In-group favoritism, sometimes known as in-group–out-group bias, in-group bias, intergroup bias, or in-group preference, is a pattern of favoring members of one's in-group over out-group members. This can be expressed in evaluation of others, in allocation of resources, and in many other ways.
This effect has been researched by many psychologists and linked to many theories related to group conflict and prejudice. The phenomenon is primarily viewed from a social psychology standpoint. Studies have shown that in-group favoritism arises as a result of the formation of cultural groups. These cultural groups can be divided based on seemingly trivial observable traits, but with time, populations grow to associate certain traits with certain behaviour, increasing covariation. This then incentivises in-group bias.
Two prominent theoretical approaches to the phenomenon of in-group favoritism are realistic conflict theory and social identity theory. Realistic conflict theory proposes that intergroup competition, and sometimes intergroup conflict, arises when two groups have opposing claims to scarce resources. In contrast, social identity theory posits a psychological drive for positively distinct social identities as the general root cause of in-group favoring behavior.
Origins of the research tradition
In 1906, the sociologist William Sumner posited that humans are a species that join together in groups by their very nature. However, he also maintained that humans had an innate tendency to favor their own group over others, proclaiming how "each group nourishes its own pride and vanity, boasts itself superior, exists in its own divinities, and looks with contempt on outsiders" (p. 13). This is seen on the group level with ingroup–outgroup bias. When experienced in larger groups such as tribes, ethnic groups, or nations, it is referred to as ethnocentrism.
Realistic conflict theory (or realistic group conflict) posits that competition between groups for resources is the cause of in-group bias and the corresponding negative treatment of members of the out-group. Muzafer Sherif's Robbers Cave Experiment is the most widely known demonstration of realistic conflict theory. In the experiment, 22 eleven-year-old boys with similar backgrounds were studied in a mock summer camp situation, with researchers posing as camp personnel.
The boys were divided into two equal groups and encouraged to bond, with the aim of fostering an in-group mentality. The researchers then introduced a series of competitive activities which pitted groups against each other for a valuable prize. Hostility and out-group negativity ensued. Lastly, researchers attempted to reverse the hostility by engaging the boys in situations of mutual interdependence, an effort which eventually resulted in relative harmony between the two groups.
Sherif concluded from this experiment that negative attitudes toward out-groups arise when groups compete for limited resources. However, he also theorised that inter-group frictions could be reduced and positive relations created, but only in the presence of an over-arching goal, which could only be achieved with the two groups' cooperation.
According to social identity theory, one of the key determinants of group biases is the need to improve self-esteem. The desire to view one's self positively is transferred onto the group, creating a tendency to view one's own group in a positive light, and by comparison, outside groups in a negative light. That is, individuals will find a reason, no matter how insignificant, to prove to themselves why their own group is superior. This phenomenon was pioneered and studied most extensively by Henri Tajfel, a British social psychologist who looked at the psychological root of in-group/out-group bias. To study this in the lab, Tajfel and colleagues created minimal groups (see minimal group paradigm), which occur when "complete strangers are formed into groups using the most trivial criteria imaginable". In Tajfel's studies, participants were split into groups by flipping a coin, and each group then was told to appreciate a certain style of painting none of the participants were familiar with when the experiment began. What Tajfel and his colleagues discovered was that—regardless of the facts that a) participants did not know each other, b) their groups were completely meaningless, and c) none of the participants had any inclination as to which "style" they like better—participants almost always "liked the members of their own group better and they rated the members of their in-group as more likely to have pleasant personalities". By having a more positive impression of individuals in the in-group, individuals are able to boost their own self-esteem as members of that group.
Robert Cialdini and his research team looked at the number of university T-shirts being worn on college campuses following either a win or loss at the football game. They found that the Monday after a win, there were more T-shirts being worn, on average, than following a loss.
