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obesity spreads through social networks

Teppo

According to a recent study obesity spreads through social networks (see the New England Journal of Medicine article here, with cool animation etc).  But, could there be reverse causality?  Self-selection? And, what specifically is the ‘content’ of the network ties? And, what are the underlying theoretical mechanisms for why obesity spreads through networks? 

This longitudinal study appears to suggest that over time there is an obesity social interaction effect, the more proximate you are to someone gaining weight, the more likely you are also to gain weight. I don’t know that the study carefully explicates the underlying theoretical mechanisms of this network effect – other than, social proximity – but, one could speculate that if over time your close friends are becoming larger and you continue to interact with them, then perhaps perceptions (and norms, as conjectured in the link above) about appropriate weight change for you as well. Or, could the reverse also occur? And, is culture lurking somewhere in the background of this network effect?

Network studies have always interested me, though I have always wondered whether they are simply descriptive snapshots rather than meaningful theoretical explanations of social life. And, self-selection and other a priori conditions  (ability, a priori propensities and various unobservables) often seem like persuasive alternative explanations for proposed network effects. 

I suspect a self-selection effect in these findings (I don’t know if bias was introduced for example by not including those “friends” who dropped out of the network before the end of the study – i.e. once one gains weight, some “friends” may no longer be friends). The high sibling correlations obviously could have much to do with genetic effects, factors associated with assortative mating may also play a role, but, presumably all these factors were controlled for. Some of these confounds with obesity (and networks) could perhaps be nicely disentangled with the Bouchard-McGue/Minnesota separated twins samples/design.

Fascinating nonetheless.

Here’s a nice succinct video by one of the authors on the key findings.

15 Responses

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  1. […] obesity spreads through social networks […]

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  2. Teppo – I think you’ve hit the main issue with network studies – what is the direction of causation? Is this really a self-selection issue, or a transmission of behavior?

    My sense about the obesity research is that there are a couple of explanations:

    – you learn food preferences from each other, think about the link between ethnic food & diet

    – self-selection – you hang out with people that have similar preferences.

    My gut hunch is that some sort of simultaneous equation model is probably going to correct. I think all of hang out with people who share habit, but we also learn from them as well.

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    fabiorojas

    July 27, 2007 at 5:36 pm

  3. Another related problem is that most network studies assume RANDOM ASSIGNMENT of individuals (or, nested a priori homogeneity) – which is quite a stretch, to say the least – surely there is a priori sorting, unobservables and some choice involved.

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    teppof

    July 27, 2007 at 5:43 pm

  4. I saw the presentation version of the article last semester (the project PI presented the research to Laszlo Barabasi’s Physics seminar here at ND). By the time the presentation was over, I was pretty convinced that self-selection and reverse-causality had little to do with the core finding (on the strength of the high quality longitudinal design, and the fact that the basic effect was robust to a huge variety of specifications). I was also convinced that the social psychological mechanism proposed was probably on the right track.

    I mean you could literally see people’s BMI increasing after making a friend with a high BMI. I was struck by the epidemiology metaphor (fat is like a virus, you can “catch” it form other people) and I thought that it provided a nice counter-intuitive way to think of the process and to bring in a sociological focus to a topic that is usually thought of in purely biological terms (the metaphor that cultural standards and ideas are like viruses has a long pedigree in cognitive anthropology; see for instance Sperber 1985).

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    Omar

    July 27, 2007 at 5:56 pm

  5. Omar, very interesting. Will have to read the paper carefully, and get back. (Would be fun to do a forum on a specific paper like this, as we do with some books.)

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    tf

    July 27, 2007 at 6:03 pm

  6. My favorite finding was that the network effect is gender-homophilous, so that if you are a boy and have a girl-friend, her weight does not affect your weight gain, but if your boy-friends gain weight you also gain weight (and vice versa). Also, the effect is also (albeit a little more weakly) SES homophilous. That is, you are only affected by the weight gain of friends with similar SES and if your are high SES you are not affected by the weight gain of friends of lower SES. Pretty cool, huh? Those effects obviously scream of a Noah Friedkin-type social influence process.

    Oh, and the effect is symmetrical (even though is probably going to be reported in as “fat friends make you fat”): that is your friends weight loss affects your weight in the expected direction (that is you also lose weight).

