Category Archives: obesity

The how of social capital

Flickr/drjausSocial capital is a powerful resource for individuals and communities.  For individuals embedded in dense social networks, these networks and the attendant norms of trust and reciprocity strongly shape individuals’ ability to land jobs, earn higher salaries, and be happier and healthier.  But, even for those not in the networks, having neighbors who know and trust one another affords benefits in some domains:  better performing local government, safer streets, faster economic growth and better performing schools, among other public goods.

For sure social capital can be used toward negative ends: Al Qaeda, the Crips and the Bloods, the Michigan Militia are all examples where group members can accomplish things that they could not accomplish individually (because of  group social capital).  That said, the literature supports that the vast majority of what social capital is used for is to produce positive ends, not negative ones.

But why?  What makes social capital so powerful?

Robert Putnam and I had always focused on information-flows as the key mechanism.  So these social networks:

  • enable individuals to access valuable information: how to get something done, hear of  job leads, learn how better to promote one’s health, find out what is happening in a community, etc.; or
  • help individuals find partners for joint economic transactions (e.g., to know with whom to partner  in business, to close a sale to a friend or a friend of a friend, to locate a neighbor with whom one can exchange tools or expertise); or
  • spread reputations of members (or neighbors or local merchants) which causes all people in these networks to behave in a more trustworthy manner and facilitates altruism.  There is always a short-term gain to be had from cheating someone, but if the social networks quickly spread the information that one cannot be trusted, this short-term gain is swamped by the lost future opportunity to do business with others; thus it becomes more rational to be honest and trustworthy in communities (physical or otherwise) with strong social networks. Individuals are also likely to be kinder and more altruistic toward others because they know that “what goes around comes around” in densely inter-connected networks and communities; and
  • facilitate collective action: it is easier to mobilize others around some shared goal like politics or zoning or improving trash pick-up if others in the  community already know and  trust you, rather than your having to build those social relationships from scratch.

But Connected (by Nick Christakis and James Fowler) raises a different frame for thinking about this issue: network effects or contagion.  Are there properties of the networks themselves that help spread practices, independent of the flow of information?  This is difficult to answer fully since much of their evidence comes from the Framingham Heart Study where  they know who people’s friends are but not what they are doing with each other or what they are saying to each other.

That said, some of their results can be explained by information flows (e.g., political influence, or getting flu shots), but some seem likely to be working through other channels and not through information-flows (e.g., happiness or loneliness cascades).

In these “network effects” or contagion, Fowler & Christakis typically find that the strongest “network” effects are directly with one’s friends (one degree of separation), but these effects also ripple out two more levels to  friends of one’s friends (two degrees) and friends of the friends of one’s friends (three degrees).  As one would expect, much like a stone dropped in a pond, the ripples get smaller as one moves out.  In fact they refer to the “Three Degrees of Influence” Rule that effects are typically only seen up to three degrees out and not further: in the spread of happiness, political views, weight gain, obesity, and smoking.  For example, in happiness, if one is happy, one degree out (controlling for other factors), one’s friends are 15% happier, at 2 degrees of separation they are 10% happier, and they are 6% happier at 3 degrees of separation.  For obesity, the average obese American is more likely to have obese friends, one, two and three degrees of separation out, but not further.  Quitting smoking has diminishing effects out to three degrees.  For political influence, they note a “get-out-the-vote” experiment that shows that knocking on a stranger’s door and urging the resident to support a recycling initiative had a 10% impact on his/her likelihood to vote for the initiative; what was noteworthy to Christakis and Fowler is that the door-knocking made the spouse (who was not at the door) 6% more likely to support the recycling initiative based on communication with his/her spouse.  They conjecture that if this 60% social pass-through rate of political appeals (6% for spouse vs. 10% for person answering door) applied to one’s friends and if everyone had 2 friends, then one person urging friends to vote a certain way would have a 10% impact on one’s friends, a 6% impact on one’s friends’ friends (2 degrees) and a 3.6% impact 3 degrees out.  Multiplying these political effects all the way through, one vote could create a 30x multiplier. [The example is eye-opening and suggests that voting and political persuasion may be less irrational than thought, but also is based on a huge number of assumptions and assumes no cross-competing messages from friends.]

In an experiment on altrusim (explained in this post) Christakis & Fowler found that $1.00 of altruism, ultimately produced $1.05 of multiplier effect ($.20 one ripple out with 3 others and $.05 of altruism two ripples out with 9 others).

Christakis and Fowler, in their book, talk about contagion effects in voting, suicide, loneliness, depression, happiness, violence, STDs, number of sexual partners, binge drinking, back pain, and getting flu shots, among others.  [One summary of many of their findings, which they note, is "You make me sick!"]

Why do these effects only reach out 3 degrees of influence?  Christakis & Fowler suggest 4 potential explanations.

