Stanford Prison Experiment - 'We should have ended it earlier'
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.