Crowd psychology is a branch of social psychology that deals with the ways in which the psychology of a crowd is different from the psychology of the individual persons who are in the crowd. The field of crowd psychology enquires into the behaviors and thought processes of both the individual members of the crowd and the crowd as a collective social entity. The behavior of a crowd is much influenced by deindividuation, a person's loss of responsibility, and the person's impression of the universality of behavior, both of which conditions increase in magnitude with size of the crowd. Notable theorists in crowd psychology include Gustave Le Bon, Gabriel Tarde, and Sigmund Freud.

Social psychology is the scientific study of how thoughts, feelings, and behaviors are influenced by the actual, imagined, or implied presence of others. Social psychologists typically explain human behavior as a result of the relationship between mental states and social situations, studying the social conditions under which thoughts, feelings, and behaviors occur, and how these variables influence social interactions.