Our genes influence our social networks

Chromosomes magnified - photo by BlueSunFlower

If you don’t have enough friends or aren’t the social butterfly of your class, now you can blame your genes.

Nick Christakis (Harvard Medical School) and James Fowler (UCSD political scientist) are back with more controversial findings suggesting some genetic determination in our social networks (both in forming friendships and determining where we are in social networks).  Christakis: “the beautiful and complicated pattern of human connection depends on our genes to a significant measure.”  Previous work by Christakis looked at how our social networks and who is in them shape our likelihood of obesity, happiness, and smoking, among other outcomes.

They researched 1,100 same-sex twins in the National Longitudinal Study of Adolescent Health (colloquially called “Add Health”). Add Health examined high school students in 1994-1995 and asked questions regarding economics, physical health and social involvement. Christakis and Fowler compared the social networks and patterns of identical same-sex twins against fraternal ones to separate nature (genes) from nurture (upbringing).

Their findings go far beyond what people might think about the genetic influence on personality traits (being outgoing, shy, etc.). For example, how often the subject was named as a friend and the likelihood that the subject’s friends knew one another were strongly genetically influenced, but interestingly not the number of friends that the subject listed. This suggests a genetic determinant of being popular (beyond a simple disposition toward being outgoing); further buttressing this interpretation, whether the subject was more the center of attention (central to these networks) or more of a social outcast (peripheral to these networks) was also heritable.

Christakis admits that some of the findings are puzzling, like the fact that the likelihood that my friends Bill and John know each other is attributable to my genes; what this likely means is that some people are genetically disposed to introduce their friends to each other more or to host or arrange social events where these friends would have chances to meet each other.

‘Given that social networks play important roles in determining a wide variety of things ranging from employment and wages to the spread of disease, it is important to understand why networks exhibit the patterns that they do,’  Matthew Jackson, a Stanford University economist, wrote in a commentary accompanying the study called “Do We Inherit Our Positions in Life?”.

James Fowler… said its implications go beyond the theoretical. For some time, scientists have suspected a genetic role in certain conditions, such as obesity. Now, Mr. Fowler wants to investigate whether the dynamics of social networks might affect public-health outcomes, for instance, by exposing people to certain behaviors, such as smoking.”

“Our work shows how humans, like ants, may assemble themselves into a ’super-organism’ with rules governing the assembly, rules that we carry with us deep in our genes,” says Nicholas Christakis.  Christakis et al. also believe that there may be an evolutionary explanation for their findings since one’s position in social networks had costs or benefits to the survival of one’s genes. Being central to a group likely contributed to survival during periods of food scarcity since one could learn where food supplies were, while being peripheral to groups helped genes survive in periods where deadly germs were being transmitted by social contact. Christakis: “It may be that natural selection is acting on not just things like whether or not we can resist the common cold, but also who it is that we are going to come into contact with.”  The paper notes: “There may be many reasons for genetic variation in the ability to attract or the desire to introduce friends.  More friends may mean greater social support in some settings or greater conflict in others.  Having denser social connections may improve groupsolidarity, but it might also insulate a group from beneficial influence or information from individuals outside the group.”  The authors note that more work is required to understand what specific genes are at work and what possible mediating mechanisms might be.

The authors acknowledge some controversy in studies comparing identical twin studies to fraternal twins, with critics noting that identical twins may have a stronger affiliation with  each other that causes them to be more influenced by each other than fraternal twins.  The authors note that twin studies have been validated by comparing identical twins raised apart versus together (suggesting that it is not the shared environment).  The authors further note that personality and cognitive differences between identical and fraternal twins persist even among twins mistakenly believed to be identical by their parents (indicating that parental patterns in raising these ‘identical twins’ can’t explain the outcome).  Finally, they note that that once twins reach adulthood, identical twins living apart tend to become more similar with age, which doesn’t fit with a notion of the importance of their shared environment.

The study appeared online in James Fowler, Christopher Dawes and Nicholas Christakis,  “Model of Genetic Variation in Human Social Networks” in Proceedings of the National Academy of Sciences journal (January 26, 2009).

“More specifically, the results show that genetic factors account for 46% [95% confidence interval 23%, 69%] of the variation in in-degree (how many times a person is named as a friend), but heritability of out-degree (how many friends a person names) is not significant (22%, CI 0%, 47%). In addition, node transitivity [the likelihood that two of a person's contacts are connected to each other] is significantly heritable, with 47% (CI 13%, 65%) of the variation explained by differences in genes. We also find that genetic variation contributes to variation in other network characteristics; for example, bertween-ness centrality [the fraction of paths through the networks that pass through a given node] is significantly heritable (29%, CI 5%, 39%).”

