Genetic similarity of friends

New research finds that friends share about 1% of their genes, equivalent to the kinship between fourth cousins.[1]  The research has been widely reported, though its full significance for social dynamics has been missed by journalists.[2]  The lead author of the research paper, Nicholas Christakis of Yale University, stated: “One per cent may not sound much to the lay person but to geneticists it is a significant number. . . . Choosing friends wisely improves your chances of survival.”

If 1% is significant, the 6% genetic kinship that is typical among members of the same ethnic group is large.  That is the figure estimated by geneticist Henry Harpending in a recent paper,[3] discussed in a previous post. The researchers must know about ethnic kinship because they controlled for it in their study.  They did so by limiting the study to friends formed within the same ethnic group.  Only by doing so could they have detected the relatively slight similarity between friends, which otherwise would have been swamped by the much greater ethnic differences.

Christakis and Fowler conclude that the phenotypical similarity of friends, on which assortment is based, reflects genetic similarity.

The result is that friends form an intermediate pool of kinship concentration lying between the ethny and the clan. Within the clan the nuclear family is the most intense concentration of kinship , while geographical races and humanity as a whole are the least concentrated. In descending order of concentration, kinship runs thus: Nuclear family, extended family (clan), friends, ethny, race, humanity. The sizes of each of these run in the opposite direction. However, the aggregate kinship within each pool does not correspond in any simple way with concentration. For example, according to Christakis and Fowler’s new data, someone with five close friends has a store of only 5% of his or her genes in them,[4] while just one child carries 25% of a parent’s genome.[5] Also, an ethnic group can hold millions of copies of a member’s genome, though it is more difficult to invest in this large aggregate kinship.[6]

Sharing genes with friends must have paid off in greater fitness in the evolutionary past, the authors argue.  By the same reasoning helping fellow ethnics can also advance fitness in multi-ethnic societies.

These new data add to the growing evidence of the important of genetic kinship as a factor in social ties beyond the family, consistent with sociobiological theory.  The fact that people show persistent assortment along the lines of genetic similarity despite fulsome praise of diversity in schools and the media should strike a cautionary note.  The finding is consistent with evidence that diversity undermines social cohesion and increases conflict.[7]

A disappointing feature of the study is its failure to mention what was, to my knowledge, the first finding of genetic similarity among friends.  That was conducted by the late J. Philippe Rushton, the Canadian evolutionary psychologist and published in a paper titled “Genetic Similarity in Male Friends” in 1989.[8]  Rushton replicated this finding using different methods in 2005.[9]  These groundbreaking studies deserve to be acknowledged.

Frank Salter



[1] Christakis, N. A. and J. H. Fowler (2014). “Friendship and natural selection.” Proceedings of the National Academy of Sciences file:///C:/Users/Frank/Documents/DataFilesHOME/2014conferences+papersHOME/Consultancy2014/HumanNatureNews140330/Posts-drafts1403/PNAS-2014-Christakis-1400825111.pdf.

[2] Hannah Devlin (2014). “Our friends are closer to us than we think”, The Australian, 16 July, p. 9. Reprinted from The Times of London.

[3] Salter, F. K. and H. Harpending (2013). “J. P. Rushton’s theory of ethnic nepotism.” Personality and Individual Differences 55: 256-260.

[4] 5 x 1%

[5] Both measures being made against the baseline of the parent’s ethnic population. For technical reasons parental kinship is 25%, half of the more intuitive measure of relatedness.

[6] Salter, F. K. (2007/2003). On genetic interests. Family, ethnicity, and humanity in an age of mass migration. New York, Transaction.

[7] Vanhanen, T. (2012). Ethnic conflicts: Their biological roots in ethnic nepotism. London, Ulster Institute for Social Research.

[8] Rushton, J. P. (1989). “Genetic similarity in male friends.” Ethology and Sociobiology 10: 361-73.

[9] Rushton, J. P. (2005). “Mate choice and friendship in twins: Evidence for genetic similarity.” Psychological Science 16(7): 555-59.


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