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Are your friends making you fat?


This article from the NYTimes summarizes a study on social contagion lead by sociologists Nicholas Christakis and James Fowler. For many years sociologists had reason to believe that certain behaviors can be contagious, similar to how an epidemic spreads, but there was never any solid proof. What Christakis and Fowler were able to do that sociologists in the past couldn’t was use actual data from an organization called the Framingham Heart Study whose purpose was to investigate the causes of heart disease based on a person’s heart rate, weight, blood levels, etc. Each of the 15,000 Framingham residents that are in the Heart Study’s data base have been given a physical check up every 4 years with their results recorded. They were also required to write down one good friend who would know of their whereabouts several years from now. Christakis and Fowler used this information to analyze how obesity spreads among a social network. They found “obesity broke out in clusters. People weren’t just getting fatter randomly.” When a Farmingham resident became obese, his friend was 57% more likely to become obese, and that friend’s friend was 20% more likely to become obese, and that friend’s friend’s friend was 10% more likely to become obese. That’s up to three degrees of influence! And obesity is just one example of a contagious behavior.


Christakis and Fowler continued to explore the realm of social contagion. They discovered smoking habits of individuals affected their friends. If a person quit smoking, members of their friendship cluster were much more likely quit smoking as well, which makes sense. Wouldn’t you feel weird being the only one smoking in a restaurant over a nice dinner with your friends? Besides smoking, Christakis and Fowler observed feelings like happiness and depression also work similarly.


Their studies have generated different reactions. Some believe that social contagion is a legitimate phenomenon and has the potential to affect and even reshape how we approach medical care. For example, it would make sense to target the “superconnector” to quit smoking, since a “superconnector” is a person who is highly connected to many others in their social network and has considerable influence. Others believe there are other significant factors that contributed to the spread patterns of behavior, such as “homophily,” the tendency of people to gravitate toward others who are like them, or shared environment.


I believe social contagion makes complete sense, especially when concepts we learned in class are applied. Take the Strong Triadic Closure Property (STCP). The property states if person A has a strong link (as in, friendship) to person B and a strong link to person C, then there exists a link between persons B and C to close that triangle because there is more opportunity and incentive. The STCP could also be used to explain why obesity or happiness spreads across groups of people who are connected to each other. For example, if Kelly is surrounded by friends who have gained weight, Kelly might feel like it is more acceptable for her to gain weight, as well. It is as if the weight gain trend by her surrounding friends had the same closing effect on Kelly as described by the STCP. In a social network, there obviously exists more than just one triangle. Since so many people are interconnected, you could see how something like obesity or smoking habit or happiness can have reverberating effects on a large group.


What I found especially interesting was the fact that Christakis and Fowler’s studies showed how behavior can spread across up to 3 degrees of separation. Even if the STCP is violated and a triangle does not close, social contagion can still persist. As in, a friend of a friend’s likeliness to gain weight still increases, even if the influencer and the influencee, if you will, are not friends.


Although skeptics raise good arguments to Christakis and Fowler’s findings on social contagion, it is still pretty neat to think that what you do and how you act can essentially affect more than 1,000 people, when you total up your friends of friends of friends. “It changes your way of seeing the world.’”



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