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The Rarely-Told True Story of Zimbardo’s Prison Experiment

Goodness, evil, and the power that may cause both.

You don’t need to have a background in psychology to know about Zimbardo’s prison experiment. Depicted in movies, television, and of course all introductory psych textbooks, the true story is rarely told of what really happened in that famous and controversial experiment. When such iconic studies become part of psychology lore, their true story often gets buried in overly simplistic interpretations. Worse still, as in the case of the equally famous Milgram obedience studies, the findings become grossly distorted. The truth behind these studies will be revealed to the first time to American audiences in September of 2013, with the publication of Gina Perry’s brilliant book, Behind the Shock Machine.

Inspired by her work, I decided to go back to the original source of some of the social psych studies I’ve written and taught about for years (actually, decades). Having read the same textbook summaries, ethical discussions, and secondary sources as thousands of psychology professors and students in the past 40 years since the actual study's first publication, I thought it was time to see what really transpired in those prison cells that Zimbardo and his team built to explore the “Interpersonal Dynamics in a Simulated Prison.” Published in 1973, this landmark study in the very first volume of the International Journal of Criminology and Penology, describes in academic, yet compelling, terms the frightening events that transpired over the course of the six-day experiment. From personality questionnaires to follow-up interviews, audio and video recordings, and the first-person experience of the “prison superintendent” (none other than Zimbardo), we learn much more than you can ever typically expect to find in your average psychology article.

Of course, it will be impossible to do justice to the entire article here, and for more personal accounts and images from the study, you can consult many online resources including Zimbardo’s own website devoted to the study. You may even have seen Professor Zimbardo on television talking about the study, or taken the video course he hosted. If you haven’t, here’s the study in a nutshell. 21 Stanford undergrads were recruited for a two-week study, divided randomly into 10 “prisoners,” and 11 ”guards.” They were placed into a prison setting, where the prisoners occupied cells, and the guards watched over them. After six days, the experiment had spun out of control as simulation turned into reality, necessitating the early release of some prisoners. Most introductory psych texts use this experiment to demonstrate the “power of the situation,” and the fact that ordinary college students could become sadistic monsters (i.e. the guards) in the space of just a few short days. That guards and prisoners were randomly assigned to condition was a key fact that added to the validity of the conclusion. Discussions of the study’s ethics today state that it could never be done now, but the fact is that Zimbardo did submit the study for approval to his institutional review board, and it met the criteria of the time (for more on the study's ethics, see Zimbardo, 1973).

First off, you should recognize that this was no hokey, clearly fake situation (a problem that seemed to plague the Milgram experiments). The authors took elaborate precautions to recreate a realistic prison environment, from the uniforms to the furniture in the cells to the food and the schedule. As a result, “within what was a surprisingly short time, we witnessed a sample of normal, healthy American college men fractionate into a group of prison guards who seemed to derive pleasure from insulting, threatning, humiliating and dehumanizing their peers…” The guards, who showed no signs of being sadistic types, were able to provoke emotional breakdowns in prisoners “selected precisely” for their emotional stability.

The guards entered the study thinking that it was the prisoners whose behavior was being studied. They received training from the Zimbardo acting out his superintendent role and an undergraduate research assistant who was the “warden.” The prisoners were told that they would be asked to take on a prisoner role but given no other information other than the fact that they would be “arrested” and brought into the prison. Neither group had very specific instructions because the point of the experiment was to see how they adapted to their respective roles.

The prisoners were “arrested” by actual police officers, “booked,” and then stripped and put in a cell yard until they received their uniforms. The uniforms were more like hospital gowns, and the men wore no clothing underneath so that they would feel “emasculated.” These steps intentionally created a feeling of deidentification or loss of their own personal identities. Perhaps the only difference between the study prisoners and real prisoners is that the Stanford prisoners were paid, and could escape if they requested to be released from the study.

