Crowd psychology (also mob 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 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.
The first debate in crowd psychology began in Rome, at the first International Congress of Criminal Anthropology, on 16 November 1885. The Congress was dominated by Cesare Lombroso and fellow Italians, who emphasized the biological determinates of the psychology of a crowd.