Stanley Milgram's classic experiments on Obedience to Authority, begun in 1962 at Yale University and involved more than 1,000 participants, adults ages 20 to 50 from New Haven and Bridgeport, Connecticut. This research included 19 separate experiments (that each varied one aspect of the social situation) leading to the conclusion: Human behavior is often under the influence of external forces in the social context rather than simply a product of one's personality.
A recent summary of these experiments that challenged our view of the nature of human nature is found in the book, Obedience to Authority: An Experimental View with a foreward by Philip Zimbardo.
Participants were led to believe that they were to play the role of "teacher" in helping another participant, their "learner," improve memory by shocking errors they made in learning new material. Shocks were apparently delivered by pressing a switch on a large shock generator, starting with only 15 volts and increasing by 15-volt increments along the 30 shock switches " up to the maximum level of 450 volts. The learner-victim began to yell out in pain and to protest continuing in the study after the shock level reached into the 100-volt levels and beyond. The "Authority" in charge, wearing a lab coat, insisted that the teacher continue to the next shock levels, ignoring those complaints, since he (the experimenter-authority) would be responsible for whatever happened to the learner.
[AD]Two of three participants (65 percent) went all the way to end of the shock board, administering the full 450 volts to their victim! Such a result was totally unexpected by psychologists, as well as the general public. This result escalated to 90 percent in a study where the participants first observed another person going all the way to the end, but decreased to only 10 percent when observing others who refused to give the full 450 volts. These results lead us to conclude that we are all powerful social models for eliciting good or evil in others by what we do or refuse to do. (Note that the learner was a confederate of the experimenter, who did not actually get shocked, and met with the teachers after they completed the experiment, by being blindly obedient to authority, or by refusing to do so.)