See also “Genes and the Friends You Make” (Wall Street Journal, 1/27/09 by Philip Shishkin)

See other articles by Christakis et. al on social networks.

Happiness is contagious

Nick Christakis (Harvard School of Public Health) and James Fowler (Univ. of Calif., San Diego), who previously used the Framingham Heart Study to show that having fat friends increasingly makes people obese, are back with a very interesting paper showing that happy friends make you happy — what the co-authors called ‘an emotional quiet riot’.

It is already established that happiness and having social capital (friendships) are linked, but this research demonstrates that it matters how happy your friends are and that it is the happy friends that are causing your happiness rather than vice versa. Conversely, having unhappy friends over time makes you less happy.

The research shows up in the latest issue of BMJ. “Dynamic spread of happiness in a large social network: longitudinal analysis over 20 years in the Framingham Heart Study.”  [The study involved 5,124 adults aged 21 to 70 who were followed between 1971 and 2003.]

They measured happiness with a 4-item construct:  “I felt hopeful about the future”; “I was happy”; “I enjoyed life” and “I felt that I was just as good as other people.”

They found that happiness is a network phenomenon, clustering in groups of people that extend out to 3 degrees of separation (the friends’ friends of one’s friends), but with greater impact on friendships that are 2 or 1 degree of separation from you.  Demonstrating the magnitude of this effect, co-author James Fowler noted, “if your friend’s friend’s friend becomes happy, that has a bigger impact on you being happy than putting an extra $5,000 in your pocket.”

They found that happiness spreads across a diverse array of social ties, from spouses to siblings to neighbors. They found no happiness effect of co-workers and found that nearby ties had a far greater influence than distant ties: for example, knowing someone who is happy, makes you 15.3% more likely to be happy, but having happy next-door neighbors makes you a full 34% more likely to be happy (much higher than having happy neighbors merely on your block). The optimal effect was for a happy friend living less than half a mile away, which boosts your chance of happiness by42%. In one of the study’s surprises, happy spouses (which one assumes live less than a half mile away!) only increased one’s chance of happiness by 8%. Part of the lower spouse effect is that happiness spreads more effectively through same sex relationships than relationships (romantic or not) between a man and a woman.  (Gays take note!)  Christakis and Fowler believe we may take emotional cues from people of our gender.

They observed that network characteristics (where you were in the network and how happy the people were around you) could independently predict which individuals would be happy years into the future.

They suggest that there may be an evolutionary basis for human emotions.  Previous work noted that emotions like laughter or smiling seemed evolutionary adapted to helping people form social bonds.  [They note: "Human laughter, for example, is believed to have evolved from the 'play face' expression seen in other primates in relaxed social situations. Such facial expressions and positive emotions enhance social relations by producing analogous pleasurable feelings in others."]

While they couldn’t prove it, they suggested 3 possible causal mechanisms:

1. happy people might share their good fortune
2. happy people might change their behavior toward others (by being nicer or less hostile)
3. happy people might exude a contagious emotion (although this would have to be over a sustained time period)

Christakis and Fowler noted that the 3-degrees of separation impact observed in happiness was the same as for smoking and obesity (which also reached out 3 degrees). They wonder whether a “3-degrees of influence” extends to behaviors like depression, anxiety, loneliness, drinking, eating, exercise and other health-related activities.

So the next time you’re unhappy realize that you may be “infecting” your friends with unhappiness as well.  Christakis’ work is suggesting that we need friends, but we also need to carefully pick friends that are happy and have healthy behaviors or we risk that their unhappiness and unhealthy behaviors will spread to us.  The New York Times notes that one of the co-authors indicated that he now thinks twice about his mood knowing that it affects others. That said, he noted: “We are not giving you the advice to start smiling at everyone you meet in New York….That would be dangerous.”

While they think that face-to-face connection is important in spreading happiness (hence the decline of these effects with distance), they did a separate study of 1,700 Facebook profiles, where they found that people smiling in their photographs had more Facebook friends and that more of those friends were smiling. While the Facebook study is just an initial foray into the online word, Christakis thinks that it shows that some of these happiness findings might extend on social networking as well.  And it would take longitudinal studies to determine whether our online activities are gradually eroding our need for face-to-face communication to spread happiness.

