological impact set the stage for the extreme stress reactions and breakdowns that followed.

First Emotional Breakdown (Prisoner #8612)

• Acute Distress: Less than 36 hours in, Prisoner #8612 began experiencing uncontrollable crying, anger, and disorganized thinking.

• Informant Offer: Guards labeled him “weak” but offered a path to stay if he became an informant. Instead, #8612 returned to the cell, warning others: “You can’t leave. You can’t quit.”

Day 4: Visitors and Mounting Tension

By Day 4, fear and suspicion dominated the prison environment. Guards used the rumored escape plot as a pretext to ramp up punishments, ensuring prisoners remained disoriented, exhausted, and compliant.

This environment of unpredictable harassment and constant humiliation would grow even more intense in the experiment’s final days.

Day 5: Religious Visit and More Emotional Collapse

By Day 5, the simulated prison’s intense emotional toll was unmistakable.

The combination of institutional practices (like random punishments, forced chanting, and solitary confinement) and psychological entrapment (viewing exit as impossible) placed inmates on the brink of collapse.

The meltdown of Prisoner #819, followed by Zimbardo’s critical reminder that “this is just an experiment,” captured both the depth of the prisoners’ immersion and the experimenters’ growing realization that the situation was spinning out of control.

The Priest’s Evaluation

• Chaplains’ Intervention: A Catholic priest, formerly a prison chaplain, was invited to assess how realistic the prison environment was. Surprisingly, half the prisoners identified themselves only by their ID numbers.

• Legal Advice: The priest told prisoners that hiring a lawyer was their best chance of leaving, emphasizing how trapped they felt in the simulated incarceration.

Climate of Entrapment

• Heightened Anxiety: By Day 5, the cumulative stress of random punishments, lost sleep, and an atmosphere of constant threat had profoundly unsettled the prisoners. Most had ceased any talk of revolt or group defiance following the failed rebellion on Day 2.

• No Concept of Quitting: Despite technically having the right to leave, prisoners often behaved as though they were bound by a real sentence. With multiple breakdowns already witnessed, fear of further punishment or dismissal as “weak” kept many resigned to their fate.

Prisoner #819’s Meltdown

• Another Breakdown: While meeting the priest, Prisoner #819 began sobbing uncontrollably. Researchers moved him to a separate room to rest.

• Public Shaming Ritual: Meanwhile, guards lined up the other inmates and led them in chanting, “Prisoner #819 is a bad prisoner. Because of what Prisoner #819 did, my cell is a mess, Mr. Correctional Officer.”

• Peer Pressure: Hearing the repeated chants, #819 was overwhelmed by guilt, believing he had let down his fellow prisoners.

• Reality Check: Zimbardo intervened, reminding #819 that none of this was real, that the guards and prisoners were simply students, and that he could leave anytime. In a moment of clarity, #819 recognized the experiment’s artificial nature and agreed to go.

Strained Authority and Doubt

• Guard Frustration: Even as they continued to impose punishments, some guards seemed increasingly frustrated by the emotional volatility of the prisoners, who they saw as “uncooperative” or “unpredictable.”

• Experimenters’ Role: The research team, including Zimbardo, still tried to maintain the study’s structure. Yet the boundary between observation and active involvement blurred as they managed each emotional crisis and sought to avert further breakdowns.

• Momentum Toward Termination: By the end of Day 5, multiple prisoners had exhibited severe stress, and the guards were escalating their techniques. These pressures set the stage for the abrupt end of the experiment on Day 6.

Day 6: Abrupt Termination

Day 6 demonstrates the tipping point at which the mounting abuses, emotional breakdowns, and an outsider’s moral alarm forced the experiment’s abrupt termination.

While it failed to run the planned two-week course, this dramatic ending cemented the Stanford Prison Experiment’s status as one of the most controversial and influential studies in social psychology.

Escalating Abuse

• Unchecked Guard Power: By Day 6, the guards had grown increasingly emboldened, imposing stricter surveillance and more arbitrary punishments. With prisoners exhausted from insomnia, constant harassment, and severe emotional strain, guard aggression reached a notable high point.

• Overt Sadism: Some guards seemed to relish their authority, devising more degrading tasks and verbally taunting prisoners who showed any sign of resistance or distress. Scenes of tearful pleas, panic, and rage had become commonplace among the inmates.

Outside Perspective: Christina Maslach

• Scheduled Interviews: Christina Maslach, a recent Stanford Ph.D., was brought in to interview the guards and prisoners. Until her arrival, most outside observers had accepted or even praised the “realism” of the simulation.

• Immediate Shock: Maslach was appalled by the guards’ behavior, witnessing verbal abuse and palpable fear among the prisoners. In her words, “It’s terrible what you are doing to these boys!”

