RESUMO
OBJECTIVE: Many recent studies have provided evidence suggesting that increases in body weight may spread via social networks. The mechanism(s) by which this might occur have become the subject of much speculation, but to date little direct evidence has been available. Building on evidence from economics, anthropology, and behavioral biology, within-household peers might influence body weight via implicit provision of income security was hypothesized. DESIGN AND METHODS: Using a sample of 2,541 working-age men from the National Longitudinal Survey of Youth (1979), the effect of cohabitation on weight gain over a 6-year period was estimated. The potential confound caused by the joint determination of economic insecurity and cohabitation status with instrumental variables that exploit variation in local and state-level macroeconomic conditions and the presence of children in the home was addressed. RESULTS: The marginal effect of cohabitation with adults on body weight is negative. Moreover, the magnitude of the effect is more than six times greater when the cohabitant is engaged in paid employment. CONCLUSIONS: Income insecurity may play an important role in peer-to-peer transmission of weight gain.