The Durk Pearson & Sandy Shaw®
Life Extension NewsTM
Volume 14 No. 1 • February 2011

Correlated Genotypes in Friendship Networks

You are probably more closely related to your friends than you realize. A new paper1 reports on a study of six available genotypes from the National Longitudinal Study of Adolescent Health to test for genetic similarity between friends. Friendship in humans is unusual compared to other mammalian species in that humans can form stable, nonreproductive unions (e.g., having nothing to do with kinship) to one or more friends.

As the study’s authors note, genetic associations have long been postulated in human self-selected social groups but there has been little evidence to support these suppositions.

A study in laying hens cited in paper #12 found that the feather condition of an individual was strongly influenced by the genotypes of its neighbors. In fact, in that study the average phenotypic (gene expression) effects of the “social” genes were more than two times greater than the genes with a direct effect on an individual’s own genome. “Interestingly, some of the genes with indirect or associative effects in hens involve the serotonin pathway, which has also been shown to influence social behavior in humans.”1

Moreover, the new paper notes that people tend to associate with other people they resemble, a process called “homophily” (“birds of a feather flock together”), whereas people may also choose friends who are different from themselves in certain selected traits, a process called “heterophily” (“negatives attract”).

The authors began their study with the accepted notion that people with similar genotypes may become associated because of living physically close to them (population stratification). “However, no work has yet established that, net of such stratification, there are any genes that are correlated (either positively or negatively) between individuals in nonreproductive, friendship unions. To study whether such correlation exists, we analyzed two independent samples with information about respondents’ genes and about respondents’ friendship ties and social networks: the National Longitudinal Study of Adolescent Health (Add Health) and the Framingham Heart Study Social Network (FHS-Net).”

“In Add Health, subjects were genotyped for one marker each in the DRD2, DRD4, CYP246, MAOA, SLC6A3, and SLC6A4 genes.” The DRD2 and DRD4 are genes for two different dopamine receptors, important in behavior such as reward. The DRD2 dopamine receptor, for example, has been associated with alcoholism1 (and perhaps other forms of addiction). CYP246 is linked to detoxification of otherwise damaging substances. Figures 1 and 2 of the paper1 illustrate how genotypes for DRD2 and CYP246 were distributed in the largest connected component of the friendship network in Add Health. “Notably significant clusters of similar genotypes for DRD2 suggest the possibility of homophily, but the substantial absence of any connection between individuals with minor alleles of CYP246 suggests possible heterophily.”

The authors summarize: “An important implication of these results is that genetic structure in human populations may result not only from the formation of reproductive unions, but also from the formation of friendship unions within a population.” As an example, they explain that an individual who is susceptible to alcoholism might choose friends with the same genotype (homophily) who would be more likely to influence her to drink and that, if so, an association between alcoholism and genotype that does not take account of the influence of friends’ genotypes could overstate the effect of an individual’s genes.

“In some sense, humans might be ‘metagenomic’ not just with respect to the microbes within them, but also with respect to the humans around them.”

We speculate that the attraction of people with different CYP246 variants might provide an advantage to a social group by having members with different abilities to deal with food toxins, thus reducing the risk of a sudden population collapse from a food toxin to which all were susceptible.


  1. Fowler et al. Correlated genotypes in friendship networks. Proc Natl Acad Sci USA 108(5):1993-7 (2011).
  2. Biscarini et al. Across-line SNP association study for direct and associative effects on feather damage in laying hens. Behav Genet 40:715-27 (2010).

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