In 1961, a psychology professor at Yale University named Stanley Milgram placed an ad in the local newspaper to recruit men for a special study about "memory and learning." Unbeknownst to the subjects, however, the study turned out to be about something else entirely.
The volunteers were assigned roles: teacher, student (or learner), and scientist (or experimenter). The teacher was instructed to ask the student questions based on a memory test, and if the student was unable to answer correctly, he was given an electric shock. The experimenter ordered the teacher to increase the power of the voltage each time the student answered incorrectly.
Unbeknownst to the volunteer playing the teacher role, both the learner and experimenter were actually members of Yale's faculty. So the electric shock was not real, but the volunteer in the teacher role did not know this. During the experiment, the experimenter continually encouraged the teacher to keep shocking the learner, even when—or in some cases, especially when—the learner seemed to be in extreme pain.
Although some teachers refused to keep "shocking" the learners, the vast majority continued, some even discharging what they were told was the highest level of shock possible to the learners. While some of the people in this majority attempted to resist, they were told by the experimenters that the experiment must continue.
Part of what inspired Milgram to conduct this study was the Holocaust. He was interested in exploring the notion of true evil versus the excuse of "just following orders." His conclusion? Most people will obey authority to do horrible things and inflict pain—out of a sense of obedience or a desire to seem appear cooperative.