This except from Think Twice: Harnessing the Power of Counterintuition encapsulates Philip Zimbardo's famous prison experiment:
Philip Zimbardo, a psychologist at Stanford University, did an experiment in 1971 that ranks with Asch and Milgram in exhibiting the power of the situation. To start, Zimbardo advertised for volunteers in a two-week-long prison experiment, offering $15 a day. He ran the seventy applicants through psychological and physical tests and ended up with twenty-four healthy, mentally stable, middle-class, male students from the Palo Alto, California, area.
By a flip of a coin, Zimbardo assigned half the volunteers to be prisoners and the rest to be prison guards. One morning, a Palo Alto police car picked up the prisoners, “charging” them with armed robbery and burglary. The guards were assigned to work one of three eight-hour shifts.
With the help of prison consultants, Zimbardo built a prison in the basement of the building that housed Stanford’s psychology department. On arrival at the jail, the guards and warden took steps to humiliate, dehumanize, and oppress the prisoners.
Although Zimbardo randomly designated the roles, the situation clearly shaped behavior. He noticed that the volunteers (and he himself) started to act out their assigned roles. The prisoners tried various tactics to gain advantage over the guards and tried to escape, while the guards schemed to keep the prisoners in check. Concerned that the guards were heaping too much abuse on the prisoners, Zimbardo questioned the situation’s morality and ended the study after only five days.
Zimbardo explains the factors that make the situation so forceful:
First, situational power is most likely in novel settings, where there are no previous behavioral guidelines. Second, rules— which may emerge through interaction or be predetermined— can create a means to dominate and suppress others because people justify their behavior as only conforming to the rules. Third, when people are asked to play a certain role for a prolonged period, they risk becoming actors who can’t break from character. Roles shut people off from their normal lives and accommodate behaviors they would generally avoid. Finally, in situations that lead to negative behavior, there is often an enemy— an outside group. This is especially pronounced when both the in-group and out-group stop focusing on individuals.
In his book, The Lucifer Effect: Understanding How Good People Turn Evil, Zimbardo offers tips for “resisting the pull of unwelcome social influence.”