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Herd mentality, or mob mentality, describes how people are influenced by their peers to adopt certain behaviors, follow trends, and/or purchase items. Examples of the herd mentality include stock market trends, fashions in apparel, cars, taste in music, superstition, religion, home décor, etc. Social psychologists study the related topics of group intelligence, crowd wisdom, and decentralized decision making.
The term herd mentality is the word herd, meaning "group of animals," and mentality, implying a certain frame of mind. However the most succinct definition would be: "how large numbers of people act in the same ways at the same times."
Herd behavior is distinguished from herd mentality because it applies to all animals, whereas the term mentality implies a uniquely human phenomenon. Herd mentality implies a fear-based reaction to peer pressure which makes individuals act in order to avoid feeling "left behind" from the group. Herd mentality is also known as "mob mentality".
Herd mentality and herd behavior have been prevalent descriptors for human behavior since people began to form tribes, migrate in groups, and perform cooperative marketing and agricultural functions. 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 Herd Instincts in Peace and War is a classic in the field of social psychology. Sociologist and Economist Thorstein Veblen's 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's financial columnist James Suroweicki published The Wisdom of Crowds.
21st-century academic fields such as marketing and behavioral finance attempt to identify and predict the rational and irrational behavior of investors. (See the work of Daniel Kahneman, Robert Shiller, Vernon L. Smith, and Amos Tversky.) Driven by emotional reactions such as greed and fear, investors can be seen to join in frantic purchasing and sales of stocks, creating bubbles and crashes.
See also 
- Argumentum ad populum
- Bandwagon effect
- Collective intelligence
- Critical mass (sociodynamics)
- Crowd psychology
- Decentralized decision making
- Delphi method
- Early adopter
- Group intelligence
- Herd behavior
- Information cascade
- Opinion leadership
- Predictive market
- Religious paranoia
- Social network
- The Wisdom of Crowds
Further reading 
- Bloom, Howard, The Global Brain: The Evolution of Mass Mind from the Big Bang to the 21st Century. (2000) John Wiley & Sons, New York.
- Freud, Sigmund's Massenpsychologie und Ich-Analyse (1921; English translation Group Psychology and the Analysis of the Ego, *1922). Reprinted 1959 Liveright, New York.
- Gladwell, Malcolm, The Tipping Point: How Little Things Can Make a Big Difference. (2002) Little, Brown & Co., Boston.
- Le Bon, Gustav, Les Lois psychologiques de l'évolution des peuples. (1894) National Library of France, Paris.
- Le Bon, Gustave, The Crowd: A Study of the Popular Mind. (1895) Project Gutenberg.
- McPhail, Clark. The Myth of the Madding Crowd (1991) Aldine-DeGruyter.
- Trotter, Wilfred, Instincts of the Herd in Peace and War. (1915) Macmillan, New York.
- Suroweicki, James: The Wisdom of Crowds: Why the Many Are Smarter Than the Few and How Collective Wisdom Shapes Business, Economies, *Societies and Nations. (2004) Little, Brown, Boston.
- Sunstein, Cass, Infotopia: How Many Minds Produce Knowledge. (2006) Oxford University Press, Oxford, United Kingdom.