Zimbardo and a team of academics from Stanford University selected a group of 22 volunteers for a two-week study of the dynamics of prison life. On the basis of a coin toss, half were designated "guards," and assigned the proper costumes. The others were labeled "prisoners" and swaddled in prison attire.
Within a day, the participants had immersed themselves in their respective roles. The guards became aggressive, hostile, and verbally abusive (physical violence was forbidden); the "prisoners" became depressed, and a few developed psycho-somatic afflictions, such as rashes.
The "guards," by way of contrast, reveled in their status, constantly inflicting whimsical punishments on the "prisoners" and looking for new and more inventive ways to restrict what little liberty they still enjoyed.
When a graduate assistant protested that it was a form of abuse to inflict suffering on poorly-paid volunteers, Zimbardo ended the experiment – after just six days.
When Dr. Zimbardo announced the end of his study, "most of the guards seemed to be distressed … and it appeared to us that they now enjoyed the extreme control and power which they had exercised and were reluctant to give it up," he wrote in an essay published by the 1973 issue of Naval Research Review.
"Being a guard carried with it social status within the prison, a group identity (when wearing the uniform), and, above all, freedom to exercise an unprecedented degree of control over the lives of other human beings," Zimbardo observed. "This control was invariably expressed in terms of sanctions, punishment, demands, and with the threat of manifest physical power. There was no need for the guards to rationally justify a request as they did [in] their ordinary life, and merely to make a demand was sufficient to have it carried out. Many of the guards showed in their behavior and revealed in post-experimental statements that this sense of power was exhilarating."