Obesity is contagious. So is happiness.
At least, these are the results coming in from long-term studies of social networks — the networks of friends and families, neighbors and colleagues that we all belong to. Such studies have found that one person’s change in behavior ripples through his or her friends, family and acquaintances. If one of your friends becomes happy, for example, you’re more likely to become happy too. If you’re great friends with someone who becomes obese, you’re much more likely to become obese as well.
And the effect doesn’t stop there. If your friend’s friend becomes happy, that increases the chance your friend will become happy — and that you will too. Conversely, if you become obese or depressed, you may inadvertently help your friends, and your friend’s friends, to become fat or gloomy. (Intriguingly, happiness and obesity seem to spread in different ways. Obesity spreads most easily between friends of the same sex who are emotionally close. Happiness spreads most readily between friends who live near each other: a happy friend on the same block makes more difference than a happy friend three miles away.)
From Social Medicine, NY Times.
And from a Wired article on the same topic a couple of months back, with good graphics:
Christakis and Fowler realized that this obsolete list of references could be transformed into a detailed map of human relationships. Because two-thirds of all Framingham adults participated in the first phase of the study, and their children and children’s children in subsequent phases, almost the entire social network of the community was chronicled on these handwritten sheets. It took almost five years to extract the data—the handwriting was often illegible—but the scientists eventually constructed a detailed atlas of associations in which every connection was quantified.
The two researchers thought the Framingham social network might demonstrate how relationships directly influence behavior and thus health and happiness. Since the study had tracked its subjects’ weight for decades, Christakis and Fowler first analyzed obesity. Clicking through the years, they watched the condition spread to nearly 40 percent of the population. Fowler shows me an animation of their study—30 years of data reduced to 108 seconds of shifting circles and lines. Each circle represents an individual. Size is proportional to body mass index; yellow indicates obesity. “This woman is about to get big,” Fowler says. “And look at this cluster. They all gain weight at about the same time.”