Haney, Craig., Banks, Curtis., & Zimbardo, Philip. (1973)

A Study of Prisoners and Guards in a Simulated Prison.




This study was funded by the US Navy, as it and the US Marine Corps were interested in finding out the causes of conflict between guards and prisoners in the naval prisons.  Attempts to explain the violent and brutal conditions often found in prisons had previously used dispositional attribution.  That is, that the state of the prison is due to the nature of the prison guards and the prisoners.

For example, it had been argued that prison guards bring to their jobs a particular ‘guard mentality’ and are therefore attracted to the job as they are already sadistic and insensitive people.    Whereas prisoners are individuals who have no respect for law and order and bring this aggressiveness and impulsivity to the prison. 

Philip Zimbardo was interested in testing this dispositional hypothesis by demonstrating that the conditions of the prisons were not a result of the type of individuals working and incarcerated in the prisons and hoped to go on to help the Navy develop training, which would eliminate the deplorable conditions in the prisons.

Zimbardo believed that the behaviour in prisons could be best explained using a situational attribution.  In particular he believed that the conditions were influenced by the social roles that prisoners and prisoner guards are expected to play.   Shakespeare put this quite well when he wrote ‘All the world’s a stage, and all the men and women players’ (As You Like It).  Suggesting that we are what we play. 

We all play many roles in society and these social roles do to some extent shape our identity.  Each role we play brings with it certain rules or expectations about how we should behave.  For example when we play the role of a student there may be very different expectations about how we should behave compared with say the role of an audience member at the Rocky Horror Picture Show.   Similarly there may be certain expectations about the role a prisoner or prison guard should play.



The aim of the study was to investigate the effects of being assigned to the role of either a prison guard or prisoner.



The study is usually described as an experiment with the independent variable being the conditions the participants are randomly allocated to.  Either prisoner or guard.  The dependent variable is the resulting behaviour.

The study can also be described as a simulation as it was attempting to create a prison like environment.

Data collected were combinations of both quantitative and qualitative data.  The main data though was qualitative and was obtained using video, audiotape and direct observation. 

The participants were respondents to a newspaper advertisement, which asked for male volunteers to participate in a psychological study of ‘prison life’ in return for payment of $15 per day. 

The 75 respondents completed a questionnaire about their family background, physical and mental health, prior experiences and attitudinal tendencies with respect to psychopathology and any involvement in crime.

Based on the results of the tests 24 men were selected.  These 24 were judged to be the most physically and mentally stable, most mature, and least involved in antisocial behaviours.  The participants were described as “normal, healthy male college students who were predominantly middle class and white.” The 24 participants did not know each other prior to the study.

A simulated prison was built in the basement of the psychology building at Stanford University.  The simulated prison comprised of:

        Three small cells (each 6 x 9 ft) with three prisoners to a cell.  The cells contained three cots (with mattress, sheet and pillow) for each prisoner. 

        A ‘solitary confinement’.  Which was an extremely small, unlit room (2 x 2 x 7 ft).  This room was across from the cells.

        Several rooms in an adjacent wing of the building were used as guards’ quarters (to change in and out of uniform and for relaxation), interview rooms and a bedroom for the ‘warden’ and ‘superintendent’ (Zimbardo). 

        A small, enclosed room, which was used as a ‘prison yard’.

        Video recording equipment was placed behind an observation screen

For the duration of the study the prisoners remained in the mock prison for 24 hours.  Three were arbitrarily assigned to each of the three cells, and the others were on stand-by at their homes.  The guards worked on three-man eight-hour shifts, and went home after their shifts.

The participants all agreed voluntarily to play the role for $15 a day for up to two weeks.  The participants signed a contract guaranteeing basic living needs, such as an adequate diet and medical care. Although it was made explicit in the contract that if they were to be assigned to the role of prisoner they would have to have some basic civil rights (e.g. privacy) suspended.      The participants were not given any information about what to expect and how to behave.

The 24 participants were randomly assigned to the role of ‘prisoner’ or ‘guard’ and informed by telephone to be available at their homes on a particular Sunday when the experiment would begin. 

