Em Griffin in his book The Mindchangers describes an experiment done by Solomon Asch with groups of 12 people. They were brought into a room where four lines of unequal length were displayed.

They had to decide which two were the same length and publicly vote for their choice. Person after person after person (11 in all) voted for the wrong line—because they had all been told to ahead of time. The one individual who was in the dark couldn’t imagine how in the world all these seemingly normal people could all choose the wrong line. When it was his turn to vote, he had to decide, “Do I go with what I know my senses are telling me, or do I go with the crowd?” 1/3 of those tested caved in to group pressure and changed their vote to agree with their peers.

(Em Griffin, The Mindchangers, Tyndale House, 1976, p. 193ff)