Correlated genotypes in friendship networks.
It is well known that humans tend to associate with other humans who have similar characteristics, but it is unclear whether this tendency has consequences for the distribution of genotypes in a population. Although geneticists have shown that populations tend to stratify genetically, this process results from geographic sorting or assortative mating, and it is unknown whether genotypes may be correlated as a consequence of nonreproductive associations or other processes. Here, we study six available genotypes from the National Longitudinal Study of Adolescent Health to test for genetic similarity between friends. Maps of the friendship networks show clustering of genotypes and, after we apply strict controls for population stratification, the results show that one genotype is positively correlated (homophily) and one genotype is negatively correlated (heterophily). A replication study in an independent sample from the Framingham Heart Study verifies that DRD2 exhibits significant homophily and that CYP2A6 exhibits significant heterophily. These unique results show that homophily and heterophily obtain on a genetic (indeed, an allelic) level, which has implications for the study of population genetics and social behavior. In particular, the results suggest that association tests should include friends' genes and that theories of evolution should take into account the fact that humans might, in some sense, be metagenomic with respect to the humans around them.
Division of Medical Genetics, Department of Political Science, University of California at San Diego, La Jolla, CA 92093, USA. jhfowler@ucsd.edu
James H
JH
Fowler
Jaime E
JE
Settle
Nicholas A
NA
Christakis
D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics; D065637:Cytochrome P-450 CYP2A6; D033062:Friends; D005838:Genotype; D006801:Humans; D017448:Receptors, Dopamine D2 / Q000235:genetics; D012944:Social Support
D017448:Receptors, Dopamine D2; D001189:Aryl Hydrocarbon Hydroxylases; C585710:CYP2A6 protein, human; D065637:Cytochrome P-450 CYP2A6
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