Spread Of Obesity In Large Social Network: Birds Of A Feather Flock Together

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In 2007 July, Nicholas Christakis, a professor of public health at Harvard Medical School, and James Fowler, an associate professor of political science at UC San Diego, published a cover article in medicine’s most prestigious journal, the New England Journal of Medicine, complete with a teaser written by Albert-Laszlo Barabasi. The article served as the debut of social network analysis in medicine, complete with a glossary that explained terms such as ‘ego,’ ‘alter’, ‘node,’ and ‘homophily’ for uninitiated medical researchers.

In the article ‘The Spread of Obesity in a Large Social Network over 32 Years’, Christakis and Fowler (CF) make a number of claims and in a later response to criticism, CF summarize their two major findings of the paper in the following two points:

(1) ‘obese persons formed clusters in the network at all time points and that these clusters extended to three degrees of separation (e.g., to a person’s friend’s friend’s friend)'(Fowler, 2007, p. 377)

(2) ‘statistical analysis suggested that the clusters were not soley attributable to the selective formation of social ties among obese persons. A person’s chances of becoming obese increased if he or she had a friend who became obese in a given time period.’ (Fowler, 2007, p. 377)

I note that this brief summary of the main findings is slightly incomplete. If one considers point (2), there are a number possible reasons why someone’s body mass index (BMI) might be linked to the BMI of his or her friends. I will begin by considering some possible causes for this clustering.

Various causes of clusters of obese people

Noting that friends often share similar environments, it could be the case that fast food chains (or fast food advertisements) have invaded our common environment, or some technological improvements such as elevators have decreased the need to be physically active. In this case, although a person’s weight gain is correlated with the weight gain of his friends, it’s not really his friend that causes him to gain weight, but rather a shared environmental factor—a ‘confounding variable.’

However, we can consider a situation in social dynamic, a person’s weight gain made an impact on his friends as they tend to gain more weight. Let’s reasonably assume that people have an incentive to eat lots of unhealthy food (e.g., it tastes good, is cheap, easy to find). Let’s imagine that my definition of an acceptable BMI is pegged to be slightly lower than the average BMI of my friends. If my friends gain a lot of weight, then my acceptable BMI will increase, allowing me to enjoy unhealthy foods without thinking, I am unacceptably overweight. One can also imagine simpler processes whereby my friends’ behavior directly affects my BMI; for example, if my friends frequently starts visiting unhealthy restaurants then there is a tendency that I may also drag into it. Let’s call these explanations of (2) cases of ‘social influence’, and note that the ‘social influence’ explanation of (2) differs from the ‘confounding variable’ explanation’ because we put the blame for our weight gain on the behavior of our friends rather than on some shared environmental change.

So far we have established two processes that could account for clusters of obese people: confounding variables (shared environmental factors such as fast food chains) and social influence. Point (2) also states that ‘the clusters were not soley attributable to the selective formation of social ties.’ To understand this statement, we must introduce a third process that can create clusters of similar people: selection (aka, ‘choice homophily’,’homophily’). Selection’s mechanism is simple: ‘birds of a feather flock together,’ that is, people befriend those who resemble themselves. In this case clustering of obesity can emerge even though everyone’s weight remains constant–overweight people tend to become friends with overweight people, and similarly, people who are not overweight tend to become friends with similarly slim people. This process, selection, also leads to obese clusters.  

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