Philip Zimbardo, the psychologist behind the controversial "Stanford Prison Experiment" in which participants playing prison guards became abusive, has died aged 91.
Stanford University announced his death on Friday, saying he died at his home in San Francisco.
Professor Zimbardo's research included persuasion, hypnosis, cults, shyness, time perspective, altruism, and compassion.
However, he was best known for an experiment he ran in 1971 that aimed to replicate prison conditions to see how participants would react.
He and a team of graduate students recruited university-age men to spend two weeks inside a mock prison in a basement on Stanford campus.
However, the study ended after just six days because the students playing the role of guards became psychologically abusive.
The participants in the "prisoner" role became anxious, emotionally depressed and enraged, according to a statement from the university.
Professor Zimbardo was criticised for taking on the role of superintendent - an active participant in the study - instead of remaining a neutral observer.
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Despite the controversy, the experiment is now used in psychology classes all over the world to study evil and the ethics of psychological research with human beings.
"Phil Zimbardo is one of the most prolific and influential psychologists of his generation - a true pioneer of the field of social psychology," said Claude Steele, professor of psychology at Stanford.
"Virtually all of Phil's research shows how important phenomena of real-life human behaviour can be studied scientifically. For a young science like social psychology, this has been an especially important contribution."
"Most people go about their daily life assuming that they have more control over their behaviour than they actually do," Professor Zimbardo said in 1971 as the prison experiment was beginning.
"We are often unaware of the tremendous power which social situations exert upon us to shape, guide, and manipulate our behaviour."
He is survived by his wife, three children and four grandchildren.
(c) Sky News 2024: Philip Zimbardo: Psychologist behind controversial 'Stanford Prison Experiment' dies aged 91
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