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The Stanford Prison Experiment

In 1975, social psychologist Philip Zimbardo conducted an experiment that showed violent and aggressive behavior could be elicited from college students simply by asking them to play the role of prison guards. Zimbardo did this to prove that situations, rather than personal traits (dispositions), ruled behavior.

The situationist vs dispositionist controversy was at the heart of the Stanford Prison Experiment. We encountered this distinction in discus­sing Walter Mischel's famous criticism of the trait concept, in Chapter 11 (Personality).

A situationist believes environments and situations elicit certain beha­viors. A dispositionist believes individual traits or personality characteristics usually determine behaviors. (Disposition is a lasting personality orientation such as "temperament, character, nature, constitution, makeup, or mentality.")

What is disposition?

In Chapter 11 we saw how American social psychologists and personality psychologists invented the concept of authoritarian personality to explain Nazi behavior. The authoritarian personality was said to be a personality type with dispositions like these:

Blind allegiance to conventional beliefs about right and wrong

Respect for submission to acknowledged authority

Belief in aggression toward those who do not subscribe to conventional thinking, or who are different

A negative view of people in general - i.e. the belief that people would all lie, cheat or steal if given the opportunity

A need for strong leadership which displays uncompro­mising power (etc)

Dispositions come from inside, pre­sumably put there mostly by education and culture (or biology). By contrast, a "situationist" perspective emphasizes the power of the environment. Situations can over­ride individual dispositions.

Stanley Milgram, who conducted the obedience research, apparently wanted to see if the "German character" encouraged obedience, which would be predicted by dispositionism. If Americans resisted the demand to obey, but Germans obeyed, it might support the dispositional perspective.

Instead, Milgram found that his subjects, all Americans, mostly obeyed situational pressures to be cruel. Milgram never did repeat the experiment in Germany.

By the 1970s, a decade after Milgram showed the potential power of the situation, Zimbardo decided to see if ordinary college students could be similarly affected. He arranged an experiment to demonstrate the power of the situation to overpower individual personality, ethics and morals.

The Experiment

Male subjects were recruited through newspaper ads offering them $15 a day to participate. Seventy-five men applied to participate, and 19 were chosen.

Zimbardo said a battery of tests were employed to select those with the most stable personalities. Volunteers were randomly assigned to play prison guard or prisoner through the flip of a coin.

The preparation and execution of the study was thorough and realistic. It began on a Sunday morning as police swept through Palo Alto, the home of Stanford University, picking up students selected to be prisoners.

Sirens blared as students tapped to play the prisoner role were arrested, placed spread-eagled against a car, searched, handcuffed, and booked at the police station. After fingerprinting and paper­work, prisoners were transported to Stanford County Prison, actually the basement of the Stan­ford psychology building, unused for a few weeks after summer term.

The prisoners were stripped and their hair was sprayed to kill lice. They were required to stand naked and listen to the guards read a list of regulations.

One of the rules was that prisoners had to follow any order from a guard. Another rule was that they had to ask for permis­sion to do anything, even to go to the toilet. After this the prisoners were issued a simple uniform and a towel and each was assigned to a cell.

What elements helped make the prison study realistic?

The guards in the study were given khaki-colored uniforms and mirrored sun­glasses, giving them an anony­mous and sinister look. They wore billy clubs, keys, and handcuffs.

Zimbardo said the guards received no special instructions. That was not quite true, as later critics pointed out.

The guards may not have received explicit instructions to be cruel, but the demand characteristics of the experi­ment were obvious. Everything about the experiment made it clear the guards were expected to act like stereotyped prison guards.

How did the prison guards start to act?

Before long, the prison guards were acting mean. For example, they decided to call roll during the middle of the night and make the prisoners do pushups, with guards putting their feet on the middle of a prisoner's back.

On the second day of the experiment, the guards crushed a rebellion and became more verbally abusive. After a day and a half, one of the prisoners had to be re­leased because of uncontrolled crying, depression, fits of rage, and disorgan­ized thinking.

A few days later, the same thing hap­pened to three additional prisoners. By the fifth day, all the volunteer prisoners asked to be released. One subject developed a skin rash over his whole body after having his appeal rejected by a mock Parole Board.

On the sixth day the exper­iment was terminated. "The experience dramatically and painfully transformed most of the participants in ways we did not antici­pate, prepare for, or expect" (Haney & Zimbardo, 1998).

Over 30 years after the research, Zim­bardo added details about how the experiment happened to be called off.

About halfway through the study, I had invited some psycho­logists who knew little about the experiment to interview the staff and participants, to get an outsiders' evaluation of how it was going. A former doctoral student of mine, Christina Maslach, a new assistant professor at the University of California at Berkeley, came down late Thursday night to have dinner with me.

We had started dating recently and were becoming romantically in­volved. When she saw the prisoners lined up with bags over their heads, their legs chained, and guards shouting abuses at them while herding them to the toilet, she got upset and refused my suggestion to observe what was happening in this "crucible of human nature."

