It started with a classified ad in the “help wanted” section of the Palo Alto Times. Between job postings for Avon reps and security system sales was an ad reading:  

“Male College students needed for psychological study of prison life. $15 per day for 1-2 weeks beginning Aug. 15. For further information & applications, come to Room 248, Jordan Hall, Stanford U.” 

It was 1971, and the ad was placed by Philip Zimbardo, a psychology professor at Stanford University. The previous semester, some of Zimbardo’s undergraduate students had built a small mock prison in their dorm for a class project studying the psychological effects of incarceration on both prisoners and guards. The results were alarming. Over just three days, the role-playing prisoners became despondent and desperate, and the guards turned into bullies.  

Zimbardo was intrigued. If an unscientific dorm room experiment could elicit such strong reactions, what about a full-blown psychological study? Twenty-four young men were recruited from the ad—a mix of Stanford students and others in Palo Alto for the summer—and were randomly divided into two groups: prisoners and guards.