1) intrinsic decay: C&F liken this to a game of telephone where as the information gets repeated, the content gets lost, or the passion and knowledge of the initiator gets dissipated.

2) Instability of ties: because of what is known as “triadic closure“, if A is friends with B and B is friends with C, it is likely that A will become friends with C.  Because of this, closer-in ties between people have more routes connecting them, and further out ties are more dependent on only one pathway connecting them.  For example, assume Abby and Fran were friends 3 degrees removed via Bert and via Charles. If any of these intervening friendships end (say Bert is no longer friends with Charles), Abby loses her tie to Fran.  Thus, these outer ties are much less stable and averaged across all the “3 degrees of influence” friendships, many more may have zero effect because the path of influence dies out as friends change.

3) cross-information:  as one gets further out away from you, say the friends of the friends of your friends, all of these folks are getting lots of cross-stimuli from lots of other sources (many of which may come from different clusters with different habits or values) and these cross-stimuli start to cancel each other out.

4) evolutionary biology: C&F note that humans evolved in small groups that had a maximum of three degrees of separation so it may be that we became more attuned to being influenced by folks who were in a position to alter our gene pool.

So what are the network influences independent of communication.  There seem like 6 possible channels, and often it is hard to separate one from the other, although some may make more sense for the spread of behaviors and others may make more sense for spread of attitudes or emotions:

1) homophily: “Homophily” is the practice of befriending others like you — “birds of a feather flock together.” Being friends with people who are different than you can be stressful.  This is why in mates and in friends we are likely to choose others with whom we have a lot in common — think of arguments you’ve had with friends about where to go for dinner or what is right or wrong with the world when those friends have very different tastes or politics.  For this reason, one reason for increased clustering over time of obese people or smokers or binge drinkers is that it is stressful to be in groups where one is the minority and either constantly noodging others to change their behavior or else your finding yourself frequently doing what your friends want to and what you do not (e.g., eat fast food, smoke, or listen to heavy metal rock music).  As a consequence, people may vote with their feet and form new ties or strengthen ties with others with whom they have more in common.

2) norms/reference groups/culture/peer pressure:   we often measure the reasonableness of our behavior against our friends.  For example, if our teen friends have all had 6 sexual partners in the last year, then repartnering seems far more normal than if one is friends with a group that is heavily monogamous.  Ditto with obesity or smoking or other possible traits or behaviors.

3) subconscious/imitation:  as suggested with “emotion” below, sometimes we mirror others’ behavior or emotions without even thinking about it.  C&F say it makes sense to think of people as subsconsciously reacting to those around them without being aware of any larger pattern.  They talk about processes by which a “wave” at a sporting event takes place, or fish swim in unison, or geese fly in a V-formation, or crickets become synchronized — all of these happen by individuals mirroring those around them.  And in the process, emergent properties of the group arise (much like a cake takes on the taste unlike any of its individual ingredients).

4) emotions: C&F note that emotions actually affect our physical being — our voices, our faces, our posture.  In experiments, people actually “catch emotions”: others become happier by spending time around happy people or sadder by hanging out with depressed individuals.  In experiments, smiling waiters get bigger tips.  It seems quite plausible that cascades like loneliness, happiness, depression, etc. could spread simply from emotional states, independent of any information flowing through these friendships.

5) social invitations for shared action: friends often invite friends to do things — that’s part of friendship. For behaviors, one of the ways they can spread through networks is that, for example, thin friends could invite friends to exercise more, or obese friends could encourage friends to get ice cream together, or smokers could encourage others to leave the dance for a cig.

Connected notes that it is often hard, for example, to tell imitation and norms apart, “When a man gives up his motorcycle after getting hitched, is he copying his wife’s behavior (she doesn’t have a motorcycle) or adopting a new norm (the infernal things are unsafe?)”

Connected also notes how behaviors or attitudes can spread several social links out, even without the intervening link changing.  They suggest that Amy could have a friend Maria who has a friend Heather.  (Amy and Heather don’t know one another.)  Heather gains weight.  Maria, who really likes Heather, becomes less judgmental of her weight and gradually less judgmental of  obesity in general.  Maria doesn’t change her behavior but when Amy stops exercising with Maria, Maria is less likely to pressure her to resume.  Thus Heather’s obesity changes Amy via Maria (by Maria no longer urging her to keep exercising), but Maria doesn’t change her behavior and Amy and Heather don’t know one another.

It’s interesting stuff to ponder and makes one think more expansively about the role and mechanisms of social capital.  It also evokes a conversation with a Saguaro Seminar participant back in 1998 concerning whether black kids and white kids doing sidewalk painting together on the steps of an art museum could promote inter-racial trust, even if the black kids and white kids didn’t know each other, didn’t talk to one another and never met again.  [My hunch is yes, depending on the strength of their pre-existing beliefs about inter-racial trust, but that talking could make the exchange far more powerful.] Another Saguaro participant wondered whether singing together in a chorus helps build social capital, even if one never has a conversation directly with another member of the chorus.  (In the latter example, in addition to being highly unlikely, you are at least getting some non-verbal information over time from the other choral members about their trustworthiness: do they come regularly and on time, do they respectfully listen to and follow the choralmeister?)

