diagnostic interviews and personality tests

Procedure
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.
The 75 applicants who answered the ad were given diagnostic interviews and personality tests to eliminate candidates with psychological problems, medical disabilities, or a history of crime or drug abuse.
24 men judged to be the most physically & mentally stable, the most mature, & the least involved in antisocial behaviors were chosen to participate. The participants did not know each other prior to the study and were paid $15 per day to take part in the experiment. 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. 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. Zimbardo observed the behavior of the prisoners and guards (as a researcher), and also acted as a prison warden.

Findings
Within a very short time both guards and prisoners were settling into their new roles, with the guards adopting theirs quickly and easily.

The prisoners soon adopted prisoner-like behavior 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 obey the rules.

Zimbardo (1973) had intended that the experiment should run for two weeks, but on the sixth day it was terminated, due to the emotional breakdowns of prisoners, and excessive aggression of the guards. Christina Maslach, a recent Stanford Ph.D. brought in to conduct interviews with the guards and prisoners, strongly objected when she saw the prisoners being abused by the guards.
Filled with outrage, she said, “It’s terrible what you are doing to these boys!” Out of 50 or more outsiders who had seen our prison, she was the only one who ever questioned its morality.
Conclusion
Zimbardo’s Stanford prison experiment revealed how 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.