I read a thought-provoking piece in the NY Times Magazine called "Are your friends making you fat?" I've been finding the idea of networks and social networks both interesting and compelling, and I wrote about it quite a bit at my job this spring. Information and infectious diseases do flow through networks of people. The math to determine exactly how they spread, and at what rate, is rather atrocious but also really fun. (I have an MPH in epidemiology- it's sort of a career hazard.)

Where I first learned this idea was in a seminar course on STDs, and a study of a syphilis outbreak in Baltimore in the 1980s. Basically, researchers used known patterns of drug use since syphilis was typically transmitted during a sex for drugs exchange. People might not always remember their partners, but they typically remembered where they got high. With this information, healthcare workers could narrow down the number of people who needed testing. The problem with translating these types of studies into non-infectious diseases is this: networks don't tell us whether the disease is a virus or bacteria, or even how the disease is transmitted. There can definitely be clues about transmission in these networks, but rarely are there specific answers. Rather, we find associations.

Which is why I find the idea of "social contagion" rather troubling. Certainly we affect each other's habits and lifestyles. I don't doubt that. But I'm not convinced from these studies that simply being friends with a fat person is going to make me fat.

An anorexic friend can't make you anorexic. An anorexic parent can't make you anorexic (though the whammy hits from both the genetic and environmental side, for double the trouble). A black friend isn't going to make me tan better- though I wouldn't mind if they helped me burn just a bit less. A tall friend isn't going to make me taller.

I've made friends with many people who also struggle with eating disorders and other mental health issues. If you just looked at my Facebook page, you might start to think that hanging around these people may have caused my eating disorder. Or maybe that I even caused theirs. We have an association (ie, interest in eating disorder advocacy), but that says nothing about cause.

Do we see trends of behaviors passing between friends and family? Absolutely. Can these have both positive and negative effects? Absolutely. But talk of causation, especially for something as complexly regulated as body weight, seems rather premature.