Correlated genotypes in friendship networks!

Correlated genotypes in friendship networks

1. James H. Fowler^a,^b,¹, 
2. Jaime E. Settle^b, and 
3. Nicholas A. Christakis^c,^d

+Author Affiliations

1. ^aDivision of Medical Genetics,
2. ^bDepartment of Political Science, University of California at San Diego, La Jolla, CA 92093;
3. ^cDepartment of Medicine and Department of Health Care Policy, Harvard Medical School, Boston, MA 02115; and
4. ^dDepartment of Sociology, Faculty of Arts and Sciences, Harvard University, Cambridge, MA 02138

1. Edited by Gene E. Robinson, University of Illinois, Urbana, IL, and approved December 15, 2010 (received for review August 6, 2010)

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Abstract

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 CYP2A6exhibits 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.

PNAS February 1, 2011 vol. 108 no. 5 1993-1997