Social Proof is the bandwagon effect, people are more likely to engage in an action if other people are doing it.
Social proof, a term coined by Robert Cialdini in his 1984 book, Influence, is also known as informational social influence. It describes a psychological and social phenomenon wherein people copy the actions of others in an attempt to undertake behavior in a given situation.
Social proof is considered prominent in ambiguous social situations where people are unable to determine the appropriate mode of behavior, and is driven by the assumption that the surrounding people possess more knowledge about the current situation.
The effects of social influence can be seen in the tendency of large groups to conform to choices which are either correct or mistaken. This is referred to in some publications as the herd behavior. Although social proof reflects a rational motive to take into account the information possessed by others, formal analysis shows that it can cause people to converge too quickly upon a single distinct choice, so that decisions of even larger groups of individuals may be grounded in very little information (see information cascades).
Social proof is one type of conformity. When a person is in a situation where they are unsure of the correct way to behave, they will often look to others for clues concerning the correct behavior. When "we conform because we believe that others' interpretation of an ambiguous situation is more accurate than ours and will help us choose an appropriate course of action", it is informational social influence. This is contrasted with normative social influence wherein a person conforms to be liked or accepted by others.
Social proof often leads not only to public compliance (conforming to the behavior of others publicly without necessarily believing it is correct) but also private acceptance (conforming out of a genuine belief that others are correct).
Social proof is more powerful when being accurate is more important and when others are perceived as especially knowledgeable.