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In social psychology, pluralistic ignorance is a situation where a majority of group members privately reject a norm, but assume incorrectly that most others accept it, also described as 'no one believes, but everyone thinks that everyone believes." In short, pluralistic ignorance is a bias about a social group, held by a social group.  Lack of public opposition then helps perpetuate a norm that may be, in fact, disliked by most people. A lot of people are wrong about something but because everyone sees this wrong idea as the perceived social norm, no one speaks up against it.
Pluralistic ignorance can be contrasted with the false consensus effect. In pluralistic ignorance, people privately disdain but publicly support a norm (or a belief), while the false consensus effect causes people to wrongly assume that most people think like them, while in reality most people do not think like them (and express the disagreement openly). For instance, pluralistic ignorance may lead a student to drink alcohol excessively because she believes that everyone else does that, while in reality everyone else also wishes they could avoid binge drinking, but no one expresses that due to the fear of being ostracized. A false consensus for the same situation would mean that the student believes that most other people do not enjoy excessive drinking, while in fact most other people do enjoy that and openly express their opinion about it.
The term pluralistic ignorance was coined by Daniel Katz and Floyd H. Allport in 1931. Krech and Crutchfield’s described it, in (1948, pp. 388–89), as the situation where 'no one believes, but everyone thinks that everyone believes.'"In 1975, O'Gorman defined pluralistic ignorance as "Erroneous belief shared by two or more people regarding the ideas, sentiments, and actions of other individuals."
Consequences of pluralistic ignorance 
Pluralistic ignorance was blamed for a perception (among American whites) that grossly exaggerated the support of other American whites for segregation in the 1960s. It has also been named[by whom?] a reason for the illusionary popular support that kept the communist regime in the Soviet Union, as many opposed the regime but assumed that others were supporters of it. Thus, most people were afraid to voice their opposition.
Another example of pluralistic ignorance in history is prohibition. Most people thought people were for it when in reality most people were against it, including those who were vocal about it's initiation.
In a series of studies conducted to test the effect of pluralistic ignorance, Prentice and Miller  studied the consequences of pluralistic ignorance at Princeton University. They found that, on average, private levels of comfort with drinking practices on campus were much lower than the perceived average. In the case of men, they found a shifting of private attitudes toward this perceived norm, a form of cognitive dissonance. Women, on the other hand, were found to have an increased sense of alienation on the campus but lacked the attitude change detected in men, presumably because norms related to alcohol consumption on campus are much more central for men than for women. Research has shown that pluralistic ignorance plagues not only those who indulge, but also those who abstain: from gambling, smoking, and drinking and among some who follow vegetarianism. The latter has found that Pluralistic Ignorance can be caused by the structure of the underlying social network, not cognitive dissonance.
Pluralistic ignorance may partially explain the bystander effect: the observation that people are more likely to intervene in an emergency situation when alone than when other persons are present. If people monitor the reactions of others in such a situation, they may conclude from the inaction of others that other people think that it is not necessary to intervene. Thus no one may take any action, even though some people privately think that they should do something. On the other hand, if one person intervenes, others are more likely to follow and give assistance.
Not much research has been done to explain why pluralistic ignorance occurs. In 1993, Noelle-Neumann suggested that media biases lead to pluralistic ignorance. Also in 1993 Prentice & Miller used “Social fear” to explain pluralistic ignorance. They said individuals are afraid to express their true feelings on issues for fear of being socially isolated. This causes the perceived social norm to persist without opposition.
See also 
- Abilene paradox
- False consensus
- Spiral of silence
- Social norms marketing
- The Emperor's New Clothes
- Katz, Daniel, and Floyd H. Allport. 1931. Student Attitudes. Syracuse, N.Y.: Craftsman
- Centola, Damon, Robb Willer, and Michael Macy. 2005. "The Emperor’s Dilemma: A Computational Model of Self-Enforcing Norms." American Journal of Sociology 110:1009
- O’Gorman, Hubert. 1975. “Pluralistic Ignorance and White Estimates of White Support for Racial Segregation.” Public Opinion Quarterly 39 (3): 313–30.
- Kuran, Timur. 1995. Private Truths, Public Lies: The Social Consequences of Preference Falsification. Cambridge, Mass.: Harvard University Press.
- Prentice, Deborah A.; Miller, Dale T. (1993), "Pluralistic Ignorance and Alcohol Use on Campus: Some Consequences of Misperceiving the Social Norm", Journal of Personality and Social Psychology (American Psychological Association) 64 (2): 243–256, doi:10.1037/0022-35220.127.116.11, ISSN 0022-3514, PMID 8433272
- Schank, R. L. 1932. “A Study of Community and Its Group Institutions Conceived of as Behavior of Individuals.” Psychological Monographs 43 (2): 1–133
- Kitts, James A. 2003. “Egocentric Bias or Information Management? Selective Disclosure and the Social Roots of Norm Misperception.” Social Psychology Quarterly 66 (3): 222–37.
- Aronson, E., Akert, R. D., and Wilson, T. D. (2007). Social psychology (6th Ed.). Upper Saddle River, NJ: Pearson Prentice Hall, p. 362.