Social proof, also known as informational social influence, is a psychological phenomenon where people assume the actions of others in an attempt to reflect correct behavior for a given situation. This effect is prominent in ambiguous social situations where people are unable to determine the appropriate mode of behavior, and is driven by the assumption that surrounding people possess more knowledge about the situation. src Wiki
The most famous study of social proof is Muzafer Sherif's 1935 experiment. In this experiment subjects were placed in a dark room and asked to look at a dot of light about 15 feet away. They were then asked how much, in inches, the dot of light was moving.
In reality it was not moving at all, but due to the autokinetic effect it appeared to move.
How much the light appears to move varies from person to person but is generally consistent over time for each individual. A few days later a second part of the experiment was conducted. Each subject was paired with two other subjects and asked to give their estimate of how much the light was moving out loud. Even though the subjects had previously given different estimates, the groups would come to a common estimate. To rule out the possibility that the subjects were simply giving the group answer to avoid looking foolish while still believing their original estimate was correct, Sherif had the subjects judge the lights again by themselves after doing so in the group. They maintained the group's judgment. Because the movement of the light is ambiguous the participants were relying on each other to define reality.
In 1951, Solomon Aisch conducted his confirmity lab experiments. Groups of 8 students particpated in a perceptual task. 7 of the 9 people were actors and only one was the true subject of the experiment
Each participant viewed a card with a line on it followed by another with 3 lines labeled as A, B and C. One of these lines were the same as that on the first card.
In the trials, the actors were at times told to give the correct answer and at times all of them would give an incorrect answer.
The subject was also tested alone with none of the other participants.
It was observed that when in a group, in 1/3 of the cases the participants were swayed by the group's wrong answers.
Also read: The Art of Thinking Clearly