Instead she ran out of the basement, and I followed, berating her for being overly sensitive and not realizing the important lessons taking place here. "It is terrible what YOU are doing to those boys!" she yelled at me.

Christina made evident in that one statement that human beings were suffering, not prisoners, not experi­mental subjects, not paid volunteers. And further, I was the one who was personally responsible for the hor­rors she had witnessed (and which she assumed were even worse when no outsider was looking).

She also made clear that if this per­son I had become—the heartless superintendent of the Stanford pri­son—was the real me, not the car­ing, generous person she had come to like, she wanted nothing more to do with me. That powerful jolt of reality snapped me back to my senses.

I agreed that we had gone too far, that whatever was to be learned about situational power was already indelibly etched on our videos, data logs, and minds; there was no need to continue. (Zimbardo, 2007)

What story did Zimbardo tell later, about why the experiment was terminated?

Every single student playing the guard role became authoritarian and abusive at least once, and many of them seemed to enjoy the role. Although Zimbardo did not explicitly tell the guards to act aggres­sive, evidently they felt this was part of the prison guard role or stereotype.

The Zimbardo prison study resembled the Milgram obedience study in several ways. It put ordinary citizens in the role of torturers or bullies, in part by appearing to remove their personal responsibility for their actions.

Like the Milgram study, the Stanford prison experiment led to protests about abusive treatment of experi­mental subjects. Like Milgram, Zimbardo claimed he exercised extraordinary care in debriefing his subjects and conducting follow-up interviews to make sure none suffered lasting harm.

The Milgram Connection

The parallels between Zimbardo's experiment and Milgram's experiment were clear from the start, and Zimbardo gave Milgram credit for inspiring the Stanford prison experiment. However, few psychologists knew about the deeper connections between Zimbardo and Milgram until years later.

Zimbardo and Milgram were class­mates at a Bronx high school. Milgram was "considered the smartest kid" and Zimbardo was voted most popular (Zimbardo, Maslach, and Haney, 1999). Zimbardo said they had an early bias toward the situational perspective

We sometimes talked about the reasons for seemingly strange or irrational behavior by teachers, peers, or people in the real world that violated our expectations. Not coming from well-to-do homes, we gravitated toward situational ex­planations and away from disposi­tional ones to make sense of such anomalies.

The rich and powerful want to take personal credit for their success and to blame the faults of the poor on their defects. But we knew better; it was usually the situation that mat­tered, by our account. (Zimbardo, Maslach, and Haney, 1999)

Both went to Yale and were influenced by some of the same professors. They stayed in touch afterwards. Milgram's study was published about a decade before Zimbardo's.

How were Milgram and Zimbardo connected by their past histories?

Milgram's research contributed to a strong tilt, in social psychology, toward situationist perspectives. You might say they showed lasting dispositions. Milgrim (the "smart one") did his research first. Zimbardo (the "popular one") tagged along and re-affirmed Milgram's discovery that situations could dominate behavior.

Criticisms of the Stanford Prison Experiment

Criticisms of Zimbardo's experiment concentrated on the demand charact­eristics of the study. The concept of demand characteristics came from Martin Orne.

Orne did research about hypnosis, among other things. He saw demand characteristics as similar to hypnosis except implicit (unspoken). They were a form of suggestion that could induce unusual displays of behavior.

Demand characteristics were found in the context or framing of an experiment, which implicitly conveyed how a parti­cipant was expected to act. Subjects will usually try to please the experimenter and do what they think is expected.

How did Orne apply the concept of suggestion to experimental settings?

Orne (1962) found that experimental subjects were "willing to tolerate a considerable degree of discomfort, boredom, or actual pain, if required to do so by the experimenter." In one famous experiment, Orne tried to find tasks so boring or frustrating that subjects would quit an experiment, such as tearing a piece of paper into smaller pieces and repeating this for hours.

But subjects did not quit; they did what they were asked to do, without complaint. Orne wrote:

Just about any request which could conceivably be asked of the subject by a reputable investigator is legitimized by the quasi-magical phrase (or cognition) "This is an experiment" [p. 777].

Banuazizi and Movahedi (1975) pointed out that Zimbardo's experiment had set up obvious demand characteristics.

(a) The subjects entered the experi­ment carrying strong social stereo­types of how guards and prisoners act and relate to one another in a real prison.

(b) In the experimental context itself, there were numerous cues pointing to the experimental hypothesis, the experimenters' expectations, and possibly, the experimenters' ideolo­gical commitment.

(c) Complying with the actual or perceived demands in the experi­mental situation, and acting on the basis of their own role-related expectancies, the subjects produced data highly in accord with the experimental hypothesis.

To test the demand characteristics of the Stanford prison experiment, Banuazizi and Movahedi gave 185 students a questionnaires with a brief description of the Stanford prison experiment, and added this:

Suppose you decide to participate, and you sign the proper release forms. On a subsequent Sunday morning, a police officer knocks on your apartment door and arrests you.

