Every time Dr. James H. Fowler and his colleague Dr. Nicholas A. Christakis publish a research paper, they trigger a media blitz. They grabbed headlines recently with news that obesity can spread through a network of friends almost like the common cold. If your friend—or a friend of that friend—puts on weight, you’re more likely to get chubby. If your friends and their friends are thin, odds are you will be, too. Their latest study, published in the British Medical Journal this month, shows that happiness spreads through extended social networks, as well.

First, what exactly is social network research?

For years scientists have studied how individuals make decisions. Social network research tries to elevate that up to look at the complex ways in which people interact. We’re not Robinson Crusoes, after all, living on desert islands. We’re part of complex societies, and those societies help define who we are. We’re looking at how behaviors and now emotions move through social networks.

How do you put something as complex as a social network under a microscope?

We were lucky to discover that other researchers had done some of the work for us. Since the late 1940s, scientists have been studying heart disease across generations as part of the Framingham Heart Study. As part of that study, to ensure that they’d be able to keep in touch with people, they asked participants to identify a close friend who would know how to contact them in two to four years. (It worked, by the way. Only ten people have been lost to follow up in that study, which includes more than 5,000 volunteers.) Because the study drew on people in a fairly limited geographic area, many of the friends were also participants in the Framingham study, so we had detailed information about friends of friends. The Framingham study gathered up all this great information about the social network of participants. It was the perfect study for us to do our work.

Why did you decide to look at happiness?

We’ve previously studied obesity and smoking. Those are both behaviors, of course. Both turn out to spread through social networks. So we were interested in looking to see if emotions also spread through networks. If your friends and even friends of friends are happy, is that likely to make you happy?

How did you define happiness?

We used conventional measures developed by psychologists, from a survey questionnaire that the Framingham study researchers used. Volunteers were asked how often they experience certain feelings during the previous week. Four of the items addressed happiness. “I felt hopeful about the future.” “I was happy.” “I enjoyed life.” “I felt that I was just as good as other people.” It turns out that the core disposition of happiness is quite stable over time.

And what did you discover about happiness?

We found distinct clusters of happy and unhappy people in the social network. Part of that could be explained by happy people choosing happy people as their friends. But we also found that people who are surrounded by many happy people are more likely to become happy in the future. When someone in the network became happy, friends and friends of friends were likely to become happy. It’s kind of a domino effect. Geography made a difference. People had to live within about a mile of one another to exert an influence. We think that may suggest that frequency of contact is important. It’s a lot like catching the flu. The closer your contact, the more likely you are to catch it. The same with happiness.

How far does the happy bug spread?

Surprisingly far, actually. A person is 15 percent more likely to be happy if a close contact is happy. For a friend of a friend, there’s a 9.8 percent likelihood, and for a friend of a friend of a friend the likelihood is still 5.6 percent. So we traced the effect out to three degrees of separation.

So even strangers can affect how happy people feel?

That’s right. The fascinating thing is that we found virtually the same thing for obesity and smoking. The effect goes to three degrees of separation.  In fact we’re beginning to formulate the three degrees rule.

Why three?

Our theory is that it’s a little like dropping a stone into a still pond. If you drop one stone, you’ll see waves going out across the entire pond. But if you drop several stones at the same time, the competing waves will cancel themselves out at a certain point before they reach the shore. Within an individual’s social network, there are happy and unhappy people, like those several stones. So the effect cancels itself out at three degrees of separation.

What about unhappiness? Does it also spread?

We looked at that, too. In fact, I had a bet with my co-author, Nicholas Christakis, at Harvard Medical School. He was convinced that unhappiness would spread more efficiently. His idea was that unhappiness is like fear, and for evolutionary reasons it’s a matter of survival that if someone is afraid, people around them become afraid. I put my money on happiness. I won that bet. Unhappiness does spread, our results showed, but happiness spreads more effectively. I think the reason it that happiness and positive emotions like trust are required for human beings to cooperate—to hunt large game when we were hunter-gatherers, for instance, and to build complex cities today.

What do you plan to study next?

Actually, the paper on happiness is the third of six papers we plan to publish. So we’ve looked at obesity and smoking, and now happiness. We have papers coming out in the next few months on loneliness and depression and how they spread through social networks. We’re also looking at the dynamic spread of alcoholism through social networks.

In a way, your latest findings are very reassuring, aren’t they?

I think so. They remind us that we’re all connected, in ways more complex than we sometimes think. The findings have certainly changed the way I live my life. Now I understand that my mood, my emotional state, can affect people I’ve never even met. Not just my son but my son’s friend and his friend. Not just my wife but her mother and her mother’s friends. Every day now on the way home I listen to one of my favorite songs to put myself in a good mood. I want my happiness to have a ripple effect.

Photo: Kent Horner