In another set of studies, done in the 1980s by Jennifer Crocker and colleagues using the minimal group paradigm, individuals with high self-esteem who suffered a threat to the self-concept exhibited greater ingroup biases than did people with low self-esteem who suffered a threat to the self-concept. While some studies have supported this notion of a negative correlation between self-esteem and in-group bias, other researchers have found that individuals with low self-esteem showed more bias toward both in-group and out-group members. Some studies have even shown that high-self-esteem groups showed more bias than did lower self-esteem groups. This research may suggest that there is an alternative explanation and additional reasoning as to the relationship between self-esteem and in-group/out-group biases. Alternatively, it is possible that researchers have used the wrong sort of self-esteem measures to test the link between self-esteem and in-group bias (global personal self-esteem rather than specific social self-esteem).
Biological basis as an effect of oxytocin
In a meta-analysis and review of the effect of oxytocin on social behavior done by Carsten De Dreu, the research reviewed shows that oxytocin enables the development of trust, specifically towards individuals with similar characteristics—categorised as 'in-group' members—promoting cooperation with and favoritism towards such individuals. This bias of oxytocin-induced goodwill towards those with features and characteristics perceived to be similar may have evolved as a biological basis for sustaining in-group cooperation and protection, fitting with the Darwinian insight that acts of self-sacrifice and cooperation contribute to the functioning of the group and hence improve the odds of survival for members of said group.
Race can be used as an example of in-group and out-group tendencies because society often categorizes individuals into groups based on race (Caucasian, African American, Latino, etc.). One study that examined race and empathy found that participants receiving nasally administered oxytocin had stronger reactions to pictures of in-group members making pained faces than to pictures of out-group members with the same expression. This shows that oxytocin may be implicated in our ability to empathize with individuals of different races, with individuals of one race potentially biased towards helping individuals of the same race than individuals of another race when they are experiencing pain.
Oxytocin has also been implicated in lying when lying would prove beneficial to other in-group members. In a study where such a relationship was examined, it was found that when individuals were administered oxytocin, rates of dishonesty in the participants' responses increased for their in-group members when a beneficial outcome for their group was expected. Both of these examples show the tendency to act in ways that benefit in-group members.
As noted in two recent theoretical reviews, the theoretical basis for the inclusion of self-identity in the theories of reasoned action and planned behavior has many similarities to social identity theory  and its extension, self-categorization theory. According to social identity theory, an important component of the self-concept is derived from memberships in social groups and categories. When people define and evaluate themselves in terms of a self-inclusive social category (e.g., sex, class, team) two processes come into play: (1) categorization, which perceptually accentuates differences between the in-group and out-group, and similarities among in-group members (including the self) on stereotypical dimensions; and (2) self-enhancement which, because the self-concept is defined in terms of group membership, seeks behaviorally and perceptually to favor the in-group over the out-group. Social identities are cognitively represented as group prototypes that describe and prescribe beliefs, attitudes, feelings and behaviors that optimize a balance between minimization of in-group differences and maximization of intergroup differences.
More specifically, according to social identity theory, there is a continuum between personal and social identity shifts along this continuum that determine the extent to which group-related or personal characteristics influence a person's feelings and actions. If a particular social identity is a salient basis for self-conception, then the self is assimilated to the perceived in-group prototype which can be thought of as a set of perceived in-group norms such that self-perception, beliefs, attitudes, feelings and behaviours are defined in terms of the group prototype. Thus, social identities should influence behaviour through the mediating role of group norms. People will be more likely to engage in a particular behaviour if it is in accord with the norms of a behaviourally relevant group membership, particularly if the identity is a salient basis for self-definition. If the group membership is not salient, then people's behaviour and feelings should be in accord with their own personal and idiosyncratic characteristics rather than group norms.
