Male participants were recruited and told they would participate in a two-week prison simulation. The team selected the 24 applicants whose test results predicted they would be the most psychologically stable and healthy. These participants were predominantly white and of the middle class. The group was intentionally selected to exclude those with criminal backgrounds, psychological impairments, or medical problems. They all agreed to participate in a 7- to 14-day period and received $15 per day (roughly equivalent to $94 per day in 2018).
The experiment was conducted in a 35-foot (10.5 m) section of a basement of Jordan Hall (Stanford's psychology building). The prison had two fabricated walls, one at the entrance, and one at the cell wall to block observation. Each cell (6 × 9 feet, or 1.8 × 2.7 m), contained only a cot for the prisoners. In contrast, the guards lived in a very different environment, separated from the prisoners. They were given rest and relaxation areas, and other comforts.
Twelve of the twenty-four participants were assigned the role of prisoner (nine plus three potential substitutes), while the other twelve were assigned the role of guard (also nine plus three potential substitutes). Zimbardo took on the role of the superintendent and an undergraduate research assistant took on the role of the warden. Zimbardo designed the experiment in order to induce disorientation, depersonalization, and deindividuation in the participants.
The researchers held an orientation session for the guards the day before the experiment, during which guards were instructed not to harm the prisoners physically or withhold food or drink. In the footage of the study, Zimbardo can be seen talking to the guards: "You can create in the prisoners feelings of boredom, a sense of fear to some degree, you can create a notion of arbitrariness that their life is totally controlled by us, by the system, you, me, and they'll have no privacy ... We're going to take away their individuality in various ways. In general what all this leads to is a sense of powerlessness. That is, in this situation we'll have all the power and they'll have none."
The researchers provided the guards with wooden batons to establish their status, clothing similar to that of an actual prison guard (khaki shirt and pants from a local military surplus store), and mirrored sunglasses to prevent eye contact. Prisoners wore uncomfortable, ill-fitting smocks and stocking caps, as well as a chain around one ankle. Guards were instructed to call prisoners by their assigned numbers, sewn on their uniforms, instead of by name.
The prisoners were "arrested" at their homes and "charged" with armed robbery. The local Palo Alto police department assisted Zimbardo with the arrests and conducted full booking procedures on the prisoners, which included fingerprinting and taking mug shots. The prisoners were transported to the mock prison from the police station, where they were strip searched and given their new identities.
The small mock prison cells were set up to hold three prisoners each. There was a small corridor for the prison yard, a closet for solitary confinement, and a bigger room across from the prisoners for the guards and warden. The prisoners were to stay in their cells and the yard all day and night until the end of the study. The guards worked in teams of three for eight-hour shifts. The guards were not required to stay on site after their shift.
Guards had differing responses to their new roles. Dave Eshelman, described by Stanford Magazine as "the most abusive guard" felt his aggressive behavior was helping experimenters to get what they wanted. John Mark, who had joined the experiment hoping to be selected as a prisoner, instead recalls "At that time of my life, I was getting high, all day every day... I brought joints with me, and every day I wanted to give them to the prisoners. I looked at their faces and saw how they were getting dispirited and I felt sorry for them. "Warden" David Jaffe intervened with Mark to get him to change his behavior, to "participate" more and become more "tough."