Yale study (1961) demonstrating how readily people obey authority and inflict apparent harm. The classic post-Nuremberg social psychology study.
From Wikipedia
The Milgram Experiment is, in the early 1960s, a series of social psychology experiments that were conducted by Yale University psychologist Stanley Milgram, who intended to measure the willingness of study participants to obey an authority figure who instructed them to perform acts conflicting with their personal conscience. Participants were led to believe that they were assisting in a fictitious experiment, in which they had to administer electric shocks to a "learner". These fake electric shocks gradually increased to levels that would have been torturous had they been real.