On the other hand, the self-identity theory poses that the self is often a reflection of expected norms and practices in a person's social role. At the center of it is the proposition that the self is made up of multi-faceted and differentiated components that exist in an organized manner for the sake of filling in roles in society. People are able to create an identity for themselves only through talking to others, and often what roles they are taking on differ from one group to another. These differing roles and positions people fill are a result of their interactions with others and are called role identities. Role identities may be self-realized, or may be facts like being a mother, a social worker, or a blood donor. Role identities lead people to act in certain ways due to assumed expectations for the roles. Because there is satisfaction in complying with expectations of the role, there is often distress behind an inability to appear congruent to one's identity as defined by societal norms. There is also an existing hierarchy of importance for roles that individuals take on, and according to the hierarchical standing of roles, people become more representative of roles that stand higher hierarchically, according to them.
Identity salience, the likelihood of role identities being invoked in different situations, is the result of role identities being placed hierarchically in different orders from person to person. People who hold the same roles may act differently because some roles are valued over others. For example, a working mother may have less time to spend with her child as opposed to a mother that does not work. Behaviors are reflective of the identities that are held higher hierarchically by people, so people act out in self-worth and self-meaning according to these hierarchies. Someone who holds the identity of being a psychologist higher than the identity of being a linguist will find that while he/she may become competitive when meeting another person that is better at psychology than he/she, he/she won't care when in contact with someone who is much better at being a linguist than he/she. In a similar way, social relationships are influenced by this salience. Self-identity often places individuals in social contexts and a commitment to the role within that context becomes a big part of perpetrating the idea of self. It also finds people relating more to others that hold similar role identities at the top of their hierarchies.
Because people have self-concepts that are derived from a role they define for themselves within the context of a group, when staying within their roles, intergroup similarities are accentuated while intergroup differences are diminished. In an attempt to assimilate oneself according to the tendencies of a group, often people reconfigure their intragroup representations or identities. Certain prototypes form about these groups that reaffirm rules that members of the group are encouraged to follow. Shared information and views are discussed more often than novel and unshared information within a group, therefore a norm is established where the majority views are perpetuated and others silenced. This norm is fluid and changing according to different contexts, but those within the group who want to keep up with the majority views in all matters have to keep an active role in affirming the views of the in-group in contest to out-groups.
Evolution of in-groups
Formation of cultural groups
Studies have shown that in-group favoritism arises endogenously, through the formation of cultural groups. Symbolic markers in certain conditions can result in trivial groupings developing into cultural groups. The formation of such cultural groups then results in a higher degree of in-group favoritism.
Efferson, Lalive and Fehr published such a study in 2008, utilising a series of coordination games to mimic cooperation between individuals. The study found that cultural groups were able to form endogenously through creation of a linkage between a payoff-relevant behaviour and a payoff-irrelevant marker. Subsequently, in-group favouritism occurred in ensuing social interactions.
Participants were first divided into one of several populations of 10 people, and then further divided into subpopulations of 5. Each group had different payoff for coordinating on one of 2 choices, behaviour A or behaviour B. In group 1, participants were awarded 41 points for coordinating (choosing A themselves and choosing another participant who also chose A) on A and 21 for coordinating on B. The payoffs were switched in the 2nd group. In both groups participants were awarded just 1 point for mis-coordinating. During each turn participants were also allowed to choose a payoff-irrelevant marker (circle or triangle). Players from both subpopulations were mixed to create a coordination problem, and every turn, an unidentified player from each subpopulation would be randomly switched.
The experiment created a situation in which participants were strongly incentivised to develop a sense of expected behaviours in his or her subpopulation, but occasionally would find themselves in a totally new situation in which their behaviours were not in-line with social norms.
The results showed that players generally developed an inclination to pair behaviour with a marker, especially if it had resulted in a positive payoff. As linkages at an individual level increase, covariation (of marker and behaviour) at an aggregate level also increases. In the experiment, there was a significant increase in participants requesting for partners with the same-shape choice as it progressed, although the initial choice of shape had no effect on payoffs. Toward the end of the experiment, this number stood at a substantial 87%, indicating the presence of in-group favouritism.
Their study supported the hypothesis that the formation of cultural groups alters selective pressure facing individuals, and thus leads to certain behavioural traits being advantageous. Thus, if such selective pressures were present in past civilisations, where membership in a certain group is correlated with a certain behavioural norm, the emergence of in-group biases where it is beneficial to act in differing manners to members of the same group is certainly plausible.
