Collective behavior is a sociological term that refers to the various activities in which large numbers of people engage. Most often, these are relatively spontaneous and relatively unstructured behaviors. The qualifying term "relatively" means that while events may be planned, or somewhat organized, the event has a tendency to move past these structures. Examples of collective behavior include crowds, mobs, riots, panics, mass hysteria, fads, and crazes.
Sociologist Robert E. Park was the first to use the term "collective behavior" in 1921, but the term was adopted and used by later thinkers to explore the ways in which events do not reflect existing social structures, but occur in a more spontaneous ways. Herbert Blumer divided collective behavior into four forms: the crowd, the public, the mass, and the social movement. There have been five major theories to explain why people behave certain ways in crowds.
Contagion Theory maintains that crowds hold a hypnotic influence over members. In large numbers, people become anonymous and drop any sense of personal responsibility. Convergence Theory, in contrast, holds that people who wish to behave a certain way form crowds. Emergent-Norm Theory states that new norms emerge within crowd situations. Value-added Theory states that collective behavior is a sort of cathartic release. Finally, Complex Adaptive Systems Theory claims that contagion, convergence, and emergent norms are simply examples of synergy.