Philip G. Zimbardo, a Stanford University social psychologist whose aborted 1971 experiment, employing college students to play prison guards and inmates, became one of the most controversial episodes in modern psychology and yielded disturbing insights into the effect of stress on human behavior, died Oct. 14. He was 91. For his prison experiment, conducted over a school break, Dr. Zimbardo recruited 24 male students, screening them to ensure that they had no history of crime or violence and that they were emotionally stable. He randomly assigned half to be guards and half to be prisoners. The guards were instructed not to physically harm the prisoners but to otherwise maintain control as they saw fit. Wearing uniforms and sunglasses, they forced the prisoners to follow strict rules, and punishments for violators included solitary confinement in a converted closet. The prisoners ... were referred to by numbers. After an attempted revolt on the second day, the guards began asserting their authority by turning fire extinguishers on the prisoners and demanding that they ... strip naked. Dr. Zimbardo [wrote] that the experiment was "a cautionary tale" about what can happen when "we underestimate the extent to which the power of social roles and external pressures can influence our actions." He saw parallels between the guards' behavior during his exercise and the conduct of U.S. soldiers who had abused prisoners at the Abu Ghraib prison.