The prison cell is small and dirty. Three men in bleached garments are cowering on the bare floor. Silent, they wince at every noise from down the corridor. Suddenly, two men in uniforms and sunglasses appear at the cell door, smacking their batons into their hands. Violence is imminent.

Six days earlier both the inmates and the guards had been regular college kids. It was 1971, and they were about to start living a two-week experiment designed by psychologist Philip G. Zimbardo. The Stanford University researcher randomly divided a group of mentally healthy students into "guards" and "inmates" who would inhabit a simulated prison on campus. Zimbardo had to stop the experiment prematurely after only six days, because the guards had become sadistic, abusing the inmates physically and mentally.

This article was originally published with the title How Group-Think Makes Killers.

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