Philip Zimbardo, who has died aged 91, was a psychologist whose studies included work on shyness, the psychological roots of evil and the effects of the past on decision-making; but he was best known for the 1971 Stanford Prison Experiment,

Philip G. Zimbardo, the psychologist behind the controversial “Stanford Prison Experiment” that was intended to examine the psychological experiences of imprisonment, has died. He was 91.

Zimbardo's "Prison Experiment," a landmark and controversial study, was shut down after six days, but its implications have had a profound effect.

Philip Zimbardo's 1971 Stanford University experiment, employing college students to play prison guards and inmates, became one of the most controversial episodes in modern psychology.

The 1971 prison study was scheduled for two weeks, but was stopped after six days when college-aged students playing guards became psychologically abusive.

The 1971 prison study was scheduled for two weeks, but was stopped after six days when college-aged students playing guards became psychologically abusive and those playing prisoners became anxious, emotionally depressed,