Adolescent weight gain and social networks: is there a contagion effect?

Previous studies on the spread of obesity in social networks have focused on the contemporaneous effect of peer weight outcomes on individuals. This article is the first to investigate the longer term effects, within adolescence and from adolescence into early adulthood, of peers on individual weight outcomes. Using data from the first three waves of the National Longitudinal Study of Adolescent Health (Add Health), and accounting for correlated effects using a number of empirical strategies including school-level fixed effects and accounting for neighbourhood preferences, we show that Body Mass Index (BMI) and overweight status in a person's friendship network influence their BMI and likelihood of being overweight. The evidence suggests that there is some persistence of the effects of past peer weight experiences on individual weight outcomes during adolescence and into early adulthood. The findings are consistent with adolescence being an important formative period of individuals’ preference for ideal physique and own body weight aspirations. We conclude that policy makers should be particularly concerned with interventions during childhood and adolescence, in order to slow the spread of obesity by promoting a healthy body image and positive health behaviours.