Proposes a theory of social impact specifying the effect of other persons on an individual. According to the theory, when other people are the source of impact and the individual is the target, impact should be a multiplicative function of the strength, immediacy, and number of other people. Furthermore, impact should take the form of a power function, with the marginal effect of the Nth other person being less than that of the (N–2)th. When other people stand with the individual as the target of forces from outside the group, impact should be divided such that the resultant is an inverse power function of the strength, immediacy, and number of persons standing together. The author reviews relevant evidence from research on conformity and imitation, stage fright and embarrassment, news interest, bystander intervention, tipping, inquiring for Christ, productivity in groups, and crowding in rats.