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In August 1971, with funding from the US Navy and the US Marine Corps, Zimbardo set out to show that people allocated to be ‘prison guards’ or ‘prisoners’ would tend to slip into those predefined roles, behaving in a way that they thought was required, rather than using their own judgement and morals. Zimbardo hoped the knowledge gained could be applied to real-life prison behaviour and the abusive interrelations that existed between prisoners and guards. 

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 (both covert and overt).

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 anti-social 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. For the duration of the study 9 ‘prisoners’ were to remain in the mock prison for 24 hours per day for up to 2 weeks. 3 were to be arbitrarily assigned to each of the 3 cells; the other participants nominated ‘prisoners’ were to be on stand-by at their homes. The ‘guards’ were to work on 3-man 8-hour shifts and go home after their shifts.  The participants signed a contract guaranteeing basic living needs, such as an adequate diet and medical care. 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 (eg: privacy) suspended. The participants were not given any information about what to expect or how to behave. The 24 participants were randomly assigned by the flip of a coin 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. (Interestingly, all the participants expressed the preference to become ‘prisoners’.)

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 researchers, the ‘superintendent’ of the prison (Zimbardo) and the ‘warden’ (an 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 enforce 16 rules to ensure “a reasonable degree of order” – eg: only eating at certain times or obtaining permission to go to the toilet or write a letter. The only constraint was that no physical punishment or physical aggression was allowed. Other than that, the guards were to run the prison as they saw fit. Zimbardo (1989) said he told the guards: “You can create in the prisoners feelings of boredom, a sense of fear to some degree, you can create a notion of arbitrariness that their life is totally controlled by us, by the system, you, me – and they’ll have no privacy…they can do nothing, say nothing that we don’t permit. We’re going to take away their individuality.” 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, etc. 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 2 groups – deindividuation. The guards’ uniform consisted of a plain khaki shirt and trousers, a whistle, a police night stick (a wooden baton) and reflecting sunglasses to make 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 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 be humiliating, serving as symbols of subservience and dependence. The ankle chain was a constant reminder that they were inmates in a correctional facility. The stocking cap removed any distinctiveness associated with hair length, colour and style (mimicking the shaving of heads in some ‘real’ prisons). 

On the chosen Sunday morning the 9 ‘prisoners’ waiting at home “to be called” for the start of the experiment found their homes were raided without warning by real local police officers! They were arrested on suspicion of burglary or armed robbery, advised of their rights, handcuffed, thoroughly searched (often in full view of their neighbours and passers by) and driven in the back of a police car to the police station. (The Palo Alto Police had agreed to help with the experiment!) As if they were real-life suspects, the prisoners had their mug shots (ID pictures) and fingerprints taken and were put in a detention cell. Throughout the 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.

The ‘prisoners’ were then blindfolded and driven by one of the experimenters and a ‘guard’ to the basement of Jordan Hall (‘Stanford County Prison’). 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  taken, the prisoner was put in his cell and ordered to remain silent. The warden read them the rules of the institution which were to be memorised and had to be followed. Prisoners were to be referred to only by the number on their uniforms – another strategy to depersonalise them.

Every day the participants were allowed 3 bland meals, 3 supervised toilet visits and given 2 hours for the privilege of reading or letter writing. Work assignments had to be carried out and 2 visiting periods per week were scheduled, as were movie rights and exercise periods. 3 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.

On the second day of the experiment the prisoners ripped the identifying numbers from their uniforms and organised a mass revolt and riot, as a protest about the conditions. They taunted and cursed the guards. Guards worked extra hours and devised a strategy to break up and put down the riot, using fire-extinguishers to hose them down. They broke into each cell, stripped the prisoners naked, took the beds out and forced the ringleaders of the prisoner rebellion into solitary confinement – an extremely small, unlit room (2 x 2 x 7 ft), nicknamed ‘the hole’ across from the cells. No prompt for this action was given by Zimbardo; the guards used their own initiative to formulate the plan.