He charges you with a felony, warns you of your constitutional rights, searches you, handcuffs you, and in the back of his car, takes you to the police station for booking. You are then fingerprinted, and left in an isolated detention cell.

After a while, you are blindfolded and sent to a prison. There, you are stripped naked, skin searched, deloused, and issued a uniform, bedding, soap, towel, toothpaste, and toothbrush. And all this time you have been pushed around, put down, and humiliated. (Banuazizi and Movahedi, 1975)

After reading that, students were asked to guess the experimental hypothesis and their expectancies about the outcome of the experiment. 35 of the 185 students already knew about the Stanford prison experiment, so their responses were excluded.

How did Banuazizi and Movahedi test the demand characteristics of Zimbardo's study?

81% of the remaining 150 respondents "were able to articulate quite accurately the intent of the experiment." Here are some of their questionnaire responses.

S1014: Experimenter is trying to prove the contention that he has about jails. He believes that people are pushed about, put down, and humiliated in jails and other correctional institutions.

S1029: Testing endurance–to see how far you go before fighting back, I think they are trying to find out what causes prison riots

S1053: He is trying to find out if any­body would fit into either of the two roles. That is, figuring that everyone is equal, a person selected as a guard will behave, act, and become like a guard; if a person is selected to be a prisoner, he will act, behave, and become like a prisoner.

Next the students receiving the ques­tionnaire were asked to guess the outcome of the research. 85% of males and 94% of females guessed that parti­cipants assigned to be guards would be oppressive and hostile.

In other words, the demand character­istics of the experiment were obvious. Banuazizi and Movahedi note, "when experimental subjects are asked to play highly stereotyped and emotion-laden roles, they bring to the experimental situation "mental sets," or dispositions, which could decisively influence their behavior.

Zimbardo (following Milgram) saw his research as demonstrating the power of situations over dispositions. In Banuzizi and Movahedi's re-interpretation, Zim­bardo was using dispositions in the experimental subjects, namely their stereotyped, pre-existing ideas about prison-guard roles.

How did Banuazizi and Movahedi portray Zimbardo as using dispositions in addition to situations?

As a separate issue, was it ethical for Zimbardo to put student participants in this situation? Again, present-day criticisms of the prison experiment echo criticisms of Milgram's experiment.

You might recall the concept of engag­ed followership raised in connection with the obedience experiment. Mil­gram's subjects were engaged followers (because they bought into their role as participants in scientific research) and Milgram's research associates were engaged followers (for the same reason; they knew they were participating in research).

All parties therefore justified what they did. Milgram's assistants (such as the man playing the role of experimenter in the famous video) no doubt felt their make-believe roles justified their cruelty in urging subjects to continue even when they were distressed. The participants who continued to shock the learner felt similarly justified in their role as research subjects.

Everybody involved (not just subjects) was engaged in what they considered a worthy enterprise: scientific research. According to Milgram and Zimbardo, the experiments were unpleasant, but they were justified by the lessons they taught us about human behavior. Zimbardo made this argument explicitly, writing:

The value of the Stanford Prison Experiment (SPE) resides in demonstrating the evil that good people can be readily induced into doing to other good people within the context of socially approved roles, rules, and norms, a legitimizing ideology, and institutional support that transcends individual agency. (Zimbardo, Maslach, and Haney, 1999)

The "engaged followership" critique turns this rationalization on its ear. The researchers were excusing unjustifiable abusive behavior toward the participants, by portraying the research as valuable.

What is the "engaged followership" analysis of both Milgram and Zimbardo's research?

Ironically, the critics would agree with Zimbardo, Maslach, and Haney's point. Good people can be readily induced to do bad things by putting bad things in a socially approved context. The actual researchers (Milgram and Zimbardo) as well as the make-believe experimenter and prison guards engaged in cruelty, justifying their activity by portraying it as a worthy scientific endeavor.


Banuazizi A. & Movahedi S. (1975). Interpersonal dynamics in a simulated prison: A methodological analysis. American Psychologist, 30, 152-160.

Haney, C., Banks, C., & Zimbardo, P. (1973). Interpersonal dynamics in a simulated prison. International Journal of Criminology and Penology, 1, 69-97.

Kompa, J. S. (2012, July) Analysis of the Stanford Prison Experiment. Retrieved from:

Orne, M. T. (1962) On the social psychology of the psychological experiment: With particular reference to demand characteristics and their implications. American Psychologist, 17,776-783.

Zimbardo P. G., Maslach C., & Haney C. (1999). Reflections on the Stanford Prison Experiment: Genesis, transformations, consequences. In Blass T. (Ed.), Obedience to Authority: Current Perspectives on the Milgram Paradigm (pp. 193-237). Mahwah, NJ: Lawrence Erlbaum.

Zimbardo, P. G. (2007) Revisiting the Stanford prison experiment: A lesson in the power of situation. Chronicle of Higher Education, 53, p.B6.

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