The Asch Conformity Experiments

Solomon Asch conducted an experiment to investigate the extent to which social pressure from a majority group could affect a person to conform.

Asch (1951) devised what is now regarded as a classic experiment in social psychology, whereby there was an obvious answer to a line judgment task.

If the participant gave an incorrect answer it would be clear that this was due to group pressure.
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The Asch Conformity Experiments:

https://www.verywellmind.com/the-asch-conformity-experiments-2794996