The Spread of Obesity in a Large Social Network Over 32 Years

Background: The prevalence of obesity has increased substantially over the past 30 years. We quantitatively explored the nature and extent of person-to-person spread of obesity as a possible contributing factor explaining this increase. Methods: We developed a densely interconnected social network of 12,067 people assessed repeatedly from 1971 to 2003 as part of the Framingham Heart Study. Measured body mass index was available for all subjects. We used longitudinal statistical models to examine whether weight gain in one person was associated with weight gain in friends, siblings, spouses, and neighbors. Results: Discernible clusters of obese persons were present in the network at all time points, and the clusters extended three people deep. These clusters were not solely due to selective formation of social ties among obese persons. A friend becoming obese in a given time interval increased a person's chances of becoming obese by 57% (95% CI: 6%-123%). Among pairs of adult siblings, one becoming obese increased the chance that the other became obese by 40% (21%-60%). Among spouses, one becoming obese increased the likelihood that the other became obese by 37% (7%-73%). Immediate neighbors did not exhibit these effects. In general, same-gender persons showed relatively greater influence on each other. The spread of smoking cessation did not account for the inter-personal spread of obesity. Conclusions: Network phenomena appear relevant to the bio-behavioral trait of obesity. Obesity appears to spread across social ties, a finding with implications for clinical and public health interventions