Wednesday, January 19, 2011

We may have more in common with friends than we think we do:

Correlated genotypes in friendship networks

Full text:

    James H. Fowlera,b,1,
    Jaime E. Settleb, and
    Nicholas A. Christakisc,d

+ Author Affiliations

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

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

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.


    1To whom correspondence should be addressed. E-mail:
    Author contributions: J.H.F., J.E.S., and N.A.C. 
designed research, performed research, analyzed data, 
and wrote the paper.
    The authors declare no conflict of interest.
    This article is a PNAS Direct Submission.
    This article contains supporting information online at

Freely available online through the PNAS open access option.

See this comment at Genetic Future by Daniel McArthur:

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