the idea of a "group mind" or "mob behavior" was first put forward by 19th-century french social psychologists gabriel tarde and gustave le bon. herd behavior in human societies has also been studied by sigmund freud and wilfred trotter, whose book "instincts of the herd in peace and war" is a classic in the field of social psychology. sociologist and economist thorstein veblen's "the theory of the leisure class" illustrates how individuals imitate other group members of higher social status in their consumer behavior. more recently, malcolm gladwell in "the tipping point", examines how cultural, social, and economic factors converge to create trends in consumer behavior. in 2004, the "new yorker" financial columnist james suroweicki published "the wisdom of crowds".