According to a study in the New England Journal of Medicine, if someone in a social network becomes obese, those closely connected to them have a greater propensity for becoming obese as well. The strongest demonstration of this effect occurs among friends, not within a family or among those sharing a household. There was no apparent consideration of groups of otherwise strangers who come together in organized settings for the purpose of weight loss. With some diet programs, social support has been shown to be beneficial.

Researchers found that if a friend is becoming obese, the chance of your becoming obese increases 57 percent. For mutual friends, there is over a 170 percent increased chance. The study analyzed data over a period of 32 years for over 12,000 adults who underwent regular medical examinations. The researchers mapped a strong social network amongst the study subjects that recorded both family members and unrelated friends. Over time, entire social networks gained weight. A likely cause is shared lifestyle and eating habits.

Researchers believe that the shift in lifestyle and appearance can be attributed to a change of norms about what is an appropriate body size. Head researcher James Fowler says, "Consciously or unconsciously, people look to others when they are deciding how much to eat, how much to exercise and how much weight is too much". Fowler also mentions however that "it's important to remember that we've not only shown that obesity is contagious but that thinness is contagious." On the latter point, a new study seems to substantiate that weight loss may also be "contagious" among social networks.

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