  An increasing number of scholars are using longitudinal social network data
to try to obtain estimates of peer or social influence effects. These data may
provide additional statistical leverage, but they can introduce new inferential
problems. In particular, while the confounding effects of homophily in
friendship formation are widely appreciated, homophily in friendship retention
may also confound causal estimates of social influence in longitudinal network
data. We provide evidence for this claim in a Monte Carlo analysis of the
statistical model used by Christakis, Fowler, and their colleagues in numerous
articles estimating "contagion" effects in social networks. Our results
indicate that homophily in friendship retention induces significant upward bias
and decreased coverage levels in the Christakis and Fowler model if there is
non-negligible friendship attrition over time.
