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In social psychology , an ingroup is a social group towards which an individual feels loyalty and respect, usually due to membership in the group. This loyalty often manifests itself as an ingroup bias. Commonly encountered ingroups include family members, people of the same race, culture or religion, and so on.
By contrast, an outgroup is a social group to which an individual does not identify. For example, people may find it psychologically meaningful to view themselves according to their race, culture, gender or religion. It has been found that the psychological membership of social groups and categories is associated with a wide variety of phenomena.
The terminology was made popular by Henri Tajfel and colleagues during his work in formulating social identity theory. The significance of ingroup and outgroup categorization was identified using a method called the minimal group paradigm. Tajfel and colleagues found that people can form self preferencing ingroups within a matter of minutes and that such groups can form even on the basis of seemingly trivial characteristics, such as preferences for certain paintings.
The psychological categorization of people into ingroup and outgroup members is associated with a variety of phenomena. The following examples have all received a great deal of academic attention.
- Main article: In-group favoritism
This refers to the fact that under certain conditions people will preference and have affinity for one’s in-group over the out-group, or anyone viewed as outside the in-group. This can be expressed in evaluation of others, linking, allocation of resources and many other ways. Research demonstrates that people often privilege ingroup members over outgroup members even when the ingroup has no actual social standing; for instance, a group of people with the same color shirts, when the other group has another color of shirt..
Discrimination between ingroups and outgroups is a matter of favoritism towards an ingroup and the absence of equivalent favoritism towards an outgroup . Outgroup derogation is the phenomena in which an outgroup is perceived as being threatening to the members of an ingroup . This phenomenon often accompanies ingroup favoritism, as it requires one to have an affinity towards their ingroup. Some research suggests that outgroup derogation occurs when an outgroup is perceived as blocking or hindering the goals of an ingroup. It has also been argued that outgroup derogation is a natural consequence of the categorization process  .
- Main article: Social influence
People have been shown to be differentially influenced by ingroup members. That is, under conditions where group categorization is psychologically salient people will shift their beliefs in line with ingroup social norms.
- Main article: Group polarization
This generally refers to the tendency for groups to make decisions that are more extreme than the initial inclination of its members, although polarization toward the most central beliefs has also been observed. It has been shown that this effect is related to a psychologically salient ingroup and outgroup categorization
- Main article: Outgroup homogeneity
Categorization of people into social groups increases the perception that group members are similar to one another. An outcome of this is the outgroup homogeneity effect. This refers to the perception of members of an outgroup as being homogenous, while members of one's ingroup are perceived as being diverse, e.g. "they are alike; we are diverse,”  . This is especially likely to occur in regards to negative characteristics. Under certain conditions, ingroup members can be perceived as being similar to one another in regards to positive characteristics. This effect is called ingroup homogeneity. 
In evolutionary psychology, ingroup favoritism is seen as an evolved mechanism selected for the advantages of coalition affiliation. It has been argued that characteristics such as gender and ethnicity inflexible or even essential features of such systems.However, there is evidence that elements of favoritism are flexible in that they can be erased by changes in social categorization. One study in the field of behavioural genetics suggests that biological mechanisms may exist which favor a coexistence of both flexible and essentialist systems.
- Ambivalent prejudice
- Benevolent prejudice
- Collective narcissism
- Common ingroup identity
- Cross-race effect
- Ethnic identity
- False consensus effect
- Group attribution error
- Group-serving bias
- Hostile prejudice
- Ingroup bias
- List of biases in judgment and decision making
- Optimal distinctiveness theory
- Out-group homogeneity
- Out-group homogeneity bias
- Social dominance orientation
- Self perception
- Social identity
- Social networks
- Social perception
- H. Tajfel, (1970). Experiments in Intergroup Discrimination.
- H. Tajfel, M. Billig, R. P. Bundy and C. Flament (April/June 1971). Social categorization and intergroup behaviour. European Journal of Social Psychology 1 (2): 149–178.
- H. Tajfel, (1974). Social Identity and Intergroup Behavior.
- Aronson, E., Wilson, T. D. & Akert, R. D. (2009). Social psychology, 7th, Upper Saddle River: Prentice Hall. ISBN 0-138-144788; ISBN 978-0-13814-478-4.
- Meagan M. Patterson, Rebecca S. Bigler. Effects of physical atypicality on children's social identities and intergroup attitudes.
- Brewers, Marilynn B. (1999). The Psychology of Prejudice: Ingroup Love and Outgroup Hate?. Journal of Social Issues 55 (3): 429-444.
- (2002) "Intergroup Bias" Richard J. Crisp Social Psychology, 323-344, New York, NY: Routledge.
- (2008). Negational categorization and intergroup behavior. Personality and Social Psychology Bulletin 34 (6): 793-806.
- (1994) Stereotypes and Social Cognition, 104-107, London: Sage Publications.
- (1980). The perception of variability within in-groups and out-groups: Implications for the law of small numbers.. Journal of Personality and Social Psychology 38 (1): 141–152.
- Jackson, Lynne M. (2011). The Psychology of Prejudice: From Attitudes to Social Action, 110-112, Washington, DC: American Psychological Association.
- L. Cosmides, J. Tooby and R. Kurzban (April 1, 2003). Perceptions of race. Trends in Cognitive Sciences 7 (4): 173–179.
- L. A. Hirschfeld (1996). Race in the Making: Cognition, Culture, and the Child's Construction of Human Kinds, Cambridge, Massachusetts: Mit Press. ISBN 0-262-08247-0; ISBN 978-0-26208-247-1.
- F. J. Gil-White (August/October 2001). Are Ethnic Groups Biological "Species" to the Human Brain? Essentialism in Our Cognition of Some Social Categories. Current Anthropology 42 (4): 515–553.
- R. Kurzban, J. Tooby and L. Cosmides (December 18, 2001). Can race be erased? Coalitional computation and social categorization. Proceedings of the National Academy of Science USA 98 (26): 15387–15392.
- G. J. Lewis and T. C. Bates (November 2010). Genetic Evidence for Multiple Biological Mechanisms Underlying In-Group Favoritism. Psychological Science 21 (11): 1623–1628.
- Ingroup loyalty, outgroup hostility: The power of tribes in evolution
- Social Development and Human Evolution: Managing the Ingroup Boundary, Michael J. Lovaglia, Chana Barron, Jeffrey A. Houser