Note: Justin Wolfers (on the Freakonomics blog) is skeptical of this research.  As he notes:

[It's possible that it is not your friends' happiness that is causing yours, but that "if you and I are friends, we are often subject to similar influences. If a buddy of ours dies, we’ll both be less happy. Or, less dramatically, if our favorite football team wins, we’ll both be happier. But this isn’t contagious happiness — it is simply a natural outcome of the shared experiences of people in the same social circles. Unfortunately, observational data cannot distinguish the headline-grabbing conclusion — that happiness is contagious — from my more mundane alternative: friends have shared emotional influences."

Wolfers notes that a very careful article by Ethan Cohen-Cole and Jason Fletcher uses the same research design to show how it can lead to silly conclusions.  Cohen-Cole and Fletcher find in another dataset that this approach shows “height, headaches, and acne are also contagious.” As Wolfers notes, it’s more likely that “the same jackhammer causing your headache is likely causing mine.” And the height finding is obviously not causal but more likely a function of homophily (people choosing similar friends).

See Clive Thompson, “Is Happiness Catching?” (NYT Sunday Magazine, 9/13/09)

Boston Globe story available here.

New York Times story available here (which also has a nice graphic showing the clustering in this network of happy and unhappy people).

L.A. Times story available here.

See Wolfers’ Freakonomics blog post here.

Do fat friends make you fat and less happy? (new evidence)

I blogged earlier about Christakis and Fowler’s 2007 research about obesity as a social epidemic.  [See blog posts here.]

David Branchflower et al have released a paper using European Barometer data (across 29 European countries) that suggests that for Europeans as well, having fat friends may increasingly make them fat.  One’s friends influences what one thinks of as fat or skinny, so having more obese friends, makes one ratchet up (subconsciously) what one thinks of as the dividing line between fat and thin.

Blanchflower and colleagues also find in German panel data that controlling for other factors, being fatter (having a higher Body Mass Index, or BMI) reduces one’s sense of subjective wellbeing (i.e., happiness).  As I noted in an earlier blog, since having friends itself is associated with higher happiness and many benefits of social capital, the conclusion is not to drop one’s overweight friends, but it does suggest that if one is not mindful to ensure that you have a healthy dose of thinner friends as well, you may well find yourself fatter and less happy overall.

See: David G. Blanchflower, Andrew J. Oswald, Bert Van Landeghem, “Imitative Obesity and Relative Utility” (NBER Working Paper No. 14337, September 2008)

See also, Clive Thompson, “Is Happiness Catching?” (NYT Sunday Magazine, 9/13/09)

Quit your habit in groups and be more popular, say scientists

Nick Christakis and James Fowler made headlines recently for their study on obesity contagion (See Can your friends affect your weight?).

Now they’re back with analogous research that shows that you are far more likely to be able to quit smoking if you do it in groups (where those around you are also quitting). It’s scientifically-proven, but something that practitioners have known for a while: why do you think Jenny Craig has dieters work in groups, or why all the self-help groups (Alcoholics Anonymous and others of that ilk) use group norms to reinforce changes in behavior.

Study co-author Fowler notes that in tracking individuals and social groups (through the Framingham Heart Study) over 30-years, the average size of each cluster of smokers was of similar size, but Fowler notes: “It’s just that there are fewer and fewer of these clusters as time goes on.”

The social contagion of quitting smoking can extend to people that the quitter didn’t know. For example if Anne quits smoking, and Anne is friends with Barb and Barb is friends with Clarissa, Clarissa’s chance of quitting increases by 30%, even if Anne and Clarissa don’t know each other.

Christakis notes that smokers have moved more to the periphery of social networks, where they often were more at the center of these social networks several decades ago. While this doesn’t say that teen non-smokers will necessarily be more popular, it does suggest over their lifespan that non-smoking is more likely to be associated with popularity than smoking.

The obesity study appears in the May 22 issue of the New England Journal of Medicine.

See also, Clive Thompson, “Is Happiness Catching?” (NYT Sunday Magazine, 9/13/09)

Social Networks: Birds of a Feather Do Flock Together

A study by Harvard and UCLA researchers on Facebook is finding that social networks tend to lead to bonding social capital (people associating with others like them). These are preliminary findings and the study is continuing through 2009.

They have found that race and gender have the largest influence on who one befriends in social networks online, and white students (especially men) have the least diverse social networks. The study also found that the size of the social network was largest for black students, followed in turn by mixed race students and white students.