• Confrontation with Zimbardo: She openly challenged Zimbardo and his team, calling out the ethical violations and emotional harm. This frank condemnation played a critical role in snapping Zimbardo out of his “prison superintendent” mindset.

End of the Experiment

• Planned vs. Reality: Though originally intended to run two weeks, the experiment ended prematurely on the sixth day due to the emotional breakdowns of prisoners, and excessive aggression of the guards.

• Zimbardo’s Realization: Zimbardo later acknowledged how fully he had internalized the warden role: “I was thinking like a prison superintendent rather than a research psychologist.” Maslach’s intervention helped him see that the study had crossed ethical lines.

Conclusion

According to Zimbardo and his colleagues, the Stanford Prison Experiment revealed how people will readily conform to the social roles they are expected to play, especially if the roles are as strongly stereotyped as those of the prison guards.

Because the guards were placed in a position of authority, they began to act in ways they would not usually behave in their normal lives.

The “prison” environment was an important factor in creating the guards’ brutal behavior (none of the participants who acted as guards showed sadistic tendencies before the study).

Therefore, the findings support the situational explanation of behavior rather than the dispositional one.

Zimbardo proposed that two processes can explain the prisoner’s “final submission.”

Deindividuation may explain the behavior of the participants; especially the guards. This is a state when you become so immersed in the norms of the group that you lose your sense of identity and personal responsibility.

The guards may have been so sadistic because they did not feel what happened was down to them personally – it was a group norm. They also may have lost their sense of personal identity because of the uniform they wore.

Also, learned helplessness could explain the prisoner’s submission to the guards. The prisoners learned that whatever they did had little effect on what happened to them.

In the mock prison the unpredictable decisions of the guards led the prisoners to give up responding.

After the prison experiment was terminated, Zimbardo interviewed the participants. Here’s an excerpt:

‘Most of the participants said they had felt involved and committed. The research had felt “real” to them. One guard said, “I was surprised at myself.

I made them call each other names and clean the toilets out with their bare hands. I practically considered the prisoners cattle and I kept thinking I had to watch out for them in case they tried something.”

Another guard said “Acting authoritatively can be fun. Power can be a great pleasure.” And another: “… during the inspection I went to Cell Two to mess up a bed which a prisoner had just made and he grabbed me, screaming that he had just made it and that he was not going to let me mess it up.

He grabbed me by the throat and although he was laughing I was pretty scared. I lashed out with my stick and hit him on the chin although not very hard, and when I freed myself I became angry.”’

Most of the guards found it difficult to believe that they had behaved in the brutal ways that they had.

Many said they hadn’t known this side of them existed or that they were capable of such things.

The prisoners, too, couldn’t believe that they had responded in the submissive, cowering, dependent way they had. Several claimed to be assertive types normally.

When asked about the guards, they described the usual three stereotypes that can be found in any prison: some guards were good, some were tough but fair, and some were cruel.

A further explanation for the behavior of the participants can be described in terms of reinforcement. 

The escalation of aggression and abuse by the guards could be seen as being due to the positive reinforcement they received both from fellow guards and intrinsically in terms of how good it made them feel to have so much power.

Similarly, the prisoners could have learned through negative reinforcement that if they kept their heads down and did as they were told, they could avoid further unpleasant experiences.

Ethical Issues

In 1973, the American Psychological Association (APA) conducted an investigation into the ethical aspects of the prison study and concluded that it had satisfied the profession’s existing ethical standards at the time.

This conclusion is noteworthy but needs to be understood within the historical context of ethical guidelines in psychological research.

Ethical standards evolve, and what was deemed acceptable in 1973 might be considered unacceptable by today’s standards.

The study has received many ethical criticisms, including lack of fully informed consent by participants as Zimbardo himself did not know what would happen in the experiment (it was unpredictable).

Also, the prisoners did not consent to being “arrested” at home. The prisoners were not told partly because final approval from the police wasn’t given until minutes before the participants decided to participate, and partly because the researchers wanted the arrests to come as a surprise.

However, this was a breach of the ethics of Zimbardo’s own contract that all of the participants had signed.

Informed Consent

For research to be ethical, participants must provide their voluntary consent based on a clear understanding of what their participation will entail, including any potential risks and discomforts.

The advertisement described a “psychological study of prison life” which might not have adequately conveyed the harsh realities that unfolded.

It is debatable whether participants were truly aware of the potential for the extreme psychological distress and dehumanising treatment they would experience.

This raises questions about whether the consent obtained was truly informed, as participants may not have been aware of the potential for severe emotional trauma and the loss of personal autonomy within the simulated prison environment.

