Key study: Haney, Banks and Zimbardo (1973) prisoners and guards

Aim: to investigate whether the brutality reported in prisons is caused by the personality of guards and prisoners (dispositional) or by the prison situation and the roles people take on.
Sample: 75 men answered a newspaper advert; 24 male college students judged most physically and mentally healthy were selected and paid $15 a day. They were randomly assigned to be guards or prisoners.
Procedure: a mock prison was built in the basement of the psychology department at Stanford University in 1971.
Prisoners were 'arrested' at home by real police, then fingerprinted, blindfolded and taken to the prison. They wore smocks with ID numbers, a chain on one ankle and a cap, and were called only by number (to deindividuate them).
Guards wore khaki uniforms and mirrored sunglasses and carried a whistle and a baton. They were told to keep order but not to use physical violence. The study was planned to last two weeks.

Results: guards became increasingly aggressive and controlling, using punishments such as press-ups and taking away privileges. Aggression increased daily and was worse at night, when guards thought they were not being watched.
Prisoners rebelled on the second day, then became passive and depressed.
Five prisoners had to be released early because of extreme emotional reactions such as crying, rage and anxiety.
Not all guards were cruel: some were fair, some harsh.
The study was stopped after six days.
Conclusion: the situation and the social roles people were given, rather than their personalities, caused the behaviour. Both groups conformed to their roles.

Strengths: participants were screened as healthy and randomly allocated, so differences in behaviour seem to come from the roles, not personality. It gave detailed, rich data and has influenced views on prison conditions.
Weaknesses: serious ethical problems: participants suffered psychological harm, and found it hard to leave. Zimbardo acted as prison superintendent, so he was not objective. Demand characteristics: guards may have been acting how they thought guards should. Small, all-male American student sample. A later study, the BBC Prison Study (Reicher and Haslam), did not find guards becoming tyrannical in the same way.