Those participants allocated the role of guards had to attend an orientation meeting the day before the induction of the prisoners.  They met the principal investigators, the ‘superintendent’ of the prison (Zimbardo) and the ‘warden’ (undergraduate research assistant).  They were told that the ‘experimenters wanted to try to simulate a prison environment within the limits imposed by pragmatic and ethical considerations’.  Their assigned task as prison guards was to ‘maintain the reasonable degree of order within the prison necessary for effective functioning’. 

The guards were instructed in their administrative details, including; the work-shifts, the completion of ‘critical incident’ reports, and the managing of meals, work and recreation programmes for the prisoners.  In order to start involving the guards in their roles even before the prisoners were incarcerated, they assisted in the final phases of completing the prison complex – putting the cots in the cells, moving furniture and so on.  However the guards were not told how to behave apart from being explicitly told that they were not allowed to use physical punishment or physical aggression.

The guards believed that the experimenters were mainly interested in studying the behaviour of the prisoners although the experimenters were just as interested in their behaviour.

The uniforms of both prisoners and guards were intended to increase group identity and reduce individuality within the two groups. 

 The guards’ uniform consisted of a plain khaki shirt and trousers, a whistle, a police night stick (a wooden batten) and reflecting sunglasses, which made eye contact impossible.   The guards’ uniforms were intended to convey a military attitude, while the baton and whistle were symbols of control and power. 

The prisoners’ uniform consisted of a loose-fitting muslin smock with an identification number on the front and back, no underwear, rubber sandals, a hat made from a nylon stocking and they had a light chain and lock around their ankle.  Each prisoner was also issued with a toothbrush, soap, soap-dish, towel and bed linen.  No personal belongings were allowed in the cell. The prisoners’ uniforms were designed to de-individuate the prisoners and to be humiliating and serve as symbols of subservience and dependence.  The ankle chain was a constant reminder of the oppressiveness of the environment.  The stocking cap removed any distinctiveness associated with hair length, colour and style (as does shaving of heads in some ‘real’ prisons).  The ill-fitting uniforms made the prisoners feel awkward in their movements; since these ‘dresses’ were worn without underwear, the prisoners were forced to assume unfamiliar postures, more like those of a woman than a man - another part of the emasculating process.

The prisoner participants were unexpectedly ‘arrested at their homes with the cooperation of the local police department.  A police officer then charged them with suspicion of burglary or armed robbery, advised them of their rights, handcuffed them, thoroughly searched them (often in full view of their neighbours and passers by) and drove them in the back of a police car to the police station. 

At the police station they had their fingerprints and photograph taken and were put in a detention cell.  Each prisoner was then blindfolded and driven to the mock prison by one of the experimenters and a guard.  Throughout this arrest procedure, the police officers involved maintained a formal, serious attitude, and did not tell the participants that this had anything to do with the mock prison study.

At the mock prison, each prisoner was stripped, sprayed with a delousing preparation (a deodorant spray) and made to stand alone and naked in the ‘yard’.  After being given their uniform and having a mug shot (ID picture) taken, the prisoner was put in his cell and ordered to remain silent.

The warden read them the rules of the institution (developed by the guards and the warden), which were to be memorised and had to be followed.  Prisoners were to be referred to only by the number on their uniforms, also in an effort to depersonalise them. 

Every day the participants were allowed three bland meals, three supervised toilet visits, and given two hours for the privilege of reading or letter writing.  Work assignments had to be carried out and two visiting periods per week were scheduled, as were movie rights and exercise periods. 

Three times a day prisoners were lined up for a ‘count’ (one on each guard work-shift).  The original purpose of the ‘count’ was to establish that all prisoners were present, and to test them on the knowledge of the rules and their ID numbers.  The first ‘counts’ lasted only about ten minutes but as conditions in the prison deteriorated, they increased in length until some lasted for several hours.  



In summary the study showed that the behaviour of the ‘normal’ students who had been randomly allocated to each condition, was affected by the role they had been assigned, to the extent that they seemed to believe in their allocated positions.  The study therefore rejects the dispositional hypothesis.

