Estimating Peer Effects on Health in Social Networks

In July of 2007, we published a paper in the New England Journal of Medicine 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.