The Solomon Asch conformity experiments were a set of experiments performed in the 1950’s by the American psychologist, Solomon Eliot Asch, in which it was demonstrated that an individual’s opinions could and would be influenced by those making up the majority of a group.
Asch’s most basic experiment was conducted on male college students who were placed into groups such that all but one of the participants were actors who had previously discussed the way in which they would respond. The true subject of the experiment, was the one person in the group who had no knowledge of the purpose and methodology of the experiment. During the experiment, participants were first shown a card with a line on it, followed by a second card which had three lines of differing lengths drawn on them. Participants were then asked to say aloud which line on the second card matched the length of the line shown on the first card. The actors had determined prior to the experiment whether to correctly or incorrectly state which lines were a match. For the first two trials of the experiment, the actors would correctly report the matching line, and the individual subject of the experiment seemed at ease. On the third trial, however, the actors would all give the same wrong answer. In other words, they would all report that a certain line on one card was the same length as the second line on the card adjacent to the first, even when it was evident that the lines were not correctly matched. The purpose of the experiment was to see whether the real participant would change his answer and respond incorrectly so as to conform to the answer of others, who formed the majority of the group. Asch reported that 75% of participants gave an incorrect answer to at least one question while only 25% never gave an incorrect response.

Source:

Asch, S.E. (1951). Effects of group pressure on the modification and distortion of judgements. In H. Guetzkow (Ed.), Groups, leadership and men (pp. 177–190). Pittsburgh: PA Carnegie Press.