Automatic bias for own gender
Rudman & Goodwin (2004) conducted research on gender bias that measured gender preferences without directly asking the participants. Subjects at Purdue and Rutgers participated in computerized tasks that measured automatic attitudes based on how quickly a person categorizes pleasant and unpleasant attributes with each gender. Such a task was done to discover whether people associate pleasant words (good, happy, and sunshine) with women, and unpleasant words (bad, trouble, and pain) with men.
This research found that while both women and men have more favorable views of women, women's in-group biases were 4.5 times stronger than those of men and only women (not men) showed cognitive balance among in-group bias, identity, and self-esteem, revealing that men lack a mechanism that bolsters automatic preference for their own gender.
Using a publics-goods game, Van Vugt, De Cremer, and P. Janssen (2007) found that men contributed more to their group in the face of outside competition from another group; there was no distinct difference amongst women's contributions.
Fershtman and Gneezy (2001) found that men showed in-group biases in a "trust" game based on ethnicity, whereas this tendency was not present in women. The study aim to identify ethnic discrimination in Israeli Jewish society, and was conducted on 996 Israeli undergraduates. Groups were separated based on whether the participant's name was typically ethnically Eastern or Ashkenazic. Similar to a dictator game, subjects were instructed to divide a sum of money (20 NIS) between themselves and another player. Player A was told that any money sent over to Player B would be tripled, and Player B would receive details of the experiment, including the name of Player A and the transferred sum. Subsequently, Player B would have a choice of whether to send any money back.
The experiment found that despite sharing similar average transfer values (10.63 for women and 11.42 for men), women did not display significant in-group biases when it came to recipients with either Ashkenazic or Eastern sounding names. However, a bias against Eastern sounding names was present amongst men.
Furthermore, men showed more bias for Ashkenazic men compared to women, but the opposite was true for Eastern names. This result may seem counter-intuitive, as participants appear to share more in common if they were both male. Thus, we would expect Eastern females to be more marginalised, but is actually consistent with other studies which studied discrimination against Afro-American women.
Fehr, Bernhard, and Rockenbach (2008), in a study conducted on children, found that boys displayed in-group favouritism from ages 3–8, whereas girls did not display such tendencies. The experiment involved usage of an "envy game", a modified version of the dictator game. A possible explanation posited by researchers relied on an evolutionary basis.
They theorised that parochialism and favouring members of the same group may have been particularly advantageous as it strengthened the individuals group position in intergroup conflicts. As males were the ones who were frequently at the forefront of such conflicts in the past, and thus bore the majority of the costs of conflicts in terms of injury or death, evolution may have favoured a greater sensitivity in males in situations which resulted in an advantageous payoff for their in-group. Thus males tended to show in-group biases from a younger age than females, as was evident in the experiment.
2008 US Presidential elections
A study conducted during the 2008 Presidential elections showcased how group identities were dynamic. The study was carried out among 395 Democrats from Cambridge, MA, using an Economics dictator game. Subjects were given $6 to divide between themselves and another person. The recipients remained anonymous, apart from which candidate they supported in the Democratic Primaries.
Data were collected in three separate periods. June 10 to 18 (after Hillary Clinton's concession speech on June 7); August 9 to 14, before the Democratic National Convention on the 25; and September 2 to 5, in the buildup to the Presidential elections. The results showed that men displayed significant in-group favouritism from June all the way to the DNC in August. This in-group bias, however, was not present in September. Women displayed no significant in-group favouritism throughout.
The experiment suggested that group identities are flexible and can change over time. Researchers theorised that in-group bias was strong in June, as the competition to be the Democratic nominee in the elections was still recent and thus salient. A lack of actual electoral conflict (against the Republicans) caused perception of salient groupings to remain throughout August. Only in September did the in-group favouritism subside as a superordinate goal shared between groups was now present.
