Using "a densely interconnected social network of 12,067 people assessed repeatedly from 1971 to 2003 as part of the Framingham Heart Study" the authors of this article in the New England Journal of Medicine tried to determine why obesity has increased so dramatically in recent decades.
The prevalence of obesity has increased from 23% to 31% over the recent past in the United States, and 66% of adults are overweight. Proposed explanations for the obesity epidemic include societal changes that promote both inactivity and food consumption. The fact that the increase in obesity during this period cannot be explained by genetics and has occurred among all socioeconomic groups provides support for a broad set of social and environmental explanations. Since diverse phenomena can spread within social networks, we conducted a study to determine whether obesity might also spread from person to person, possibly contributing to the epidemic, and if so, how the spread might occur.
Whereas obesity has been stigmatized in the past, attitudes may be changing. To the extent that obesity is a product of voluntary choices or behaviors, the fact that people are embedded in social networks and are influenced by the evident appearance and behaviors of those around them suggests that weight gain in one person might influence weight gain in others. Having obese social contacts might change a person's tolerance for being obese or might influence his or her adoption of specific behaviors (e.g., smoking, eating, and exercising). In addition to such strictly social mechanisms, it is plausible that physiological imitation might occur; areas of the brain that correspond to actions such as eating food may be stimulated if these actions are observed in others. Even infectious causes of obesity are conceivable.
According to their findings, being friends with me would increase your chances of becoming obese with 57%. Marrying me would only give you odds of 37%, though, what does this say about the other 20% of spouses? So much for this ideal marriage some people try hitting you over the head with. They're not even friends with each other! Simply being neighbours doesn't influence people, nor do they have an increased risk of getting fatter if they don't consider themselves friends with someone
(no matter what the other thinks).
A person's chances of becoming obese increased by 57% if he or she had a friend who became obese in a given interval. Among pairs of adult siblings, if one sibling became obese, the chance that the other would become obese increased by 40%. If one spouse became obese, the likelihood that the other spouse would become obese increased by 37%.
The study's authors suggest that this occurs by adopting the other person's habits (eating habits spring to mind) and increased tolerance of obesity from being exposed to them (making it easier to 'let yourself go' without worrying about social stigma).