Social Proof

The effects of social influence can be seen in the tendency of large groups to conform to choices which may be either correct or mistaken, a phenomenon sometimes referred to as 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 choice, so that decisions of even large groups of individuals may be grounded in very little information. http://en.wikipedia.org/wiki/Social_proof