Philip Zimbardo the psychologist behind the Stanford Prison Experiment
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Stanford Prison Experiment - 'We should have ended it earlier'

17 August 2011 Last updated at 18:40 BST

Forty years ago, a group of students hoping to make a bit of holiday money turned up at a basement in Stanford University, California, for what was to become one of the most notorious experiments in the study of human psychology.

The idea was simple - take a group of volunteers, tell half of them they are prisoners, the other half prison wardens, place them in a makeshift jail and watch what happens.

The Stanford Prison Experiment was supposed to last two weeks but was ended abruptly just six days later, after a string of mental breakdowns, an outbreak of sadism and a hunger strike.

Philip Zimbardo was the psychologist who led the experiment. Here, he and some of those who took part look back on what happened.