He converted a basement of the Stanford University
psychology building into a mock prison. He advertised
for students to play the roles of prisoners and guards for
a fortnight. 21 male college students (chosen from 75
volunteers) were screened for psychological normality
and paid $15 per day to take part in the experiment.
The aim was to see how readily people would react
and if the prisoners would conform to the guards' rules.
Participants were randomly assigned to either the role of prisoner
or guard. It was kept as “real life” as possible. Prisoners were
arrested at their own homes, without warning, and taken to the local
police station. Guards were also issued a khaki uniform, together
with whistles, handcuffs and dark glasses, to make eye contact
with prisoners impossible. No physical violence was permitted.
Here they were treated like every other criminal. They were
fingerprinted, photographed and ‘booked’. Then they were
blindfolded and driven to Stanford University. The prison had
barred doors and windows, bare walls and small cells. Here
the deindividuation process began. When they arrived at the
prison they were stripped naked, deloused, had all their
personal possessions removed and locked away, and were
given prison clothes and bedding. They were issued a uniform,
and referred to by their number only. Their clothes comprised
a smock with their number written on it, but no underclothes.
They also had a tight nylon cap, and a chain around one
ankle. There were 3 guards to the 9 prisoners, taking shifts of
eight hours each (the other guards remained on call).
Within a very short time both guards and prisoners were
settling into their new roles, the guards adopting theirs
quickly and easily. Within hours of beginning the experiment
some guards began to harass prisoners. They behaved in a
brutal and sadistic manner, apparently enjoying it. Other
guards joined in, and other prisoners were also tormented.
The prisoners were taunted with insults and petty orders,
they were given pointless and boring tasks to accomplish,
and they were generally dehumanised.
The prisoners soon adopted prisoner-like behaviour too.
They talked about prison issues a great deal of the time.
They ‘told tales’ on each other to the guards. They started
taking the prison rules very seriously, as though they were
there for the prisoners’ benefit and infringement would spell
disaster for all of them. Some even began siding with the
guards against prisoners who did not conform to the rules.
As the prisoners became more
dependent, the guards became
more derisive towards them.
They held the prisoners in
contempt and let the prisoners
know it. As the guards’
contempt for them grew, the
prisoners became more
submissive. As the prisoners
became more submissive, the
guards became more aggressive
and assertive. They demanded
ever greater obedience from the
prisoners. The prisoners were
dependent on the guards for
everything so tried to find ways
to please the guards, such as
telling tales on fellow prisoners.
One prisoner had to be
released after 36 hours
because of uncontrollable
bursts of screaming, crying
and anger. His thinking
became disorganized and
he appeared to be entering
the early stages of a deep
depression. Within the next
few days three others also
had to leave after showing
signs of emotional disorder
that could have had lasting
consequences. (These
were people who had been
pronounced stable and
normal a short while
before.)
Zimbardo had intended that the experiment should run for a
fortnight, but on the sixth day he closed it down. There was
real danger that someone might be physically or mentally
damaged if it was allowed to run on. After some time the
subjects were called back for a follow-up, debriefing session.
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. The “prison”
environment was an important factor in creating the guards’
brutal behaviour (none of the participants who acted as guards
showed sadistic tendencies before the study). Therefore, the
roles that people play can shape their behaviour and attitudes.
The study has received many ethical criticisms, including lack of
fully informed consent by participants and the level of humiliation
and distress experienced by those who acted as prisoners. The
consent could not be fully informed as Zimbardo himself did not
know what would happen in the experiment (it was
unpredictable). Also, participants playing the role of prisoners
were not protected from psychological and physical harm. For
example, the prisoner who had to be released after 36 hours.