Video of one of Asch’s conformity experiments.  Below is a paraphrase from Wikipedia:

 The Asch conformity experiments, published in 1951, were a series of studies that starkly demonstrated the power of conformity in groups.

Experiments led by Solomon Asch asked students to participate in a “vision test.” In reality, all but one of the participants were confederates of the experimenter, and the study was really about how the remaining student would react to the confederates’ behavior.

The participants — the real subject and the confederates — were all seated in a classroom where they were told to announce out loud their judgment of the length of several lines drawn on a series of displays. They were asked which line was longer than the other, which were the same length, etc. The confederates had been prearranged to all give an incorrect answer to the tests.

While most subjects answered correctly, many showed extreme discomfort, and a high proportion (32%) conformed to the erroneous majority view of the others in the room when there were at least three confederates present, even when the majority said that two lines different in length by several inches were the same length. When the confederates were not unanimous in their judgment, subjects were much more likely to defect than when the confederates all agreed. Control subjects with no exposure to a majority view had no trouble giving the correct answer.

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