The experiment had to be stopped after just six days instead of the planned 14 days, mainly because of the pathological reactions of the participants.  Five prisoners had to be released even earlier because of extreme emotional depression.

In general, the guards and prisoners showed a marked tendency towards increasingly negative emotions, and their overall outlook became increasingly negative.  Despite the fact that guards and prisoners were essentially free to engage in any form of interaction, the nature of their encounters tended to be negative, hostile, insulting and dehumanising.  The prison was internalised by both the prisoners and the guards, that is, they started to believe in it.  They adopted very contrasting behaviours, which were appropriate for their respective roles.  The guards started most of the interactions, most of which were in the form of commands or verbal affronts, while the prisoners adopted a generally passive response mode.  Although it was clear to all participants that the experimenters would not permit physical violence to take place, varieties of less direct aggressive behaviour were often observed. 

One of the most dramatic evidence of the impact of this situation upon the participants was when five prisoners had to be released early due to extreme emotional depression, crying, rage and acute anxiety.  Of the remaining prisoners, only two said they were not willing to forfeit the money they had earned in return for being ‘paroled’.  When the simulation was terminated after only six days instead of the projected fourteen days all of the remaining prisoners were delighted by the news, but most of the guards seemed to be distressed by the premature end to the study - it appeared that they had become sufficiently involved in their role that they now enjoyed the extreme control and power which they exercised.   This is referred to by Zimbardo as pathology of power.

However there were individual differences in styles of coping with this novel experience.  Half of the prisoners endured the oppressive atmosphere, and not all the guards resorted to hostility, some guards were tough but fair, while some went far beyond their roles to engage in creative cruelty and harassment.



Zimbardo believes that the study demonstrate the powerful effect roles can have on peoples’ behaviour.  Basically the participants were playing the role that they thought was expected of, either a prisoner or prison guard.  (It is in fact a simulation of what we expect prison life to be, rather than what it is, as none of the participants had previously been in prison as a guard or prisoner).

Zimbardo then went on to explain the prison guards’ behaviour and the prisoners’ behaviour

Zimbardo explains that the reason for the deterioration in guard behaviour was power.   The guards were given control over the lives of other human beings and did not have to justify their displays of power as they would normally have to in their daily lives.  They started to enjoy this power very earlier on in the study (pathology of power) as demonstrated that even after the first day all prisoner rights became redefined as privileges, and all privileges were cancelled. 

Zimbardo explains the social deterioration of the prisoners as the pathological prisoner syndrome.  At the beginning of the study, the prisoners rebelled against their conditions, but the guards undermined every attempt at rebellion, and any solidarity between the prisoners collapsed.  Half of the prisoners responded by becoming sick, and eventually had to be released before the study was finally brought to a conclusion.  The remaining prisoners became passive, dependent and had flattened emotions.  Zimbardo suggested that there were a number of processes that contributed to the pathological prisoner syndrome:

         The loss of personal identity – the prisoners were de-individuated by being stripped of their individuality, their name, dress, appearance, behaviour style, and history.  Living among strangers who do not know your name or history, dressed like all the other prisoners, all led to the weakening of self-identity among the prisoners.  The prisoners became de-individuated not only to the guards, but to themselves;

         The arbitrary control exercised by the guards - on post-experimental questionnaires, the prisoners said they disliked the way that the way they were subjected to the arbitrary and changeable decisions and rules of the guards as this made life unpredictable and unfair.  For example, smiling at a joke could be punished in the same way that failing to smile might be.  As the environment became more unpredictable, the prisoners’ behaviour showed signs of learned helplessness, that is, as the prisoners’ previously learned assumptions about a just and orderly world were no longer functional, they ceased to initiate any action;

         Dependency and emasculation - the prisoners were made to be totally dependent on the guards for commonplace functions such as going to the toilet, reading, lighting a cigarette and this emasculated them.  The smocks, worn without underwear, lessened their sense of masculinity.    This was taken to the extent that when the prisoners were debriefed they suggested that they had been assigned to be prisoners because they were smaller than the guards.  In fact there was no difference in average height between the prisoners and guards, and the perceived difference was a response to the prisoners’ perception of themselves and their lack of power.