I welcome your thoughts.

For more on the network effects, read pp. 24-30, 25-43 and 112-115 in Connected.

Having few friends predicts early death as much as smoking or alcoholism

“Low social interaction as high a risk factor for early death as smoking 15 cigarettes daily or being an alcoholic, and twice the risk factor of obesity.”

Julianne Holt-Lunstad, a psychologist at BYU, published a recent meta-analysis with Timothy Smith and J. Bradley Layton (that culls from learning across 148 longitudinal health studies covering over 300,000 individuals). They showed that increased involvement in social networks on average reduces one’s chance of mortality over the period of any particular study by 50+%, a greater effect than either stopping smoking or eliminating one’s obesity/physical inactivity.

The study “Social Relationships and Mortality Risk: A Meta-analytic Review” appears in the journal PLoS Medicine.  They controlled for baseline health status,  and found consistent results for friendships with family, friends, neighbors and colleagues across age, gender, initial health status, cause of death, and follow-up period.

The life-protective benefits of friendship were strongest for complex measures of social integration and lowest for simple measures of residential status (e.g., living alone versus with others) .  In studies that had greater dimensions of social involvement (whether one was in a network, the kinds of social support one got, etc.), the life-protecting benefits of friendships were higher, likely corresponding to the multiple pathways through which friendships provide benefits.

Low social interaction, according to the authors, was as high a risk factor for early death as smoking 15 cigarettes a day or being an alcoholic.  Low social interaction was a higher risk factor than not exercising and twice as high a risk factor for early death as obesity.

Co-author Tim Smith noted: “We take relationships for granted as humans – we’re like fish that don’t notice the water….That constant interaction is not only beneficial psychologically but directly to our physical health.”

The longitudinal studies they analyzed tracked health outcomes and social interaction for a period of seven and a half years on average.

The 50% increased survival rate is quite likely an underestimate: these longitudinal studies don’t track relationship quality but only one’s inclusion in a social network, so they include negative relationships as well. Survival benefits of friendships are likely to be much higher if one could isolate only positive and healthy social relationships.

Holt-Lunstad speculated that the pathways of social relationships to improved longevity stem range from  “a calming touch to finding meaning in life.” She believes that those who are socially connected take greater responsibility for others’ and their own lives and take fewer risks.

Here is key Figure 6 from their study:

Unlike some other work, such as Eric Klinenberg’s Heat Wave, where shut-in elderly were especially at risk of death in Chicago’s 1995 heat wave, the findings of Holt-Lunstad are generalizable to all age groups.

Only the lonely die young (UPDATED 5/2013)

(picture by moann)

(picture by moann)

John Cacioppo (from Univ. of Chicago) has a new interesting trade book out (with William Patrick) called Loneliness: Human Nature and the Need for Social Connection.

His interview with the Boston Globe appeared in the Sunday Magazine over the weekend. Among his quotes/observations:

  • “Social isolation has an impact on health comparable to the effect of high blood pressure, lack of exercise, obesity, or smoking,”
  • Loneliness is not simply a matter of being alone.  The lonely often spend as much time with others as the less lonely.  The key difference is that the lonely have the feeling that any real connection with others is lacking.
  • Loneliness is half heritable and half environmental, but the heritable part seems to be associated with how much disconnection hurts.
  • “In 1984, the question was asked [in the GSS survey], ‘How many confidants do you have?’ And the most frequent answer was three. That question was repeated 20 years later, in 2004, and the most frequent response was zero.”

Cacioppo and Patrick highlight some interesting experiments among the lonely, among them:

  1. That faced with a task of trying enough cookies to rate their flavor, on average, people who have been told that co-workers didn’t like working with them ate twice as many cookies as people who had been told that co-workers loved working with them.
  2. Those who are lonely, for example in playing the Ultimatum Game, settle for far worse outcomes or  distributions than those who are not lonely (similar to people with low self-esteem choosing partners or dates who mistreat them, subconsciously justifying that they are not worthy of better treatment).
  3. The lonely sleep less well and less efficiently.
  4. The lonely can’t think as clearly.
  5. The lonely were more likely to describe a gadget anthropomorphically and the lonely were more likely to believe in the supernatural (e.g., God, angels or miracles), and believed in the supernatural more when they were feeling lonely.
  6. Lonely people had higher levels of chronic inflammation, a condition associated with heart and artery disease, arthritis, Alzheimer’s and other illnesses.

The Boston Sunday Globe interview with Cacioppo can be read here.

See article “The Science of Loneliness” (New Republic, Judith Shulevitz, May 2013)

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)

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 dropping 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)