Milgram experiment
Participants were told they would be helping out in a "learning experiment". The participant played the role of a "teacher" in which they had to ask questions to the "learner". Every time the learner got a question wrong or failed to answer, the teacher had to press a switch to give the learner an electric shock. The voltage increased each time. In the original version of the experiment, the teacher and learner were in separate rooms, but could talk to each other through the wall.