Social Mkt

Model of genetic variation in human social networks
James H. Fowlera,1, Christopher T. Dawesa, and Nicholas A. Christakisb
aDepartment

of Political Science, University of California, San Diego, CA 92093; and bDepartment of Health Care Policy, Harvard Medical School, and Department of Sociology, Harvard University, Cambridge, MA 02138 Edited by Colin F. Camerer, California Institute ofTechnology, Pasadena, CA, and accepted by the Editorial Board November 29, 2008 (received for review July 15, 2008)

Social networks exhibit strikingly systematic patterns across a wide range of human contexts. Although genetic variation accounts for a significant portion of the variation in many complex social behaviors, the heritability of egocentric social network attributes is unknown. Here, weshow that 3 of these attributes (in-degree, transitivity, and centrality) are heritable. We then develop a ‘‘mirror network’’ method to test extant network models and show that none account for observed genetic variation in human social networks. We propose an alternative ‘‘Attract and Introduce’’ model with two simple forms of heterogeneity that generates significant heritability and other importantnetwork features. We show that the model is well suited to real social networks in humans. These results suggest that natural selection may have played a role in the evolution of social networks. They also suggest that modeling intrinsic variation in network attributes may be important for understanding the way genes affect human behaviors and the way these behaviors spread from person to person.evolution of cooperation heritability twins

same-sex dizygotic (DZ) twins who share only 50% on average. Under the assumptions of the twin study design, if genetic variation is contributing to variation in the trait, then MZ twins should be significantly more similar than DZ twins. Although some scholars object to the assumptions of the design (see SI), it has been widely used to show thatgenes play a role in personality (16), intelligence (17–18), and several other behavioral traits (14–15, 19–23). Turkheimer suggests as a ‘‘first law of behavior genetics’’ that all human behavioral traits are heritable (24). We should therefore not be surprised to learn that individual social network characteristics have a partly genetic basis. However, as we will show, not all networkcharacteristics are significantly heritable, and, more pertinently, specific estimates of heritability can provide a means to test theoretical models of human social networks. Results The fundamental building blocks of a human social network are egocentric properties of each individual in the network: the degree (the number of a person’s contacts, or social ties) and transitivity (the likelihood that two of aperson’s contacts are connected to each other, also called the clustering coefficient). A wide variety of social networks can be constructed by altering the distribution of degree and transitivity between individuals (the nodes of the network), and these two attributes also have a strong influence on other network properties such as betweenness centrality (the fraction of paths through the networkthat pass through a given node). For example, a higher degree is positively correlated with greater centrality. To measure how much variation in these node-level measures can be attributed to genetic variation, we used an additive genetic model (see SI) to analyze 1,110 twins from a sample of 90,115 adolescents in 142 separate school friendship networks in the National Longitudinal Study ofAdolescent Health (the ‘‘Add Health’’ study; see SI for description). The results show that genetic factors account for 46% [95% confidence interval (C.I.) 23%, 69%] of the variation in in-degree (how many times a person is named as a friend), but heritability of out-degree (how many friends a person names) is not significant (22%, C.I. 0%, 47%). In addition, node transitivity is significantly...