While this finding is consistent with findings across lots of sociological settings that show that we tend to form friendships with others who are like us (what researchers call ‘homophily’), it is a blow to Internet utopianists who hoped that the Internet would somehow make it far easier for us to form friendships with those who are different than us. [As the dog using the Internet in the famous New Yorker cartoon articulated, "no one knows you're a dog on the Internet."]

The interesting research project is being conducted by Jason Kauffman and Nicholas Christakis at Harvard University and by Andreas Wimmer (a sociology professor at UCLA). [We wrote earlier about Nick Christakis' research on how obesity spreads through social networks.]

Putting a positive spin on the fact that facebook tends to lead toward like befriending like, Kevin Lewis (a third year PhD working on the project) asserted that this finding may buttress the case that the friendships formed online are real, if they exhibit traits (like homophily) that we see in real-world friendships.  Harvard-UCLA researchers are also examining”triadic closure” with these data: the tendency found by socialists for people who have friends in common to themselves become friends over time.

This study is part of an emerging field of computational social science (analyzing the vast data trails that Americans leave with their e-mail, their online friendships, their call-logs, etc.). My colleague David Lazer recently convened a meeting at Harvard of scholars doing computational social science or interested in doing more.  (For a brief post, see here.)  Some of the projects are quite fascinating including one by a scholar who captured all of his child’s communication and utterances from infancy through toddlerhood through an always-on digital camera, and then transcribed all the conversation to observe patterns of speech development.

And the New York Times yesterday in their article, “On Facebook, Scholars Link Up With Data” (NYT, 12/18/07) mentioned not only the Kaufman et al. study but other interesting recent studies. “Scholars at Carnegie Mellon used the site to look at privacy issues. Researchers at the University of Colorado analyzed how Facebook instantly disseminated details about the Virginia Tech shootings in April….Social scientists at Indiana [Eliot Smith], Northwestern [Eszter Hargittai], Pennsylvania State [S. Shyam Sundar], Tufts, the University of Texas and other institutions are mining Facebook to test traditional theories in their fields about relationships, identity, self-esteem, popularity, collective action, race and political engagement.”

This is all a wonderful development as we hope it will help to sort out some of the ethereal claims on social networks from the actual practices observed.  And given that these networks are longitudinal, one can actually watch friendships being made and see what factors at time 1 predicted friendships at time 2 which is quite exciting from a social science perspective.

Becker-Posner on social obesity epidemic

I’ve posted earlier on the issue of social obesity (i.e., your friends can make you fat) here, here and here.

Economist Gary Becker and Judge Richard Posner have a point-counterpoint blog that focuses on various issues, but most recently raises a few additional issues relating to the Christakis et al Framingham study.

They agree that few of us know our objective obesity (which is defined in body mass index of weight relative to height) and that people who are overweight tend to believe that they are more normal weight than they are.  It thus makes sense that ones friends (if they are similar in obesity or thin-ness) help one to shape one’s self-conception of the average weight of society.

Becker points out that the ideal study would look at college freshman randomly assigned roommates and seeing whether those assigned to larger roommates were more likely to put on pounds over the freshman year since that would rule out the motivations of why one chooses friends and unexplained variables.

Becker also points out that the study by Christakis doesn’t account for the social multiplier on weight gain (your friends’ weight gain increases your weight, at some percentage less than 100%.  Then in turn your subsequent gain increases their weight.  The weight gains from this multiplier decline with each transmission, but still account for weight gains larger than the original weight gain.)

You can read their posts here, and here.

Your friends may also depress you?

I’ve written previously about Christakis et al.’s remarkable study that traces the spread of obesity through social networks (i.e., your friends, although generally same gender friends).  Christakis et al also believe that smoking is transmitted through social networks (with friends making you more comfortable lighting up or not, depending on what they do).

In Sunday’s New York Times Week in Review, Gina Kolata indicates that Christakis and Fowler are investigating whether depression also travels in social networks (depressed friends make you more depressed or happy friends make you less depressed).  There is apparently some evidence on this score from another scholar, Columbia’s Peter Bearman, using the federal National Longitudinal Study of Adolescent Health, “a study of tens of thousands of teenagers that asks them to name their friends and follows them for years. It turned out…that certain friendships increased the likelihood of suicide or suicidal thoughts in teenage girls.