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    Omar

    July 27, 2007 at 6:14 pm

  7. Stoopid Question: Why do skinny people get fat instead of fat people getting skinny? In other words, why is the network transmitting fatness norms instead of healthy eating norms?

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    fabiorojas

    July 27, 2007 at 6:17 pm

  8. It works either way as I remember. But the problem is that there is a secular trend towards fatness (everybody is getting fatter at the same time).

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    Omar

    July 27, 2007 at 6:19 pm

  9. Doesn’t that beg the question? Why is fatness normative? Seriously, why don’t people say “my friends eat well, I should eat well?” I say its safe to say that eating norms are socially transmitted, but I still don’t get why it’s *this* norm.

    Is there research showing anorexia and bulimia are socially transmitted among young women? If so, why isn’t that the norm for everyone? Why don’t we get “infected” by anorexia via social ties with this subpopulation?

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    fabiorojas

    July 27, 2007 at 6:42 pm

  10. Anorexia probably is also socially contagious…or?

    I know a clinician in the eating disorder space, and, interestingly enough (need to find the source) – the cultural and social explanation of anorexia was recently seriously thrown into question by some scholar who found similar rates of these types of eating disorders in places like Tonga, Guinea etc.

    (Hmm, need to dig up some of this research to see what the findings were.)

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    tf

    July 27, 2007 at 6:45 pm

  11. If anorexia is not socially transmitted, then that makes this whole issue more confusing to me. Why do networks promote fatness and not skinnyness?

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    fabiorojas

    July 27, 2007 at 6:54 pm

  12. My hunch would be that taste, for once, is a “learned” thing, developed in childhood when everything develops that later makes the habitus of young aldults and the hexis of the person, and the physical statue that comes out is strongly influenced by significant others in teenage days (e.g. teenage idols), then people of (near-) same milieus become couples, and as they get older they increasingly look the same, and this goes far beyond nutrition and clothing alone.
    The second aspect to my guess is a milieu aspect – Pierre Bourdieu has this image on the cover his elite study where all the guys of a french elite university not only look the same (facial expression, body language, clothing etc.) but have pretty much similar physical statue, and they also develop very similar patterns in taste which corresponds to the first argument. Similar to this, Gerhard Schulze has found in his 1980’s milieu study in Nürnberg/Germany, that people from same milieus (e.g. Niveaumilieu, Harmony milieu) have amazingly similar physical appearances. In the Niveaumilieu (> 40 years, higher education, mostly high status) and in the Harmoniemilieu (> 40, low educational level, often low social status), the differences in the physical experience were striking, to the disadvantage of the Harmoniemilieu were many people were fat and in really bad phyical shape. With regard to the overall milieu pattern (including economic aspects, aspects of living etc.) the study suggested that people with higher education and better income have the knowledge, the means, the time and the monetary resources to invest in cooking and eating, and are generally more concerned about their health, physical shape and appearance.
    BTW: Germany was recently announced the fattest nation in Europe.

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    Tina

    July 28, 2007 at 10:13 pm

  13. I agree with Omar’s take on the article. The longitudinal design, combined with regression modelling, and their multiple tests for selection and directional effects, sold me on the quality of the statistics. The study also measures friendship in multiple ways and combines social and physical proximity measures to examine the potential impact of actual interaction with relevant others.

    I do believe that the researchers have an implicit bias against obesity, and the paper lacks theoretical foundation. The research interests me because it supports my pet theory: structural symbolic interactionism. Stryker predicts that it is network embeddedness which impacts behavior and cognition, through its impact on identity salience. Though social structure and the individual have reciprocal impacts on one another, the nexus of this influence is interaction. To the extent that one can control one’s weight through specific behaviors and cognitions, SI would predict that your friends would have an impact on that.

    I also take issue with fabiorojas’s equating fatness with anorexia or bulimia. The latter two are psychological disorders; the first is not, and to state that an effect for one should imply an effect for the other makes it sound like being fat is a psychological disorder, as well. That said, the study finds a similar effect for thinness.

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    anomie

    July 30, 2007 at 11:57 pm

  14. I feel that weight gain throughout the world has become a serious problem . I believe that instead of putting the blame onto others ..we need to take responsibility for our own actions. I find it very surprising how little people actually know about what they are putting into their own mouths!

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    Obesity info

    November 15, 2007 at 12:41 pm

  15. […] a comment » Happiness, like obesity and everything else (ok, so, I’m still doubtful), spreads through […]

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