A former student of mine, Peter Kasper, very astutely caught the fascinating result in this news article. Obesity is a social contagion. No surprise. But, the effect is stronger for friends in your network than for family AND neighbors. Whats more, even two degrees away has an effect!
Your best friend could be making you fat – Diet & Nutrition – MSNBC.com
The effect held for three degrees of separation. If a person became obese, their friends were more likely to become obese, but also friends of friends.
The original research comes from the New England Journal of Medicine and is by Nicholas Christakis and James Fowler. They used a very original research design. A long term health study of Framingham, MA, had each person list alternate contacts. As the sample included almost the whole town, these alternative contacts constitute a social network for whom they also had health data from 1948-1971. Very Clever.
The big result is that if one person names another as a contact (unidirectional tie) then if one became obese in teh 32 year period, the other was 57% more likely. If the tie is bidirectional- they each listed each other- then the odds of obesity in one if the other became obese was 157% greater than the baseline risk. WOW. Add a typical degree if clustering and transitivity, and you can see how one fat friend can lead to straining elevator cables in short order.
I wonder what they used to produce it? The caption says yellow is obese and node size is BMI. Heh. So, you see big clusters of fatty nodes/cells in an overall sparse network.
The news article (I didn’t have time to read the original) is a but thin in explaining why obesity is contagious. It would also be interesting to know how it compares to smoking, drug use, or sexual behavior. In my class, we read Malcolm Gladwell’s account of smoking and suicide as social contagions.
But back to the why question… Is it just seeing a friend engorge? Does eating behavior change? Is there an unmeasured change in environment (sendentary jobs, food sources, income) that effects dyads of friends and in turn leads to weight gain? We have good evidence that obesity is spreading, but we still don’t know why.
What this does reinforce is that social geography is more important than physical. In other words, you live in, your helath is a function of, the social net you have woven around you.