Is obesity contagious?

We started discussing work by Nicholas Christakis and James Fowler that first appeared in the New England Journal of Medicine in July 2007. The title of this paper is “The Spread of Obesity in a Large Social Network Over 32 Years.” Christakis and Fowler have followed this up with a series of papers claiming that other aspects of our human condition: happiness, loneliness, decrease in addictive behaviors (e.g., smoking), divorce, etc. can all be shown to spread through networks of friends. Christakis and Fowler have received lots of press and recognition for this work. Some examples I am familiar with are:

  • This TED talk by Christakis titled “The hidden influence of social networks.”
  • This article in the “Wired” magazine.
  • This interview by Stephen Colbert!

There has been some push back against the main conclusions that Christakis and Fowler derive from their data and analysis. For example, statisticians have argued that it may be impossible to distinguish between an effect that has spread over the edges of the network from the possibility that individuals may have formed relationships (edges) with other individuals having similar habits or tastes. This latter tendency is called homophily by social scientists and what some statisticians are arguing is that what Christakis and Fowler are seeing is their data might just be homophily. For example, international students arriving at the University of Iowa tend to associate with other international students. We definitely don’t think of being “international” as a contagion that spread on a network! A couple of somewhat technical articles by statisticians and a mathematician criticizing the Christakis-Fowler methods and conclusions are:

As you can tell from the snarky title of the latter article, Prof. Lyons is quite upset by what he claims is flawed analysis. This push back has been picked up by the popular press as well. Here are some examples:

Christakis and Fowler have since responded to these criticisms in a paper titled: “Social Contagion Theory: Examining Dynamic Social Networks and Human Behavior,” which you can find under publications at Fowler’s webpage. I suspect this is not the last we will hear of this fascinating debate.


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