Are friends more genetically similar to one another than to randomly selected peers? Humans tend to form social relationships with others who resemble them, and recent research has evaluated the possibility that unobserved genotypes may play an important role in the creation of homophilous relationships. Researchers Benjamin Domingue (Stanford), Daniel Belsky (Duke), CDHA affiliate Jason Fletcher (public affairs; sociology), Dalton Conley (Princeton), Jason Boardman (University of Colorado Boulder), and Kathleen Mullan Harris (University of North Carolina at Chapel Hill), have extended this line of research in a new study appearing in the Proceedings of the National Academy of Sciences.
The team analyzed genome-wide and social network data from the National Longitudinal Study of Adolescent to Adult Health (Add Health) and found that friendship pairs tend to be more genetically similar to one another compared to random pairs of individuals. The genetic similarity stemmed from two potentially complementary processes: social homophily and social structuring.
Social homophily-related genetic similarity may arise because individuals form social bonds on the basis of shared characteristics, many of which have genetic origins. The friends studied in Add Health tended to share educational trajectories and also had similar education-associated genotypes. Structural-level genetic similarity may arise because social bonds tend to form between people who share social environments. In the case of Add Health, friends spent time together at schools, an environment that may be partly influenced by the genetics of students.
The researchers also found that the genetics of a person’s social network may affect their risk of obesity and their educational attainment. Notably, such social genetic effects can bias analyses of genetic associations if the social genome is not taken into account. For social scientists, the results provide evidence of environmental transmission of peer- and school-level influences on adolescents’ outcomes.