In the McArdle vs. Frum diavlog I alluded to earlier there was a dispute centered around two seemingly contradictory results. First, the fact that heritability of obesity is rather high, in twins-separated-at-birth studies the correlation between monozygotic twins raised apart is on the order of ~0.75. And yet people tend to track the weight of their peer group. The causality here has to be teased apart of course, but consider this study, The presence of friends increases food intake in youth:
Design: Twenty-three overweight and 42 nonoverweight youths had the opportunity to play and eat with a friend (n = 26) or with an unfamiliar peer (n = 39). The dependent variables of interest were the amount of nutrient-dense and energy-dense foods children consumed and their total energy intake.
Results: Participants eating with a friend ate substantially more than did participants eating with an unfamiliar peer. Furthermore, overweight youth, but not nonoverweight youth, who ate with an overweight partner (friend or unfamiliar peer) consumed more food than did overweight participants who ate with a nonoverweight eating partner. Matching of intake was greater between friends than between unfamiliar peers.
Naturally twins raised apart were not placed into a “design” whereby they were forced to eat with strangers. Rather, they selected their peer groups. Heritability of many traits increases with age because individuals seek out particular environments which eventually dampen the “noise” which reduces the correlation between those with similar genetic propensities. Assortative friendship by weight then might result in amplifiers of mean deviation from phenotypic norms; that is, thin peer groups might model specific behaviors and apply certain pressures which differ greatly from overweight peer groups. Gene-environment correlation. So naturally in the interests of public health we need integration across weight classes….
Also see ScienceDaily.