The experimenters collected almost everything that transpired as “data,” leaving them with an enormous amount of information to sift through, code, and analyze. At the end of it all, Zimbardo believed that their data supports and validates the anecdotal evidence given by ex-convicts of life in prison. All the students involved experienced changes in moods including intense negative affect, an overall pessimistic outlook, and greater and greater declines in their self-esteem. Although the prisoners and guards could interact in any way they wished, the “characteristic nature of their encounters tended to be negative, hostile, affrontive, and deumanising.” Prisoners became passive and guards barked commands and hurled insults (they were prohibited from any form of physical violence). Four prisoners had to be released as early as the second day due to extreme emotional reactions, and one developed a psychosomatic rash soon later which also required that he be released. What was particularly telling is that when the experiment was halted, the prisoners took the news like any prison escapees would—with glee. The guards were angry and distressed.

Not everyone reacted equally badly to their roles, though, and some guards were “tough but fair.” For the most part, these differential reactions couldn’t be predicted by the personal tests that the participants completed at the start of the study. Some of the prisoners high on “authoritarianism” coped better with prison life, but other than this, there were few indications from the individual data about who would conform and who would rebel or be unable to continue in their role.

The guards resorted to various forms of harassment short of physical punishment. They called prisoners by number, not name, and hurled streams of insults at them. The evening shift was the harshest—it was then that guards gave out the most commands and were most insulting and controlling.

For their part, the prisoners essentially stopped communicating with each other about anything other than prison life (talk which occupied 90% of all their private conversations). They tried to set up a grievance committee, develop escape plans, wonder what the other prisoners were doing, and express concern about being placed in solitary confinement. The dehumanization they experienced from the guards carried over to their own interpersonal interactions, and soon they started deprecating each other. When they had the opportunity to talk to a priest, they referred to themselves by number, not name, showing just how far their deidentification had penetrated. Fortunately, all of these bad experiences, including their negative moods, bounced back to normal after the end of the study. Their depression and stress were restricted entirely to the situation, further reinforcing the idea that the “power” of the situation to overwhelm individual differences in mood and personality.

The study highlights what social psychologists continually drum into our heads, which is that the situation can and does shape our behaviors, thoughts, and feelings at even the most private of levels. Yet, as Zimbardo points out, if this were a true prison, people would forget the dehumanizing and deindividuating nature of the environment and attribute everyone’s behavior (guards and prisoners) to personal disposition. The prisoners would not be seen as antisocial by nature and the guards as little better. Not everyone reacts similarly to even this harsh of an environment, of course, as people learn different coping methods to help them adapt, but the range is much smaller than we would imagine.

To sum up, the Zimbardo study sheds the harshest light of perhaps any research ever done on the nature of power and its pervasive corruptive powers.

From the standpoint of the guards, the pathology of power led them to exert an “unprecedented degree of control” that was “self-aggrandising” and “self-perpetuating.” Prisoners in the real world want to wrest back some of this control by any means necessary, and so when released, they “will take action to establish and assert a sense of power.”

While in prison, though, the loss of personal identity and control led the prisoners to adopt the pathological prisoner syndrome, a reaction that took several forms of coping strategies. They went through stages from disbelief to rebellion; when these didn’t work, they tried to work the system (the “grievance committee”). When those efforts to gain control failed, it was every man for himself as each tried to find ways to preserve their own self-interests and identity. For some, this meant further rebellion and for others it meant becoming excessively obedient, even to the point of siding with the guards against intransigent prisoners.

Zimbardo and his co-authors hoped that the emotional and human price of the study would provide a model for improvements in the penal system as a whole. You might not agree with his conclusion, but it’s a thought-provoking one, so I’ll offer it here: “… since prisoners and guards are locked into a dynamic, symbiotic relationship which is destructive to their human nature, guards are also society’s prisoners.”

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Copyright Susan Krauss Whitbourne, Ph.D. 2013.


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

Zimbardo, P. G. (1973). On the ethics of intervention in human psychological research: With special reference to the Stanford prison experiment. Cognition, 2(2), 243-256. doi: 10.1016/0010-0277(72)90014-5