<b>Complex Systems</b><br><i>Homophily, Contagion and Confounding: Pick Any Three</i>

Speaker: Cosma Shalizi (Carnegie-Mellon University)

One person's behavior can often be predicted from that of their neighbors in a social network. This is sometimes explained by homophily, the tendency to form social ties with others because we resemble them.

It is also sometimes explained by social contagion or social influence, the tendency to act like someone because they are our neighbor. We show that, generically, these two mechanisms are confounded with each other, and with the causal effect of an individual's attributes on their behavior. Distinguishing them requires strong assumptions on the parametrization of the social process or on the adequacy of the covariates used (or both). In particular, simple examples show that asymmetries in regression coefficients cannot identify causal effects, and that 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 non-identifiability results. (Joint work with Andrew Thomas)