“The risky friendships are what Dr. Bearman calls a contradictory network — a teenage girl has two friends who dislike each other. ‘It tells you about the importance of social relationships for girls’ health and self-esteem,’ Dr. Bearman says. ‘If you are in an unstable triad, it makes it much more difficult to fit in.’”

Bearman and his colleagues are now studying the social spread in the diagnosis of autism. They believe that a large portion of the spread of autism may reflect the increased diagnosis of autism rather than an increase in the disease itself.  Bearman believes that if a friend’s child is treated for autism, you are more likely to test your child if he/she shows at all similar behaviors.  Then as the concentration of autistic kids spreads in a locality or school, the school becomes sensitized which also further increases diagnosis of autism.  So in effect, according to this theory, a social norm of testing and diagnosing for autism spreads through social networks.

As Gail Collins warned, can picking friends based on them being non-smoking, thin, and happy be far behind?  And are we soon going to hear criminal defendants citing as their defense that the social norm that killing is OK was spread to them through their friends?  And all this work on the negative influence of friends, threatens to overshadow a much longer and more durable scholarly thread (and ultimately more generalizable) in public health about how social support in general is a critical driver of health, happiness, and longevity.

See article text at “You, Your Friends, Your Friends of Friends” (NYT, Week in Review, 8/5/07, Gina Kolata)

See also, Clive Thompson, “Is Happiness Catching?” (NYT Sunday Magazine, 9/13/09)

Overweight friends as social contagion (II)

Yesterday, we reported about Christakis’ and Fowler’s top-rate study of how obesity spreads in social networks.

In today’s NYT, Gail Collins has a humorous Op-Ed (“Fat comes in on little cat feet“) about the ramifications of such a study.  Included are the following:

“8 p.m. — ‘Friends.’ In a much-anticipated reunion special, the gang has all bought condos in the same strangely affordable Manhattan apartment building. Tension mounts as Phoebe and Rachel notice that Monica is putting on weight. Well aware of the new study showing that obesity travels through friendship networks, they evict her. ‘The body mass of the many is more important than the survival of the one,’ says a saddened Ross. ‘Even if she is my sister.’ Later, the rest of the group reminisces about good times past with their now-shunned buddy. Nicole Richie guest stars as Chandler’s new love interest.”

Collins notes that the study found that obesity spreads through social networks and Christakis and Fowler “believe this is true even if said friend lives in Bangkok. The far-away friend has far more influence on your weight than relatives in the same house. And your neighbors can gain or lose the equivalent of several persons without it having any impact whatsoever.”

Collins doesn’t blame the researchers for their findings but notes that this is unlikely to be the “kind of information that’s going to brighten up anybody’s day. I’ve been overweight my entire life, and although I’ve had a lot of friends, I can’t think of one who got fat while hanging around with me. But if there’s anybody out there, I really do apologize. I’d have dropped you ages ago if only I’d known.”

Given the social contagion finding, Collins speculates:” Can you imagine how mean the high school mean girls are going to get if they think they have scientific evidence that ostracizing the chubby kids is a blow for physical fitness?….And now that his theory about leprosy-bearing Mexicans sneaking across the border has been completely debunked, Lou Dobbs will be hyperventilating about obese illegal immigrants ingratiating themselves and their fat into American communities.”

Christakis and Fowler are clear to indicate that they do not recommend droppiong fat friends from one’s social network since friendship has many other health benefits.  A Slate magazine article (“Maybe fat people should be stigmatized“) thinks that they authors are being too PC and avoiding people taking responsibility for their decisions.    And Christakis and Fowler compare having an overweight friend to having no friend and conclude that the former confers more health benefits (even with the accompanying increased risk of obesity);  if their choice set was a fat friend or a thin friend, they would have gotten a different result since the thin friend would still confer the health benefits of friends without the obesity risk.  To this, Christakis said in a phone interview with Collins that “The network of fat-influencing relationships are so dense that in the end ‘your weight status might depend on the weight difference of your sister’s brother’s friend.’ “   Sounds like a bit of a cop-out.  Like arguing that someone shouldn’t avoid risk factors because there are so many other risks out there. 

But maybe in our days of increased social isolation, people should hang on the friends they have since so many Americans are losing their close friends.

David Lazer has an interesting post about how Christakis et al. deftly handle the issue of causality in their paper using longitudinal data.

And Ellen Goodman had an interesting post on this called Obesity Contagion (Boston Globe, 8/3/07)

The NEJM article by Christakis et al. on the spread of obesity through networks is available here.