Research analyzing articles about 35 inter-group conflicts (e.g., Falklands War) by comparing the corresponding language versions of Wikipedia (e.g., English, Spanish) found evidence for in-group favoritism: While the "in-group" was systematically preferred and presented in a more favorable light, the "out-group" was presented as more immoral and more responsible for the conflict. There were substantial variations between conflicts, however, and additional analyses revealed in-group favoritism to be more pronounced in more recent conflicts and in articles written predominantly by "in-group" members.
Versus out-group negativity
Social psychologists have long made the distinction between ingroup favouritism and outgroup negativity, where outgroup negativity is the act of punishing or placing burdens upon the outgroup. Indeed, a significant body of research exists that attempts to identify the relationship between ingroup favouritism and outgroup negativity, as well as conditions that will lead to outgroup negativity. For example, Struch and Schwartz found support for the predictions of belief congruence theory. The belief congruence theory concerns itself with the degree of similarity in beliefs, attitudes, and values perceived to exist between individuals. This theory also states that dissimilarity increases negative orientations towards others. When applied to racial discrimination, the belief congruence theory argues that the perceived dissimilarity of beliefs has more of an impact on racial discrimination than does race itself.
Oxytocin is not only correlated with the preferences of individuals to associate with members of their own group, but it is also evident during conflicts between members of different groups. During conflict, individuals receiving nasally administered oxytocin demonstrate more frequent defense-motivated responses toward in-group members than out-group members. Further, oxytocin was correlated with participant desire to protect vulnerable in-group members, despite that individual's attachment to the conflict. Similarly, it has been demonstrated that when oxytocin is administered, individuals alter their subjective preferences in order to align with in-group ideals over out-group ideals. These studies demonstrate that oxytocin is associated with intergroup dynamics.
Further, oxytocin influences the responses of individuals in a particular group to those of another group. The in-group bias is evident in smaller groups; however, it can also be extended to groups as large as one's entire country leading toward a tendency of strong national zeal. A study done in the Netherlands showed that oxytocin increased the in-group favoritism of their nation while decreasing acceptance of members of other ethnicities and foreigners. People also show more affection for their country's flag while remaining indifferent to other cultural objects when exposed to oxytocin. It has thus been hypothesized that this hormone may be a factor in xenophobic tendencies secondary to this effect. Thus, oxytocin appears to affect individuals at an international level where the in-group becomes a specific "home" country and the out-group grows to include all other countries.
Cross-cultural studies have found that in-group derogation, the tendency to criticize members of one's own group or culture more harshly than members of outside groups, is more common among members of disadvantaged and minority groups than among members of the majority or dominant group. According to psychology professor Christine Ma-Kellams et al. (2011), system justification theory seeks to explain why "minorities sometimes endorse system-justifying views of their group". They said their research into in-group favoritism and derogation partially supported this theory, but that the theory failed to address all of the nuances.
Ma-Kellams et al. also found that, compared to individualist cultures, people from collectivist cultures, such as East Asian cultures, tended to judge their own group members less favorably than they judged outsiders, whereas people from individualist cultures were inclined to judge members of their own group more favorably than they judged outsiders. Social identity theory and Freudian theorists explain in-group derogation as the result of a negative self-image, which they believe is then extended to the group. Ma-Kellams et al. theorized that "ingroup derogation may be more culturally normative and less troubling for East Asians" as evidenced by the fact that East Asians were also likely to report high levels of positive affect (emotion) towards members of their in-group, demonstrating ambivalence towards the unfavorable characteristics they had acknowledged about their in-group. According to Ma-Kellam et al., culturally-ingrained attitudes and beliefs, rather than low self-esteem, may play a role in collectivist cultures' in-group derogation, due to their ability to tolerate holding seemingly contradictory views.
- Aronson, E., Wilson, T. D., & Akert, R. (2010). Social psychology. 7th ed. Upper Saddle River: Prentice Hall.
