“Is Happiness Catching?” is the feature article in this week’s New York Times Magazine. Clive Thompson writes about the Framingham Heart Study, which has followed 15,000 people starting back in 1948. Originally framed as a study of physical disease, the data are now being turned to social ends.

Nicholas Christakis, a medical sociologist and doctor at Harvard, and James Fowler, a political scientist at UC San Diego, have taken this data set to examine a question that dates back to Durkheim and his ideas about collective effervescence, anomie, and suicide – how do our social relationships affect what we experience and do? As Thompson frames it:

By analyzing the Framingham data, Christakis and Fowler say, they have for the first time found some solid basis for a potentially powerful theory in epidemiology: that good behaviors — like quitting smoking or staying slender or being happy — pass from friend to friend almost as if they were contagious viruses. The Framingham participants, the data suggested, influenced one another’s health just by socializing. And the same was true of bad behaviors — clusters of friends appeared to “infect” each other with obesity, unhappiness and smoking. Staying healthy isn’t just a matter of your genes and your diet, it seems. Good health is also a product, in part, of your sheer proximity to other healthy people.

Their research shows that common explanations for problem behavior, such as individual being at fault or peer pressure, are inadequate. What we experience and how we act spreads further than we think. Take a major illness affecting a mother late in life, and the strain and stress her daughter experiences caring for her mother. That strain can affect the daughter’s husband, who in turn shapes his friend’s life.

Christakis saw this through his clinical experience, and with Fowler, decided to study the impact of social networks. One of their main findings is that in the Framinghamn study, “drinking spread socially, as did happiness and even loneliness. And in each case one’s individual influence stretched out three degrees before it faded out. They termed this the ‘three degrees of influence’ rule about human behavior: We are tied not just to those around us, but to others in a web that stretches farther than we know.”

When a Framingham resident became obese, his or her friends were 57 percent more likely to become obese, too. Even more astonishing to Christakis and Fowler was the fact that the effect didn’t stop there. In fact, it appeared to skip links. A Framingham resident was roughly 20 percent more likely to become obese if the friend of a friend became obese — even if the connecting friend didn’t put on a single pound. Indeed, a person’s risk of obesity went up about 10 percent even if a friend of a friend of a friend gained weight.

Christakis and Fowler’s work provides an in-depth description of the functioning of social networks – not a examination of why loneliness spreads so much as an examination of how it does. In other words, there is not a theory of social contagion of behaviors, but an examination of the role of social networks in loneliness. As they write in an in-press paper co-authored with John Cacioppo:

Results indicated that loneliness occurs in clusters within social networks, extends up to three degrees of separation, and is disproportionately represented at the periphery of social networks. In addition, loneliness appears to spread through a contagious process even though lonely individuals are moved closer to the edge of social networks over time. The spread of loneliness was found to be stronger than the spread of perceived social connections, stronger for friends than family members, and stronger for women than for men.

For why social contagion works, for this “more than peer pressure” effect (that third degree of influence – a friend of a friend of yours), Christakis and Fowler throw out a range of neuropsychological ideas such as “subconscious social signals that we pick up from those around us, which serve as cues to what is considered normal behavior” and “the spread of good or bad feelings, they say, might be driven partly by “mirror neurons” in the brain that automatically mimic what we see in the faces of those around us — which is why looking at photographs of smiling people can itself often lift your mood.”

But there are also intriguing signs of everyday dynamics that help or hinder the spread of behaviors (in other words, don’t blame it all on the brain):

The Framingham findings also suggest that different contagious behaviors spread in different ways. For example, co-workers did not seem to transmit happiness to one another, while personal friends did. But co-workers did transmit smoking habits; if a person at a small firm stopped smoking, his or her colleagues had a 34 percent better chance of quitting themselves. The difference is based in the nature of workplace relationships, Fowler contends. Smokers at work tend to cluster together outside the building; if one of them stops smoking, it reduces the conviviality of the experience. (If you’re the last smoker outside on a freezing afternoon, your behavior can seem completely ridiculous even to yourself.) But when it comes to happiness, Fowler said, “people are both cooperative and competitive at work. So when one person gets a raise, it might make him happy, but it’ll make other people jealous.”

Other explanations include homophily, or like people gravitating towards each other, and shared environment, for example, a McDonalds opening which people then visit. Jason Fletcher, for example, has shown that using the social contagion methodological approach, one can show that very implausible things appear contagious according to the Christakis/Fowler analysis – such as tallness (hmm, maybe they play basketball together?).

A more significant limitation is that the Framingham data is good on family but not so much on friends – respondents were required to only list one significant friend over time. Thus, what might appear as contagion could be due to missing data.

With drug use, it is clear that when adolescents take drugs, two effects happen: they start to proactively seek out friends who also use drugs (largely because they approve of their behavior) and those friends can reinforce the initial behavior itself (often leading to a rise in substance use). In other words, our social networks have reciprocal effects, us on out, and others on in.

Christakis and Folwer demonstrate that the directionality of these reciprocal effects matter. If I consider you a good friend, but you don’t consider me a good friend, your behavior is more likely to influence me than vice versa. “According to their data, if Steven becomes obese, it has no effect on Peter at all, because he doesn’t think of Steven as a close friend. In contrast, if Peter gains weight, then Steven’s risk of obesity rises by almost 100 percent. And if the two men regard each other as mutual friends, the effect is huge — either one gaining weight almost triples the other’s risk.”

The article then goes into some very interesting public health considerations, with a focus on smoking and obesity. But I’ll leave you to read those yourself, and end with a more basic piece of advice. The Christakis/Fowler research suggests an addition to an old motto. Don’t worry, be happy… by getting connected.

Their findings show that the gamble of increased sociability pays off, for a surprising reason: Happiness is more contagious than unhappiness. According to their statistical analysis, each additional happy friend boosts your good cheer by 9 percent, while each additional unhappy friend drags you down by only 7 percent.

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Christakis and Fowler have a book coming out later this month, Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives. It comes complete with a fancy website, Connected the Book, complete with a 3 minute view, discussion boards, reviews, and more.

Here’s their 2007 New England Journal of Medicine article, The Spread of Obesity in a Large Social Network over 32 Years.

You can also read an in-press article written with John Cacioppo, entitled Alone in the Crowd: The Structure and Spread of Loneliness in a Large Social Network (pdf), if you want to get more at the science side.

And here’s the NY Times Clive Thompson article, Is Happiness Catching?