Can your friends affect your weight?

Nicholas Christakis (at the Harvard School of Public Health) (together with James Fowler at U. Cal. San Diego) has done very interesting research with the landmark Framingham Heart Study looking at the likelihood of obesity. They find, after tracking some 12,000 individuals through the 32-year long study (1971-2003), that even controlling for all the genetic markers for obesity (like parents’ obesity) and weight at the study’s baseline, having fat friends increases the chance that you will be fat.  [The Framingham study participants could list only up to 3 friends, so the friends being discussed here are closer, stronger friends, rather than weaker ties.]

Having an obese friend increased one’s likelihood of obesity by 57%, with a smaller effect for siblings and spouses. This risk of developing obesity rose to 171% for the closest mutual friendships. Having very large neighbors had no effect on obesity if those neighbors weren’t friends of the individual in question.

The study found that the network impact of friends on obesity could be seen in as small as 2-4 year increments.

Christakis discounted other likely factors such as environmental (since it didn’t matter how close geographically ones’ friends were, making it less likely that they are both responding to something in the neighborhood, for example).  They did code the data for density of fast food restaurants and it did not make this effect go away.  [The fact that long-distance close friends influence obesity as much as nearby close friends is very surprising;  for sure, if respondents are still listing this long-distance friend as one of their three closest friends, they must have stayed in regular contact, but I would have thought that obese friends made respondents feel more comforable being obese partly by physically seeing an obese friend, and this is presumably less common with geographically distant friends.

And Christakis thinks the mechanism is “induction”; having an overweight person list YOU as their friend doesn’t increase your likelihood of being obese, but your listing an overweight person as YOUR friend does.  The impact of these friendships decreases with social distance (in other words, your friends affect your weight more than your friends’ friends, which is more consequential than the weight of your friends’ friends’ friends, etc.) but they still have an impact out to three degrees of separation.  And same sex friends influence your weight gain more than opposite sex friends.

Christakis thinks that people with heavier friends either come to think of themselves as less fat or else it validates their obesity in a way that wouldn’t be the case if their friends were thinner.  (But the study showed that it wasn’t the simple story of a change in physical exercise or eating habits from these friends.)  And there is homophily in obesity — in other words, fat people are more likely to choose overweight friends and thin people are more likely to pick thin friends.  And if overweight people have thin friends or vice-versa, these relationships tend to be less stable over time (the thin people are more likely to drop their overweight friends or become more obese).

The New York Times notes that: “Science has shown that individuals have genetically determined ranges of weights, spanning perhaps 30 or so pounds for each person. But that leaves a large role for the environment in determining whether a person’s weight is near the top of his or her range or near the bottom. As people have gotten fatter, it appears that many are edging toward the top of their ranges. The question has been why. ” (“Find Yourself Packing It On? Blame Friends, NYT, 7/26/07).  The study suggests that social contagion of obesity through networks may be the explanation.

Christakis thinks that the role of social networks may be one of the explanations for the increasing obesity in America (along with other factors like exercise, change in eating habits like supersizing of food and more fast food and soft drinks, etc.).  The social networks may have changed norms and made weight gains more acceptable, even outside of any change in behavior.    Moreover Christakis thinks that we might be able to use the structure of social networks to fight obesity, by for example fighting obesity in groups rather than with individuals.  If there can be a social contagion of obesity, how might we start a social contagion of weighing less?  Co-author James Fowler noted that having a friend that was able to lower his or her weight down made it easier for one to lose weight; that’s why weight loss programs often function using groups, to reinforce the attempted change in behavior. But this question of spurring a social contagion for good is exactly the kind of question that Malcolm Gladwell in The Tipping Point considers.

Christakis and colleagues have also found similar network properties in this study to the spread of smoking, but the cessation of smoking was not a factor in individuals gaining weight or not.

Finally Christakis thinks that we may undervalue health interventions since we look only at the impact of the intervention on one individual rather than examining the multipliers on this investment through his/her social network.

The authors note that there are many social and health benefits of  friendship so their study is not a reason not to develop friends with anyone.

Follow-up posts on this study available here, and here.

The Christakis et al article is published today in the New England Journal of Medicine and a summary of some of the findings is available in this Harvard Gazette story.

There’s a neat visual representation of the obesity spreading through the social networks over time in this video.

See also, Clive Thompson, “Is Happiness Catching?” (NYT Sunday Magazine, 9/13/09)