A new study from researchers at UC Davis, Pittsburgh and the US Air Force Academy uses randomized assignment of students at the Air Force Academy to squadrons to estimate the impact of social networks on fitness:

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. [NBER Working Paper 16518]