Estimating peer effects on health in social networks: a response to Cohen-Cole and Fletcher; and Trogdon, Nonnemaker, and Pais.

In July of 2007, we published a paper in theNew England Journal ofMedicine that used dynamic data over 32 years from the Framingham Heart Study Social Network (FHS-Net) to study the conditions under which obesity might spread from person to person (Christakis and Fowler, 2007, hereafter, CF).We found that obese persons formed clusters in the network at all time points and that these clusters extended to three degrees of separation (e.g., to a person’s friend’s friend’s friend). Moreover, statistical analyses suggested that the clusters were not solely attributable to the selective formation of social ties among obese persons. A person’s chances of becoming obese increased if he or she had a friend who became obese in a given time period. Our analyses were restricted to adults, so a natural question to ask is whether or not the results would generalize to a population of adolescents. The existence of social norms regarding weight in both adults and adolescents should not be surprising, (Chang and Christakis, 2003) but, of course, causal inference in dyads, let alone in broader social networks, is difficult (Manski, 1993). These issues are addressed by two papers in this issue, “Is Obesity Contagious? Social Networks versus Environmental Factors in the Obesity Epidemic,” by Ethan Cohen-Cole and Jason Fletcher (hereafter CCF) and “Peer Effects in Adolescent Overweight,” by Justin Trogdon, James Nonnemaker, and Joanne Pais (hereafter TNP), and by a third working paper “Identifying Endogenous Peer Effects in the Spread of Obesity” by Timothy J. Halliday and Sally Kwak (Halliday and Kwak, 2007)(hereafter HK). All three of these papers analyze the same dataset and population, the National Longitudinal Study of AdolescentHealth (AddHealth), albeitwith differentmethods and assumptions. Unlike the FHS-Net,which followed adults over a 32-year period (average age 38 in 1971), Add Health followed adolescents over a 7-year period (average age 16 in 1995). All three sets of authors take the possibility of peer effects seriously, advancing the study of this important area. HK summarize their results by noting that they are able to replicate the pattern of results in our study, although their results are sensitive to specification of the dependent variable. If weight is characterized by a dichotomous variable indicating overweight (BMI >25), then an association between friends is significant, but the association in the continuous measure of BMI is not. Similarly, TNP use a variety of econometric strategies to conclude that there are peer effects for obesity in the Add Health sample, especially among females and among adolescents with high BMI. For example, they use an