When it comes to obesity and related behaviors, new research suggests that what you weigh is in part influenced by who your friends are.

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PROBLEM: Other research has observed that obese students tend to run in the same social circles. Is this just a case of like attracting like, or might there be social influences at play?

METHODOLOGY: David Shoham and a team of researchers at Loyola University revisited the National Longitudinal Study of Adolescent Health from the mid-1990s, applying a sophisticated form of statistical analysis to data collected from students at two large high schools. Taking into account the adolescents' body mass index (BMI), time spent in front of television and computer screens, and participation in active sports, they isolated social influence from factors like homophily (the tendency to bond with people similar to oneself) and shared environmental influences and measured its influence on obesity and related behaviors.

RESULTS: Students who were borderline overweight were 40 percent more likely to decrease their BMI over the course of the school year if they reported having lean friends, while similar students were 56 percent more likely to show an increase in BMI if their friends were obese. In one of the schools, social influence was shown to affect screen time, and in both schools it affected the playing of active sports. Homophily, on the other hand, was not correlated with screen time in either school, although it was linked to the playing of active sports in one.

CONCLUSION: Social influence, along with homophily, plays a significant role in weight loss and weight gain.

IMPLICATION: While Shoham points out several limitations of the study -- the data is self-reported, and it comes from the dark ages before online social networking -- this new emphasis on social influence indicates that weight-loss strategies may need to look beyond the individual. It also underscores the importance of prevention: students showed a greater tendency to gain weight among obese friends than to lose weight among thin friends. While there's a lot that needs clarifying about how social influence actually functions in this context, these findings suggest that its power can be harnessed in efforts to combat obesity.

SOURCE: The full study, "An Actor-Based Model of Social Network Influence on Adolescent Body Size, Screen Time, and Playing Sports," is published in the journal PLoS-ONE.