Protecting Participants from Harm

One of the earliest and most prominent ethical criticisms came from Erich Fromm in 1973, who directly addressed the unethical nature of the harsh conditions imposed on the prisoners.

This critique highlights the fundamental question of whether the potential insights gained from the experiment justified the psychological distress and suffering experienced by the participants.

Although guards were explicitly instructed not to physically harm prisoners at the beginning of the Stanford Prison Experiment, they were allowed to induce feelings of boredom, frustration, arbitrariness, and powerlessness among the inmates.

The prisoners were subjected to cruel and dehumanising abuse at the hands of their peers acting as guards.

This included instances of being taunted, stripped naked, deprived of sleep, and forced to use plastic buckets as toilets

One prisoner had to be released after 36 hours because of uncontrollable bursts of screaming, crying, and anger.

These conditions, imposed within a simulated environment, raised serious ethical questions about the responsibility of the researchers for the well-being of their participants.

Here’s a quote from Philip G. Zimbardo, taken from an interview on the Stanford Prison Experiment’s 40th anniversary (April 19, 2011):

“In the Stanford prison study, people were stressed, day and night, for 5 days, 24 hours a day. There’s no question that it was a high level of stress because five of the boys had emotional breakdowns, the first within 36 hours.

Other boys that didn’t have emotional breakdowns were blindly obedient to corrupt authority by the guards and did terrible things to each other. And so it is no question that that was unethical. You can’t do research where you allow people to suffer at that level.”

However, in Zimbardo’s defense, the emotional distress experienced by the prisoners could not have been predicted from the outset.

Approval for the study was given by the Office of Naval Research, the Psychology Department, and Stanford’s Human Subjects Research Committee.

This Committee also did not anticipate the prisoners’ extreme reactions that were to follow.

Alternative methodologies were looked at that would cause less distress to the participants but at the same time give the desired information, but nothing suitable could be found.

Dual Role

The dual role of Philip Zimbardo as the principal investigator and the prison superintendent also presents significant ethical concerns.

By actively participating in the experiment as the superintendent, Zimbardo arguably blurred the lines between researcher and participant-influencer.

His focus shifted from objective observation to managing the simulated prison environment.

This immersion in the role may have compromised his ability to maintain a detached, ethical oversight of the study and to recognise and respond appropriately to the escalating harm experienced by the prisoners.

Zimbardo (2008) later noted, “It wasn’t until much later that I realized how far into my prison role I was at that point — that I was thinking like a prison superintendent rather than a research psychologist.“

This led him to prioritize maintaining the experiment’s structure over the well-being and ethics involved, thereby highlighting the blurring of roles and the profound impact of the situation on human behavior.

“After the first one broke down, we didn’t believe it. We thought he was faking. There was actually a rumor he was faking to get out. He was going to bring his friends in to liberate the prison. And/or we believed our screening procedure was inadequate, [we believed] that he had some mental defect that we did not pick up.

At that point, by the third day, when the second prisoner broke down, I had already slipped into or been transformed into the role of Stanford Prison Superintendent. And in that role, I was no longer the principal investigator, worried about ethics.

When a prisoner broke down, what was my job? It was to replace him with somebody on our standby list. And that’s what I did.

There was a weakness in the study in not separating those two roles. I should only have been the principal investigator, in charge of two graduate students and one undergraduate.”

Critics suggest that Zimbardo’s active involvement and encouragement of the guards to create an atmosphere of powerlessness  inadvertently contributed to the abusive behaviours observed.

His own admission of becoming “the superintendent of the Stanford county jail” and adopting behaviours consistent with that role highlights the potential for researcher bias and a failure to maintain ethical distance.

Withdrawal 

Ethically, participants should be free to withdraw their participation at any time without penalty.

However, in the context of the SPE, the archival materials reveal that Zimbardo had planned from the beginning to make the prisoners believe they could not leave except for emergency reasons.

The protocol submitted to Stanford’s ethics committee stated that “Prison subjects will be discouraged from quitting”.

This intentional discouragement of withdrawal directly contravenes the ethical principle of voluntary participation and the right to terminate involvement without coercion.

Moreover, prisoner accounts suggest they felt trapped and believed that attempting to leave prematurely might jeopardise their payment.

The contract signed by participants also stated that they would “only be released from participation for reasons of health deemed adequate by the medical advisers”, further limiting their perceived autonomy to leave.

This perceived inability to withdraw freely contributed to the distress experienced by the prisoners and raises serious ethical questions about the voluntariness of their continued participation

Debrief 

Arrangements were made for all individuals involved, the prisoners, the guards, and the staff (presumably including the researchers themselves), to be interviewed on the Friday following the Thursday termination of the experiment.

