Encyclopedia of Evolutionary Psychological Science

Living Edition
| Editors: Todd K. Shackelford, Viviana A. Weekes-Shackelford

In-Group Versus Out-Group

  • Robert LynchEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-16999-6_157-1



The evolution of the cognitive architecture humans use to sort themselves into coalitions of us versus them.


Considerable attention across the social sciences has been paid to understanding how people categorize individuals into groups and create group identities. Throughout human history, conflict between groups was founded on categorizing relationships into “us” and “them” and social psychologists have long recognized the importance of group membership to human social organization. The “minimal group paradigm,” established as a method for investigating the minimal conditions required for discrimination to occur between groups, has continued to show that even virtually meaningless distinctions between groups, such as shirt color, can trigger a tendency to favor one’s own group over others. It is clear that the ability to readily distinguish between in-groups and out-groups played a key role in our evolutionary history.


Understanding the potency of human group identification is often constructed around theories of kin selection – favoring one’s genetic relatives at a cost to one’s own survival and reproduction (Hamilton 1964). If ancestral human groups were composed primarily of closely related individuals, then a shared culture, background, or belief system may have served as a reliable phenotypic marker of genetic relatedness (Swann et al. 2014), and kinship may therefore have played an important role in building the cognitive architecture employed in group identification and formation. Anthropological studies of group fissioning along kin-lines, such as occurred within Yanomamo tribes of the Amazon, confirm this prediction. A cross-cultural study of 32 hunter gatherer groups, however, suggests that kinship may be overstated by evolutionary psychologists and that inclusive fitness (Hamilton 1964) – the ability of an individual organism to transfer genes to the next generation either directly through offspring or indirectly through the offspring of close relatives – cannot explain the composition and cooperation of hunter gatherer social structures. For example, close kin (e.g., siblings, spouses, parents) often make up less than 10% of a residential group and in some groups like the Ache of Paraguay, the mean genetic coefficient of relatedness (r) between adults was estimated to be 0.054 suggesting that kinship may not fully explain group cohesion (Hill et al. 2011).

Kin Psychology

Regardless of the average relatedness of early human groups, many explanations of self-sacrifice for a large group of unrelated individuals are built on kin selection and rely on something called “kin psychology” – forming kinship like bonds with unrelated individuals. These types of explanations tend to focus on cultural adaptations which harness cognitive mechanisms that direct our interactions with genetic relatives. For example, researchers have suggested that applying kinship terminology to a group of close friends (e.g., a band of brothers) or a nation (e.g., “the motherland”) can trigger emotional responses that are similar to those activated by kin selection. Building on these ideas, Whitehouse (2018) developed the theory of “identity fusion,” arguing that extreme self-sacrifice results when individuals align their own identity with that of the group through bonding over intense shared experiences (e.g., the horrors of war) or from perceptions of shared biology (e.g., recognizing putative kin by looking for individuals with similar traits called “phenotypic matching”). As a result, certain social connections might enable one to view others with such shared experiences or traits as kin. Interestingly, identity fusion is expected to be particularly powerful when combined with a strong outgroup threat. Together, these theoretical arguments and empirical results suggest that extreme self-sacrifice is made possible by exploiting psychological pathways dedicated to kin-biased altruism.

Parochial Altruism

A long history of lethal between-group competition is also likely to have played an important role in generating attitudes towards in-groups and out-groups in humans and Bowles theorized that these biases depend on the frequency of conflict between rival bands in the evolutionary past. The willingness to sacrifice for an in-group, combined with out-group hostility, has been called “parochial altruism” and this phenomenon may help to explain why clear boundaries between human groups are so pervasive. Using simulations, Choi and Bowles (2007) found that groups whose members are hostile towards other groups and are altruistic towards their own groups are more likely to succeed when the frequency of lethal intergroup conflict is high.

Phenotypic Matching and Strategic Coalitions

What cues do humans use to infer shared genes (i.e., phenotypic matching), beliefs, or interests (i.e., shared experiences) and thereby create boundaries between in and out-groups? There is considerable evidence that when being asked to remember an unknown person, humans are automatically able to remember three “primary” traits – sex, age, and race. Hirschfeld (1998) argued that categorizing individuals by their race is a byproduct of essentialism – the idea that every organism has a set of qualities that forms their identity – and evolved due to our need to reason about natural types, and Gil-White (2001) has suggested that humans process racial and ethnic groups like species. Some ethnographies provide tentative support for this sort of essentialist thinking. For example, the name Yąnomamö that a group of horticulturalists living in the Amazon rainforest call themselves is often translated as “human beings” and other enemy tribes or non-Yanomamo groups are not considered to be human. Other researchers see in-group biases as evolving to reap the advantages derived from forming and being a member of a coalition. Here, categorizing people by their race is viewed as byproduct of cognitive structures that evolved to detect coalitional alliances. The likelihood that humans would rarely, if ever, have traveled far enough to have encountered individuals from a population different enough to qualify as a race lends support for this view. Both essentialism based on shared beliefs or ancestry and the ability to form flexible strategic coalitions based on shared interests are likely to play an important role in human group affiliation, however.


Humans also form coalitions around shared beliefs, and religion has the ability to redraw boundaries between groups and create group identities. Indeed, several studies have suggested that a key function of religion is to bind individuals into groups, and members of religious groups often favor individuals who share their faith. Furthermore, religions frequently have specific rules that promote the defense of their group while simultaneously propagating hatred of out-groups. In a Jamaican sample, hostility towards out-groups was positively predicted by extrinsic religiosity (i.e., using religion to achieve non-religious goals) but negatively associated with religious devotion (e.g., belief in God) (Lynch et al. 2017).

Political Beliefs

Although religious beliefs, race, sex, ethnicity, and age are some of the most salient cues humans use to categorize other people into groups, other cues may also be relied on as a reflection of an individual’s beliefs. For example, in recent years, political views have often been used to assess group membership. In 1960, a poll found that 5% of Republicans and 4% of Democrats would “be unhappy” if their child married someone from the opposing party. In 2010, half of Republicans and a third of Democrats surveyed reported that they would be either “somewhat upset” or “very upset” if this happened. Today, implicit association tests (which measure how quickly people categorize positive and negative words with groups) have shown that people show greater prejudices against political foes than those of different races or religions.

Humor and Laughter

Laughter and sense of humor are also frequently used to categorize individuals into groups. For example, laugh tracks amplify real human-generated laughter only when individuals believe it comes from members of an in-group. This suggests that laughter may function to reinforce within group social bonds. People also report liking others who laugh at their jokes and share their sense of humor, and laughter is more common in the presence of others, especially friends, which suggests that it may be connected to both social bonding and communication. Sharing a sense of humor has also been shown to be extremely important in mate selection and sense of humor may help to assess in-group status or potential mates by determining when others share our unconscious biases and beliefs (Lynch and Trivers 2012; Lynch 2010).


The formation of boundaries between human groups and groupish behavior can be understood within an evolutionary framework. In general, it is thought that a decreased trust of unknown individuals and groups would have provided fitness advantages to our ancestors and helped to protect them and their offspring from risks posed by other humans. If this interpretation is correct, then we should expect that cognitive structures specifically designed to quickly categorize people into “us” and “them” would have been targeted by natural selection. Although many of the mechanisms remain unclear, such as whether in-group loyalty and outgroup hate are linked, an evolutionary viewpoint provides a functional explanation for why these biases are so fundamental to human behavior.



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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.University of TurkuTurkuFinland

Section editors and affiliations

  • Tara DeLecce
    • 1
  1. 1.Department of PsychologyOakland UniversityRochesterUSA