Studies of network contagion in health outcomes and behaviors (such as obesity and smoking) are all the rage these days. So it is interesting to read this paper in the current BMJ by Cohen-Cole and Fletcher that uses Add-Health data to establish some statistically significant but substantively rather implausible effects of just this sort:
Objective To investigate whether “network effects” can be detected for health outcomes that are unlikely to be subject to network phenomena. Design Statistical analysis common in network studies, such as logistic regression analysis, controlled for own and friend’s lagged health status. Analyses controlled for environmental confounders. Setting Subsamples of the National Longitudinal Study of Adolescent Health (Add Health). Participants 4300 to 5400 male and female adolescents who nominated a friend in the dataset and who were both longitudinally surveyed. Measurements Health outcomes, including headache severity, acne severity, and height self reported by respondents in 1994-5, 1995-6, and 2000-1. Results Significant network effects were observed in the acquisition of acne, headaches, and height. A friend’s acne problems increased an individual’s odds of acne problems (odds ratio 1.62, 95% confidence interval 0.91 to 2.89). The likelihood that an individual had headaches also increased with the presence of a friend with headaches (1.47, 0.93 to 2.33); and an individual’s height increased by 20% of his or her friend’s height (0.18, 0.15 to 0.26). Each of these results was estimated by using standard methods found in several publications. After adjustment for environmental confounders, however, the results become uniformly smaller and insignificant. Conclusions Researchers should be cautious in attributing correlations in health outcomes of close friends to social network effects, especially when environmental confounders are not adequately controlled for in the analysis.