In his article General Characteristics of Crowds, Gustav LeBon discusses how individuals change within a crowd, creating a crowd mind that is more than the sum or average of their collective personalities.

"[T]he fact that they have been transformed into a crowd puts them in possession of a sort of collective mind which makes them feel, think, and act in a manor quite different from that in which each individual of them would feel, think , and act were he in a state of isolation." (LeBon, 122)

An excellent example of this sort of change are soccer hooligans. 1 2 3 I was first introduced to this violent manifestation of the crowd several years ago when living in England. I was very surprised to hear my relatives, who were not normally violent people, talking about fights after soccer games and what can only be called hooliganism. After watching several television programs on this phenomenon, I came to realize that many hooligans were lawyers, businessmen, and other productive members of the community. It seemed strange to me that otherwise peaceful people could change so much so easily. Perhaps LeBon's theory explains it.

LeBon, Gustav. "General Characteristics of Crowds--Psychological Law of Their Mental Utility."