Friendship and natural selection

Abstract

More than any other species, humans form social ties to individuals who are neither kin nor mates, and these ties tend to be with similar people. Here, we show that this similarity extends to genotypes. Across the whole genome, friends’ genotypes at the single nucleotide polymorphism level tend to be positively correlated (homophilic). In fact, the increase in similarity relative to strangers is at the level of fourth cousins. However, certain genotypes are also negatively correlated (heterophilic) in friends. And the degree of correlation in genotypes can be used to create a “friendship score” that predicts the existence of friendship ties in a hold-out sample. A focused gene-set analysis indicates that some of the overall correlation in genotypes can be explained by specific systems; for example, an olfactory gene set is homophilic and an immune system gene set is heterophilic, suggesting that these systems may play a role in the formation or maintenance of friendship ties. Friends may be a kind of “functional kin.” Finally, homophilic genotypes exhibit significantly higher measures of positive selection, suggesting that, on average, they may yield a synergistic fitness advantage that has been helping to drive recent human evolution.

Individual olfactory perception reveals meaningful nonolfactory genetic information

“…It has been suggested that several vertebrate species use this information to select mates with dissimilar HLA (33)(34)(35)(36), and for this to occur, the olfactory system should have access to the information of self HLA makeup. In here finding that HLA is also linked to olfactory perception alone, our results combine with a previous effort (19) to provide a path for such self-recognition, and in this close a loop likely subserving behavioral selection mechanisms that rely on the sense of smell (20).…”

“…It has long been known that information on HLA is available in body odor (16,17). Linking HLA to the sense of smell alone (18,19) would close a functional loop potentially subserving behavioral mechanisms of selection based on the sense of smell (20). With all this in mind, we set out to test if we could generate individual olfactory fingerprints, and whether these inform on HLA.…”

An empirical examination of echo chambers in US climate policy networks

“…For the attribute component in each of these terms, agreement was coded as the exact same value in the 5-point response scale. Although we might expect high levels of transitivity to be beneficial in some social relationships, like friendship (46)(47) and cooperation (48)(49), these structures have a very different impact in networks of information transmission. For communication networks, the repeated nature of the ties may give members the impression that an issue is decided when there continues to be debate.…”

Evaluating reproductive decisions as discrete choices under social influence

“…For much of human prehistory, social learning within smaller communities was strategically directed towards familiar experts or best-informed members. In these long-term contexts, the benefits of social learning and friendship are substantial enough to have been a factor in human evolution [9][10][11]. In a typical collective-action problem, such as changing fertility norms, the transparency about what others are doing is essential [12].…”