Evaluation of Procedure/Method


The main criticism of Zimbardo’s study is on ethical grounds.

However Zimbardo does defends the experiment in a number of ways:

         The only deception involved was to do with the arrest of the prisoners at the beginning of the experiment.  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.

         When Zimbardo realised just how much the prisoners disliked the experience, which was unexpected, the experiment was abandoned.

         Approval for the study was given from the Office of Naval Research, the Psychology Department and the University Committee of Human Experimentation.  This Committee also did not anticipate the prisoners extreme reactions that were to follow.

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

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

        Zimbardo also strongly argues that the benefits gained about our understanding of human behaviour and how we can improve society should out balance the distress caused by the study.  However it has been suggested that the US Navy was not so much interested in making prisons more human and were in fact more interested in using the study to train people in the armed services to cope with the stresses of captivity. 

The study can also be criticised for its unrepresentative sample. Since the experiment was conducted using 24 normal, healthy, male college students who were predominantly middle class and white (one was described as oriental), we have to be careful generalising the results to other people. 

Importantly the study has been criticised for lacking ecological validity.  For practical and ethical reasons the simulated prison could not be totally realistic.  Many particularly unpleasant aspects of prison life were absent, such as involuntary homosexuality, racism, beatings and threats to life.  Also, the maximum anticipated sentence was just two weeks.  It is therefore possible that the study does not serve as a meaningful comparison to real prison environments.  However, there is considerable evidence that the participants did react to the situation as though it was real.   For example 90% of the prisoners’ private conversations, which were monitored by the researchers, were on the prison conditions, and only 10% of the time were their conversations about life outside of the prison.  The guards, too, rarely exchanged personal information during their relaxation breaks - they either talked about ‘problem prisoners’, other prison topics, or did not talk at all.  The guards were always on time and even worked overtime for no extra pay.  When the prisoners were introduced to a priest, they referred to themselves by their prison number, rather than their first name.  Some even asked him to get a lawyer to help get them out. 


A main strength of the study was the way it managed to maintain some degree of control and some ecological validity.   The situation was very tightly controlled e.g. guards and prisoners were randomly allocated and were selected using a stringent criterion.   The study still had ecological validity in the way that Zimbardo went to great extremes in making the study as true to life as possible, for example in the way that he had the prisoners arrested from their homes.

A further strength was in the way that Zimbardo collected data.  He used a number of qualitative approaches such as observation (sometimes overt and sometimes covert) interviews and questionnaires. 



Evaluation of Explanation

Zimbardo’s study was clearly trying to give a situational explanation for behaviour.   He argued that the study demonstrates the powerful effect roles can have on peoples’ behaviour. 

However some psychologists believe that he has over emphasised the situational explanation.  They state that the behaviour of both prisoners and guards may have arisen from the stereotyped expectations of how prisoners and guards should behave.  That is, the participants were only role-playing.  However, Zimbardo would strongly suggest that the participants’ experiences were all too real and that even if they were only role-playing at the beginning of the study, as the study progressed they were internalising these roles and they could no longer differentiate between role-playing and self.

It is also worth noting that Zimbardo’s argument can be seen as too deterministic.  For example in Zimbardo’s study not all of the participants behaved in the same way.  For example, some of the guards were less willing to abuse their power.   Perhaps the reason why some of the participants were less willing was something to do with their personalities.



Haney, C., Banks, W.C. & Zimbardo, P.G. (1973) A study of prisoners and guards in a simulated prison. Naval Research Review, 30, 4-17.











GROSS, R. (1999) Key Studies in Psychology, 3rd Edition. London: Hodder and Stoughton

BANYARD, P. AND GRAYSON, A. (2000) Introducing Psychological Research; Seventy Studies that Shape Psychology, 2nd Edition. London: Macmillan







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