These interviews were intended to be conducted by other faculty members and graduate students who had not been directly involved in the six-day simulation.

Extensive group and individual debriefing sessions were held, and all participants returned post-experimental questionnaires several weeks, then several months later, and then at yearly intervals.

These sessions served several potential purposes, including gathering further insights into the participants’ experiences, understanding the psychological impacts of the simulation, and beginning the process of addressing any emotional distress caused by their involvement.

Zimbardo concluded there were no lasting negative effects.

Zimbardo also strongly argues that the benefits gained from our understanding of human behavior and how we can improve society should outbalance the distress caused by the study.

However, it has been suggested that the US Navy was not so much interested in making prisons more humane and were, in fact, more interested in using the study to train people in the armed services to cope with the stresses of captivity.

Contributions to psychology

The BBC Prison Study: A Partial Replication

One of the most compelling follow-ups to the Stanford Prison Experiment is the BBC Prison Study (2002) conducted by social psychologists Stephen Reicher and Alexander Haslam.

While it replicated certain elements of Philip Zimbardo’s original setup, the differences in methodology and results are profound.

1. Similar Roles, New Conditions

Like the Stanford Prison Experiment, the BBC Prison Study randomly assigned participants to be either “guards” or “prisoners” in a simulated prison environment.

These roles were meant to mirror typical power structures found in real-life prisons, thus allowing researchers to observe how individuals react to hierarchy and confinement.

However, the BBC Prison Study diverged in a critical way:

• No Scripted Instructions: The guards were not explicitly guided to employ authoritarian tactics. Instead, they were left to devise their own strategies for maintaining order.
  
  This key difference contrasted with the Stanford Prison Experiment, where the guards – unintentionally or otherwise – received suggestions and cues on how to exert power.

2. Different Outcomes

Whereas in Zimbardo’s experiment the guards rapidly became harsh and the prisoners sank into learned helplessness, the BBC Prison Study took an unexpected turn:

• Prisoners Banding Together: Rather than succumbing to distress and low morale, many prisoners formed a cohesive group. They challenged the guards’ authority, negotiated for better treatment, and maintained solidarity.

• Guards Not Becoming Tyrannical: Without top-down encouragement to be punitive, the BBC guards did not uniformly adopt cruel or domineering behaviors.
  
  Some guards were uncertain about how far they could go in asserting authority, while others attempted to employ more constructive forms of leadership.

3. Why the Discrepancy Matters

The BBC findings spotlight the power of leadership cues and group dynamics in shaping behavior.

Critics of the Stanford Prison Experiment have argued that Zimbardo’s own prison superintendent role and other situational cues led guards to believe tyrannical methods were expected.

The BBC replication, on the other hand, shows that in the absence of direct encouragement—or a perceived need to act brutal—many people might not devolve into oppressive power structures.

Moreover, the BBC Prison Study suggests that situational influences alone may not guarantee abuse.

Social identity, negotiation between groups, and individual interpretations of a leadership role also play a pivotal part.

This has led some researchers to revise the notion that humans “naturally” slip into sadistic behaviors given a position of power.

Instead, individuals may need explicit cues, leadership norms, or a social context that condones aggression to enact it so willingly.

4. Implications for Understanding Power and Authority

Together, the Stanford Prison Experiment and the BBC Prison Study illustrate how environmental factors (e.g experimental instructions, institutional roles, and collective identities) can yield strikingly different outcomes from a similar setup.

For students learning about social psychology, this contrast underscores two critical lessons:

• Context Matters: Subtle differences in instructions, oversight, and group identity can shape dramatically divergent behaviors.

• Roles vs. Interpretation: People do not mindlessly conform to roles; they also interpret what those roles mean within their social and ethical framework.

From an ethical standpoint, the BBC Prison Study imposed firmer oversight and required stricter institutional review.

It also highlights how modern replications of earlier controversial experiments can offer new insights without recreating the same levels of participant distress.

Ultimately, the BBC Prison Study stands as a fascinating counterpoint to the Stanford Prison Experiment.

By removing explicit prompts to dominate, it challenges some of Zimbardo’s original claims about how easily ordinary individuals become cruel in hierarchical roles – and it encourages us to question how leadership and social identity shape group behavior.

Discussion Questions

1. What are the effects of living in an environment with no clocks, no view of the outside world, and minimal sensory stimulation?

2. Consider the psychological consequences of stripping, delousing, and shaving the heads of prisoners or members of the military. What transformations take place when people go through an experience like this?

3. The prisoners could have left at any time, and yet, they didn’t. Why?

4. After the study, how do you think the prisoners and guards felt?

5. If you were the experimenter in charge, would you have done this study? Would you have terminated it earlier? Would you have conducted a follow-up study?