Obesity in Social Networks

Deeply fascinating NEJM article I somehow missed last year on obesity and social networks:

Results Discernible clusters of obese persons (body-mass index [the weight in kilograms divided by the square of the height in meters], ≥30) were present in the network at all time points, and the clusters extended to three degrees of separation. These clusters did not appear to be solely attributable to the selective formation of social ties among obese persons. A person’s chances of becoming obese increased by 57% (95% confidence interval [CI], 6 to 123) 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% (95% CI, 21 to 60). If one spouse became obese, the likelihood that the other spouse would become obese increased by 37% (95% CI, 7 to 73). These effects were not seen among neighbors in the immediate geographic location. Persons of the same sex had relatively greater influence on each other than those of the opposite sex. The spread of smoking cessation did not account for the spread of obesity in the network.

Conclusions Network phenomena appear to be relevant to the biologic and behavioral trait of obesity, and obesity appears to spread through social ties.

Put simply, obesity is partially viral, at least in a social sense, with people around others with high BMIs tending to develop high(er) BMIs themselves. I have cited similar sorts of things in stocks as well. So intriguing.


  1. This is very interesting. It makes sense if you think about it. Eating is a social thing. If someone is obese, much of their time is spent eating which leads to picking friends that have similar interests. I know I like to eat with my friends. We also tend to eat similar things. My friends happen to be health freaks to juice wheatgrass and do yoga but that’s my network and they certainly have an influence. I’m grateful for it and consider them a rider on my health insurance policy in life. They keep me on track.

  2. This works in reverse too. I’ll bet sports clubs (e.g., running groups, cycling groups, etc.) would lead to similar results in weight loss or other measurable changes in a person’s health. When considered that way, it seems obvious.

  3. Colman Stephenson says:

    I don’t see how the second part of the conclusion can be drawn: “and obesity appears to spread through social ties” unless there is time data in the study – which is not mentioned in the abstract.
    Perhaps obese people like to hang out with similar type in which case obesity does not ‘spread’ through social ties.
    1. There is plenty of evidence that people are attracted to people like themselves in various categories (age, race, sexuality, language)
    2. Perhaps it makes them feel better and more normal.

  4. I do not believe that there is anything conclusive from the study – despite the fact that the sample size is large enough, it is taken out of the context and other variables are not controlled to draw causal relationship.
    The context could be one/all the following – with productivity tool such as computer, people simply tend to have more leisure time, so they spend more time eating. It could also be because market share of likes of ConAgra and Kraft are increasing thus more use of HFCS and low fat high sugar food is taking bigger market share. As for siblings, yeah, they grow up together and they eat same food and probably same preferences for food, and the same goes to couples – only one cooks for the couple.
    The report, is inconclusive per se. Too bad that it could be used in accordance with the growing “blame everything/everybody but myself” mentality. Look, eat less, exercise more is the only way to keep fit. Claiming obesity is viral totally misses the point – you blame your brother for making you fat? Why do not you blame your mother for even giving birth to your brother? It will never end.