To study the roles people play in prison situations, Zimbardo converted a basement of the Stanford University psychology building into a mock prison. He advertised asking for volunteers to participate in a study of the psychological effects of prison life,
More than 70 applicants answered the ad and were given diagnostic interviews and personality tests to eliminate candidates with psychological problems, medical disabilities, or a history of crime or drug abuse. The study comprised 24 male college students (chosen from 75 volunteers) who were paid $15 per day to take part in the experiment.
Participants were randomly assigned to either the role of prisoner or guard in a simulated prison environment. There were two reserves, and one dropped out, finally leaving ten prisoners and 11 guards. The guards worked in sets of three (being replaced after an 8-hour shift), and the prisoners were housed three to a room. There was also a solitary confinement cell for prisoners who 'misbehaved.' The prison simulation was kept as “real life” as possible.
Prisoners were treated like every other criminal, being arrested at their own homes, without warning, and taken to the local police station. They were fingerprinted, photographed and ‘booked.’ Then they were blindfolded and driven to the psychology department of Stanford University, where Zimbardo had had the basement set out as a prison, with barred doors and windows, bare walls and small cells. Here the deindividuation process began.
When the prisoners 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. The use of ID numbers was a way to make prisoners feel anonymous. Each prisoner had to be called only by his ID number and could only refer to himself and the other prisoners by number. Their clothes comprised a smock with their number written on it, but no underclothes. They also had a tight nylon cap to cover their hair, and a locked chain around one ankle.
All guards were dressed in identical uniforms of khaki, and they carried a whistle around their neck and a billy club borrowed from the police. Guards also wore special sunglasses, to make eye contact with prisoners impossible. Three guards worked shifts of eight hours each (the other guards remained on call). Guards were instructed to do whatever they thought was necessary to maintain law and order in the prison and to command the respect of the prisoners. No physical violence was permitted.
Zimbardo observed the behavior of the prisoners and guards (as a researcher), and also acted as a prison warden.