On Aug. 14, 1971, police in Palo Alto, Calif., arrested 10 undergraduates who were guilty of no crimes.

The students were being handcuffed as part of their roles as prisoners in the now-famous Stanford prison experiment, conducted by Philip Zimbardo GRD ’59. The conditions of the experiment, which occurred 40 years ago at Stanford University, inspired a national movement for improved regulation of human subject testing.

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Zimbardo and his colleagues recruited 21 college-aged, male volunteers and divided them into two groups — “prisoners” and “guards” — by mandating outfits and rules intended to “deindividualize” them. Guards were given wide discretion to maintain order, and prisoners were instructed not to use their real names and to obey all orders from guards or face punishment. Within two days, the study began to go awry.

Prisoners came to internalize their performed passivity and hopelessness as guards became increasingly hostile toward them. Guards arbitrarily harassed inmates, ordered them to clean toilets with their bare hands and encouraged them to perform sexual acts. The researchers terminated the experiment early, after seeing signs of severe depression and distress in several prisoners.

The unmoderated conditions of the study, which psychology professor Marianne LaFrance said likely violate today’s rules governing the treatment of human subjects, demonstrated that normal people can be induced to act maliciously, even sadistically, when put in certain roles.

“Most of us put into that situation would probably behave in a like manner,” LaFrance said, “even though when we’re asked to think about how we’d act, we say, ‘Never!’”

Zimbardo’s experiment helped inform the rules that present-day researchers follow, and independent review boards now consider outlier situations more carefully, Susan Bouregy, vice chair of Yale’s human subjects committee, said.

“Even if the current standards were in place [back then], everyone would still have been surprised,” she said, adding that Stanford’s human subjects committee had approved Zimbardo’s research proposal.

In 2004, Zimbardo testified for the defense in the trial of Sgt. Ivan Frederick, who was one of the officers court-martialled for his involvement in the Abu Ghraib prisoner abuses.