SAN FRANCISCO (AP) — 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. Stanford University announced Friday that Zimbardo died Oct. 14 at his home in San Francisco. A cause of death was not provided.

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,