From the “social network” page on Wikipedia:
Some researchers have suggested that human social networks may have a genetic basis. Using a sample of twins from the National Longitudinal Study of Adolescent Health, they found that in-degree (the number of times a person is named as a friend), transitivity (the probability that two friends are friends with one another), and betweenness centrality (the number of paths in the network that pass through a given person) are all significantly heritable. Existing models of network formation cannot account for this intrinsic node variation, so the researchers propose an alternative “Attract and Introduce” model that can explain heritability and many other features of human social networks.
 # ^ “Genes and the Friends You Make”. Wall Street Journal. January 27, 2009. http://online.wsj.com/article/SB123302040874118079.html.
 # ^ Fowler, J. H. (10 February 2009). “Model of Genetic Variation in Human Social Networks” (PDF). Proceedings of the National Academy of Sciences 106 (6): 1720–1724. doi:10.1073/pnas.0806746106. http://jhfowler.ucsd.edu/genes_and_social_networks.pdf