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 guards started most of the interactions, many 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. Lengthy prisoner counts became a trial of ordeal – with sleep deprivation a result of early-morning roll calls – and the prisoners were subjected to ritual humiliations, with forced exercise and physical punishments becoming more and more common. Mattresses having been confiscated from the prisoners, they were forced to sleep on cold, hard floors.

Initially punishments took the form of loss of privileges but the guards rapidly increased total control of each prisoner’s life, including going to the toilet. Prisoners were often not allowed to use the toilet and forced to urinate or defecate in a bucket in their cell – but not allowed to empty the buckets. Repeatedly guards also punished prisoners by forcing them to do push-ups, jumping jacks, cleaning out toilet bowls with their bare hands and acting out other degrading scenarios. (One prisoner was forced to do push-ups with a guard standing on his back!) Solitary confinement in ‘the hole’ was used. Often they also coerced prisoners to become snitches in exchange for reduced abuse. Especially when they were bored or thought that the experimenters were not watching, their treatment of the prisoners would escalate and became more pornographic. 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. Prisoners were also often stripped and subjected to sexual humiliation, including simulated homosexual sex, as a weapon of intimidation.

The humiliation and dehumanisation got so severe, that the experimenters had to frequently remind the guards to refrain from such tactics.

Prisoners became passive, excessively obedient, showed flattened mood and distorted perception of self, often slouching and keeping their eyes fixed to the ground, They had become institutionalised very quickly and adapted to their roles. One third of the guards began to show an extreme streak of sadism; and some of them were so enthusiastic that they volunteered to work extra hours without pay. Zimbardo himself started to become internalised in the experiment. The prisoners started to experience acute emotional disturbance, acute anxiety, crying and rage. They exhibited disorganised thinking, uncontrollable crying, withdrawing and behaving in pathological ways.

Overall, researchers had to release 5 prisoners from the experiment prematurely – the first one on the second day (with fits of crying and rage) and another 3 over the following 3 days! Of the remaining prisoners, only 2 said they were not willing to forfeit the money they had earned in return for being ‘paroled’.  A replacement prisoner was introduced and was instructed to go on hunger strike as a protest about the treatment of his fellow inmates and as an attempt to obtain early release. Surprisingly, his fellow inmates viewed him as a troublemaker rather than a fellow victim trying to help them. When the inmates were informed that, if the rest of their prisoners gave up their blankets, he would be released from solitary confinement in ‘the hole’, all but one refused to give up their blanket.

Other people connected to the experiment were also sucked in by the situation. The experimenters forgot that they were there to observe and collect data. Instead, they started to assume the role of prison staff and supervisor. A priest who visited the prison started to contact parents of the prisoners about arranging lawyers to bail them out. The parents, who had visited the prison themselves, seem to also have forgot that their sons had the right to withdraw from the experiment. They actually started to arrange lawyers. And a lawyer actually came…with 5 prisoners appearing before a ‘parole board’! (One prisoner developed a rash over his entire body when his ‘parole’ was rejected!) However, when on the fourth day, Zimbardo and the guards attempted to move the prisoners to the more secure local police station on the basis that some prisoners were talking about trying to escape, the officials at the station said they could no longer participate in the experiment.

The experiment carried on for 6 days until an outside psychologist, Christina Maslach – also Zimbardo’s  girlfriend (whom he later married) – was brought in to interview guards and prisoners and was shocked by the scenes that she was witnessing. At her insistence Zimbardo terminated the experiment early – it was designed to run for 14 days – and noted that out of over 50 external visitors, Maslach was the only one to raise concerns about what was happening.

All of the remaining prisoners were delighted by the end of the experiment; 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 the experience. Half the prisoners endured the oppressive atmosphere. Not all the guards resorted to hostility; some were tough but fair while some went far beyond their roles to engage in creative cruelty and harassment.