That would be the late Yale psychologist Stanley Milgram, who, in light of the many atrocities of obedience in the 20th century, wondered how and under what conditions ordinarly people would follow orders to inflict harm on people they had nothing against.

Short answer: it was WAY easier than he initially thought.

To briefly recap, Milgram set up a situation in which subjects thought they were participating in an experiment on the relationship of punishment or negative reinforcement to learning. They were to give a task invovling word memory to a learner who was in fact an actor. Each time the learner/actor made a “mistake,” the subjects were asked to give them an electric shock of steadily increasing severity.
Part I
Part II
Part III
Part IV