Peer effects and fitness

This paper addresses whether fitness is contagious arising from recent claims that such peer or contagion effects may partly explain the increase in obesity. It is worth noting that the data is drawn from students at the US Air Force Academy, a very selective institution with a generally pretty fit bunch of people. It seems doubtful that this data will allow one to identify anything about obesity in the population.

Is Poor Fitness Contagious? Evidence from Randomly Assigned Friends
Scott E. Carrell, Mark Hoekstra, James E. West
NBER w16518
The increase in obesity over the past thirty years has led researchers to investigate the role of social networks as a contributing factor. However, several challenges make it difficult to demonstrate a causal link between friends’ physical fitness and own fitness using observational data. To overcome these problems, we exploit data from a unique setting in which individuals are randomly assigned to peer groups. We find statistically significant peer effects that are 40 to 70 percent as large as the own effect of prior fitness scores on current fitness outcomes. Evidence suggests that the effects are caused primarily by friends who were the least fit, thus supporting the provocative notion that poor physical fitness spreads on a person-to-person basis.