Is Happiness Contagious?

If those riding intellectual fads are sometimes guilty of sloppy reasoning, imagine what happens when two fads collide.

That’s what happened when the British Medical Journal elected to publish a study analyzing 1) happiness in 2) social networks. The study, by James Fowler and Nicholas Christakis, concludes that happiness is contagious within social networks.

According to the authors, your happiness depends on the happiness of your friends, and their friends, and their friends. It’s a fascinating finding, and it was duly reported by hundreds of newspapers. Indeed, according to Fowler, “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.”

Unfortunately, it’s probably not true. Here’s the crux of the research: the authors show that your happiness is positively related to the happiness of your friends, and that this holds even after accounting for a number of other variables, including how happy you and your friends were a few years back. That’s correlation; what about causation?

There are (at least) three reasons why happiness is correlated within social networks. It may be that — as the authors posit — happiness is contagious. Or perhaps people with similar dispositions are more likely to be friends. Economists call this the confounder “selection effects,” while medical journals call it “homophily.” The authors partly account for this by adding statistical controls for the past happiness of both you and your friends.

The third reason is perhaps the most likely: 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.

Interestingly, the same issue of the BMJ contained a very careful article by Ethan Cohen-Cole and Jason Fletcher making precisely this point. They employ a pretty cheeky research strategy: if you want to show that a research design is silly, show that it leads to silly conclusions.

They use Fowler and Christakis’s approach on another dataset, and show that it leads to the unlikely conclusion that height, headaches, and acne are also contagious. The more likely explanation, of course, is that all are subject to similar environmental influences. For instance, the same jackhammer causing your headache is likely causing mine.

I bet that a similar analysis would show that stories about happiness being contagious are, well, contagious. After all, what else explains last week’s epidemic, with stories in The New York Times; The Boston Globe, and The Washington Post? Of course, it may just be that this “epidemic” reflects a shared environmental influence, like each newspaper receiving the same press release.

So we have two studies drawing two conclusions. The first finds that happiness is contagious; the second finds that researchers can too easily draw false conclusions about contagion. Guess which one grabbed the attention of headline writers.