- Taylor, Donald M.; Doria, Janet R. (April 1981). "Self-serving and group-serving bias in attribution". Journal of Social Psychology. 113 (2): 201–211. doi:10.1080/00224545.1981.9924371.
- Efferson, Charles; Lalive, Rafael; Fehr, Ernst (2008-09-26). "The Coevolution of Cultural Groups and Ingroup Favoritism" (PDF). Science. 321 (5897): 1844–1849. Bibcode:2008Sci...321.1844E. doi:10.1126/science.1155805. PMID 18818361.
- Fu, Feng; Tarnita, Corina E.; Christakis, Nicholas A.; Wang, Long; Rand, David G.; Nowak, Martin A. (2012-06-21). "Evolution of in-group favoritism". Scientific Reports. 2: 460. Bibcode:2012NatSR...2E.460F. doi:10.1038/srep00460. PMC 3380441. PMID 22724059.
- Sumner, William Graham. (1906). Folkways: A Study of the Social Importance of Usages, Manners, Customs, Mores, and Morals. Boston, MA: Ginn.
- Sherif, M.; Harvey, O.J.; White, B.J.; Hood, W. & Sherif, C.W. (1961). Intergroup Conflict and Cooperation: The Robbers Cave Experiment. Norman, OK: The University Book Exchange. pp. 155–184.
- Billig, Michael; Tajfel, Henri (1973). "Social categorization and similarity in intergroup behaviour". European Journal of Social Psychology. 3 (1): 27–52. doi:10.1002/ejsp.2420030103.
- Cialdini, Robert B.; Borden, Richard J.; Thorne, Avril; Walker, Marcus Randall; Freeman, Stephen; Sloan, Lloyd Reynolds (1974). "Basking in Reflected Glory: Three (Football) Field Studies". Journal of Personality and Social Psychology. 34 (3): 366–375. doi:10.1037/0022-35184.108.40.2066.
- Crocker, Jennifer; Thompson, Leigh L.; McGraw, Kathleen M.; Ingerman, Cindy (May 1987). "Downward Comparison, Prejudice, and Evaluations of Others: Effects of Self-Esteem and Threat". Journal of Personality and Social Psychology. 52 (5): 907–916. doi:10.1037/0022-35220.127.116.117.
- Abrams, Dominic; Hogg, Michael A. (1988). "Comments on the motivational status of self-esteem in social identity and intergroup discrimination". European Journal of Social Psychology. 18 (4): 317–334. doi:10.1002/ejsp.2420180403.
- Sachdev, Itesh; Bourhis, Richard Y. (September 1987). "Status differentials and intergroup behaviour". European Journal of Social Psychology. 17 (3): 277–293. doi:10.1002/ejsp.2420170304.
- Rubin, Mark; Hewstone, Miles (February 1998). "Social Identity Theory's Self-Esteem Hypothesis: A Review and Some Suggestions for Clarification". Personality and Social Psychology Review. 2 (1): 40–62. doi:10.1207/s15327957pspr0201_3. PMID 15647150.
- De Dreu, Carsten K.W. (2012). "Oxytocin modulates cooperation within and competition between groups: An integrative review and research agenda". Hormones and Behavior. 61 (3): 419–428. doi:10.1016/j.yhbeh.2011.12.009. PMID 22227278.
- Sheng F, Liu Y, Zhou B, Zhou W, Han S (February 2013). "Oxytocin modulates the racial bias in neural responses to others' suffering". Biological Psychology. 92 (2): 380–6. doi:10.1016/j.biopsycho.2012.11.018. PMID 23246533.
- Shalvi S, De Dreu CK (April 2014). "Oxytocin promotes group-serving dishonesty". Proceedings of the National Academy of Sciences of the United States of America. 111 (15): 5503–7. Bibcode:2014PNAS..111.5503S. doi:10.1073/pnas.1400724111. PMC 3992689. PMID 24706799.
- Hogg, Michael A.; Terry, Deborah J.; White, Katherine M. (1995). "A Tale of Two Theories: A Critical Comparison of Identity Theory with Social Identity Theory". Social Psychology Quarterly. 58 (4): 255–269. doi:10.2307/2787127. JSTOR 2787127.
