Homophily and Contagion Are Generically Confounded in Observational Social Network Studies

We consider processes on social networks that can potentially involve three factors: homophily, or the formation of social ties due to matching individual traits; social contagion, also known as social influence; and the causal effect of an individual's covariates on their behavior or other measurable responses. We show that, generically, all of these are confounded with each other. Distinguishing them from one another requires strong assumptions on the parametrization of the social process or on the adequacy of the covariates used (or both). In particular we demonstrate, with simple examples, that asymmetries in regression coefficients cannot identify causal effects, and that very simple models of imitation (a form of social contagion) can produce substantial correlations between an individual's enduring traits and their choices, even when there is no intrinsic affinity between them. We also suggest some possible constructive responses to these results.Comment: 27 pages, 9 figures. V2: Revised in response to referees. V3: Ditt

Behavioral Communities and the Atomic Structure of Networks

We develop a theory of `behavioral communities' and the `atomic structure' of networks. We define atoms to be groups of agents whose behaviors always match each other in a set of coordination games played on the network. This provides a microfoundation for a method of detecting communities in social and economic networks. We provide theoretical results characterizing such behavior-based communities and atomic structures and discussing their properties in large random networks. We also provide an algorithm for identifying behavioral communities. We discuss applications including: a method of estimating underlying preferences by observing behavioral conventions in data, and optimally seeding diffusion processes when there are peer interactions and homophily. We illustrate the techniques with applications to high school friendship networks and rural village networks

The Social Contagion of Adolescent Depression: Applying a Differential Susceptibility Model

Recent research investigating social risk factors of depression has found evidence for a social contagion effect. The research comes from a surge in popularity of using social network analyses to examine the spread of various health outcomes such as obesity, smoking, substance use, and sleep. Although the finding of depressive contagion represents a significant contribution to the literature on the social etiology of depression, this is only the first step in providing meaningful research useful for the practical application of curbing the growing rates of depression especially among adolescents. Rather than simply acknowledging the existence of contagion effects, researchers must begin to answer the question of whether certain individuals or environments are more susceptible to the effects of depressive contagion. As a result, this dissertation applies a differential susceptibility model to examine three moderators of depressive contagion: social network structure, racial/ethnic identity, and genotypic variation in the serotonin transporter (SLC6A4) gene. The research in this dissertation uses data from the National Longitudinal Study of Adolescent Health (Add Health). First, the results reveal that adolescents who are popular and/or embedded in dense peer networks are more susceptible to depressive contagion. Additionally, depressive contagion is more salient in schools characterized by dense social networks and high reciprocity in social ties. Second, racial homophily plays an important role in the effect of depressive contagion. Adolescents embedded in racially homophilous peer networks are more susceptible to depressive contagion. Further analysis shows that this effect applies primarily for Asians and Hispanics. Finally, results indicate a significant gene-environment interaction (GxE) effect between a polymorphism in the serotonin transporter system (5-HTTLPR) and depressive contagion. Adolescents carrying one or two short alleles in the 5-HTTLPR are more susceptible to depressive contagion

Three Essays on Social Learning

Social learning broadly refers to learning through the acquisition of information from social sources. In the three essays of my dissertation, I investigate the various underlying drivers of social learning and how such learning can impact purchase decisions. In Essay 1, I investigate the link between social learning and sales of experiential products. In particular, I focus on how social capital (i.e., the propensity for people to trust and communicate with each other) moderates the level of social learning for experiential products and thus impacts aggregate sales. In Essay 2, I study how social learning operates differently across the various stages of physician prescription - trial and repeat of a new prescription drug. Given that the mechanisms of social influence varies across trial and repeat stages, the second essay further assesses who is most influential and who is most influenceable across stages. In Essay 3, I examine how consumers make purchases of experiential products and link it to their active search for information from interdependent social sources. Essay 3 assesses the impact of the pattern of similarity of preferences in individual-level social networks (homophily, i.e., the tendency of individuals to associate with similar others, and structural balance, i.e., the congruency of preference in a social network) on consumer search, learning, and purchase