Behavioral contagion is a form of social contagion involving the spread of behavior through a group. It refers to the propensity for a person to copy a certain behavior of others who are either in the vicinity, or whom they have been exposed to. The term was originally used by Gustave Le Bon in his 1895 work The Crowd: A Study of the Popular Mind to explain undesirable aspects of behavior of people in crowds. In the digital age, behavioral contagion is also concerned with the spread of online behavior and information. A variety of behavioral contagion mechanisms were incorporated in models of collective human behavior.
Behavioral contagion has been attributed to a variety of different factors. Often it is distinguished from collective behavior that arises from a direct attempt at social influence. A prominent theory involves the reduction of restraints, put forth by Fritz Redl in 1949 and analyzed in depth by Ladd Wheeler in 1966. Social psychologists acknowledge a number of other factors, which influence the likelihood of behavioral contagion occurring, such as deindividuation (Festinger, Pepitone, & Newcomb, 1952) and the emergence of social norms (Turner, 1964). In 1980, Freedman et al. have focused on the effects of physical factors on contagion, in particular, density and number.