- Tajfel, Henri (1974-04-01). "Social identity and intergroup behaviour". Social Science Information. 13 (2): 65–93. doi:10.1177/053901847401300204.
- Turner, John C. (1985). "Social categorization and the self-concept: A social cognitive theory of group behavior". In Lawler, E. J. (ed.). Advances in Group Processes: Theory and Research, Vol.2. Greenwich, CT: JAI. pp. 77–122. ISBN 9780892325245. OCLC 925165493.
- Tajfel, Henri; Turner, John C. (1979). "An integrative theory of intergroup conflict". In Austin, W. G.; Worchel, S. (eds.). The Social Psychology of Intergroup Relations. Brooks-Cole. pp. 33–47. ISBN 9780818502781.
- Stryker, Sheldon; Serpe, Richard T. (1982). "Commitment, Identity Salience, and Role Behavior: Theory and Research Example". In Ickes, William; Knowles, Eric S. (eds.). Personality, Roles, and Social Behavior. Springer New York. pp. 199–218. doi:10.1007/978-1-4613-9469-3_7. ISBN 978-1-4613-9471-6.
- Nuttbrock, Larry; Freudiger, Patricia (1991). "Identity Salience and Motherhood: A Test of Stryker's Theory". Social Psychology Quarterly. 54 (2): 146–157. doi:10.2307/2786932. JSTOR 2786932.
- Callero, Peter L. (1985). "Role-Identity Salience". Social Psychology Quarterly. 48 (3): 203–215. doi:10.2307/3033681. JSTOR 3033681.
- Tajfel, H. (1959). "Quantitative Judgement in Social Perception". British Journal of Psychology. 50 (1): 16–29. doi:10.1111/j.2044-8295.1959.tb00677.x. PMID 13628966.
- Larson, James R.; Foster-Fishman, Pennie G.; Keys, Christopher B. (1994). "Discussion of shared and unshared information in decision-making groups". Journal of Personality and Social Psychology. 67 (3): 446–461. doi:10.1037/0022-3518.104.22.1686.
- Rudman, Laurie A.; Goodwin, Stephanie A. (2004). "Gender differences in automatic in-group bias: Why do women like women more than men like men?" (PDF). Journal of Personality and Social Psychology. 87 (4): 494–509. doi:10.1037/0022-3522.214.171.1244. PMID 15491274. Archived (PDF) from the original on July 18, 2014. Retrieved September 2, 2016.
- Van Vugt, Mark; De Cremer, David; Janssen, Dirk P. (2007). "Gender differences in cooperation and competition: the male-warrior hypothesis". Psychological Science. 18 (1): 19–23. doi:10.1111/j.1467-9280.2007.01842.x. PMID 17362372.
- Fershtman, Chaim; Gneezy, Uri (2001). "Discrimination in a Segmented Society: An Experimental Approach". The Quarterly Journal of Economics. 116 (1): 351–377. doi:10.1162/003355301556338.
- Epstein, Cynthia Fuchs (1973). "Positive Effects of the Multiple Negative: Explaining the Success of Black Professional Women". American Journal of Sociology. 78 (4): 912–935. doi:10.1086/225410. JSTOR 2776611.
- Fehr, Ernst; Bernhard, Helen; Rockenbach, Bettina (2008). "Egalitarianism in young children" (PDF). Nature. 454 (7208): 1079–1083. Bibcode:2008Natur.454.1079F. doi:10.1038/nature07155. PMID 18756249.
- Rand, David G.; Pfeiffer, Thomas; Dreber, Anna; Sheketoff, Rachel W.; Wernerfelt, Nils C.; Benkler, Yochai (2009-04-14). "Dynamic remodeling of in-group bias during the 2008 presidential election". Proceedings of the National Academy of Sciences. 106 (15): 6187–6191. Bibcode:2009PNAS..106.6187R. doi:10.1073/pnas.0811552106. PMC 2664153. PMID 19332775.
