Stanford Prison Experiment: how roles reshape identity

In 1971, Philip Zimbardo assigned Stanford students the roles of guards or prisoners in a mock jail. The study was designed to run two weeks but was halted after six days because the 'guards' had become psychologically abusive and the 'prisoners' showed signs of genuine trauma. The experiment demonstrated how powerfully situational roles can override personal character — ordinary students became oppressors simply because their environment demanded it. Zimbardo later used the findings to explain abuses at Abu Ghraib.

Source: Zimbardo, The Lucifer Effect (2007)