- Oeberst, Aileen; Beck, Ina; Matschke, Christina; Ihme, Toni Alexander; Cress, Ulrike (December 2019). "Collectively biased representations of the past: Ingroup Bias in Wikipedia articles about intergroup conflicts". British Journal of Social Psychology: bjso.12356. doi:10.1111/bjso.12356. ISSN 0144-6665.
- Bourhis, Richard Y.; Gagnon, André (2003). "Social Orientations in the Minimal Group Paradigm". In Brown, R.; Gaertner, S. L. (eds.). Blackwell Handbook of Social Psychology: Intergroup Processes. Oxford, UK: Blackwell Publishers Ltd. pp. 89–111. doi:10.1002/9780470693421.ch5. ISBN 978-0-470-69342-1.
- Mummendey, Amélie; Otten, Sabine (2003). "Aversive Discrimination". In Brown, R.; Gaertner, S. L. (eds.). Blackwell Handbook of Social Psychology: Intergroup Processes. Oxford, UK: Blackwell Publishers Ltd. pp. 112–132. doi:10.1002/9780470693421.ch6. ISBN 978-0-470-69342-1.
- Turner, John C.; Reynolds, Katherine J. (2003). "The Social Identity Perspective in Intergroup Relations: Theories, Themes, and Controversies". In Brown, R.; Gaertner, S. L. (eds.). Blackwell Handbook of Social Psychology: Intergroup Processes. Oxford, UK: Blackwell Publishers Ltd. pp. 133–152. doi:10.1002/9780470693421.ch7. ISBN 978-0-470-69342-1.
- Struch, Naomi; Schwartz, Shalom H. (March 1989). "Intergroup Aggression: Its Predictors and Distinctness From In-Group Bias". Journal of Personality and Social Psychology. 56 (3): 364–373. doi:10.1037/0022-35126.96.36.1994.
- West, Jeremy (February 2018). "Racial Bias in Police Investigations" (PDF). Working Paper.
- Depew, Briggs; Eren, Ozkan; Mocan, Naci (2017). "Judges, Juveniles, and In-Group Bias" (PDF). Journal of Law and Economics. 60 (2): 209–239. doi:10.1086/693822.
- De Dreu CK, Shalvi S, Greer LL, Van Kleef GA, Handgraaf MJ (2012). "Oxytocin motivates non-cooperation in intergroup conflict to protect vulnerable in-group members". PLOS ONE. 7 (11): e46751. Bibcode:2012PLoSO...746751D. doi:10.1371/journal.pone.0046751. PMC 3492361. PMID 23144787.
- Stallen M, De Dreu CK, Shalvi S, Smidts A, Sanfey AG (2012). "The herding hormone: oxytocin stimulates in-group conformity". Psychological Science. 23 (11): 1288–92. doi:10.1177/0956797612446026. PMID 22991128.
- De Dreu CK, Greer LL, Van Kleef GA, Shalvi S, Handgraaf MJ (January 2011). "Oxytocin promotes human ethnocentrism". Proceedings of the National Academy of Sciences of the United States of America. 108 (4): 1262–6. Bibcode:2011PNAS..108.1262D. doi:10.1073/pnas.1015316108. PMC 3029708. PMID 21220339.
- Ma X, Luo L, Geng Y, Zhao W, Zhang Q, Kendrick KM (2014). "Oxytocin increases liking for a country's people and national flag but not for other cultural symbols or consumer products". Frontiers in Behavioral Neuroscience. 8: 266. doi:10.3389/fnbeh.2014.00266. PMC 4122242. PMID 25140135.
- Ma-Kellams, Christine; Spencer-Rodgers, Julie; Peng, Kaiping (2011). "I Am Against Us? Unpacking Cultural Differences in Ingroup Favoritism via Dialecticism". Personality and Social Psychology Bulletin. 37 (1): 15–27. doi:10.1177/0146167210388193. PMID 21084525.