Advice for anyone who wants to be happier: pick the right friends.

For the increasing number of Americans who view happiness as a goal in and of itself rather than (sorry to be so old-fashioned) the result of, oh, leading a rewarding life or helping others or achieving something—a trend I bemoaned recently—the latest study provides a simple recipe. Happiness, conclude political scientist James Fowler of UC San Diego and sociologist Nicholas Christakis of Harvard Medical School, spreads far and wide through social networks. Not only can one person’s happiness be infectious for those in her immediate circle, but happiness can spread to friends of friends of friends (that is, three degrees of separation). Therefore, pick happy people to be your friends.

The same team reported last year that obesity, too, can spread through social contagion. As Fowler told Newsweek then, obesity is “spreading through ideas about what appropriate behaviors are, or what an appropriate body image might be.” Or as Christakis said, “If I see you gaining weight, and I respect you, and want to emulate you in other ways, that changes my ideas about what is an acceptable body size. I think, ‘All my buddies are getting obese, so it’s OK for me to be obese too’.”

In the case of happiness, the scientists are reporting in a paper published online in BMJ (formerly the British Medical Journal) this evening, the basic idea is that “one of the key determinants of human happiness is the happiness of others,” said Christakis. Just as with their obesity-is-contagious study, the scientists used data from the Framingham Heart Study to map out the social networks of 4,739 people whose happiness they measured from 1983 to 2003 by asking how strongly four statements described them: “I felt hopeful about the future”; “I was happy”; “I enjoyed life”; and “I felt that I was just as good as other people.”

On average, they find, for every one happy friend in your social network, your own chance of being happy rises by 9 percent. Every unhappy friend decreases your chance of being happy by 7 percent. Not surprisingly, the fewer degrees of separation between you and a happy person the stronger their influence on your own mood. Being friends with a happy person makes you 15 percent more likely to be happy; having a friend who is a friend of a happy person makes you 10 percent more likely to be happy, and having a friend whose friend’s friend is happy makes that 6 percent.

I can see it now: Americans from coast to coast dumping their depressed, dour, unhappy friends, shunning them like lepers. As if the unhappy didn’t have enough to make them miserable.

The key question, of course, is whether the correlation the researchers are reporting is causal. In other words, let’s accept that your chance of being happy is a function of the number of happy people among your friends and friends’ friends. But are those cheery pals causing your happiness?

There is one head-scratching finding in the data. If one person becomes happy (or happier), a friend living within a mile has a 25 percent greater-than-otherwise chance of becoming happy. But if your spouse become happy, you have only an 8 percent increased chance at moving up the happiness meter. If happiness is contagious, shouldn’t spouses make more of a difference?

Alternative hypothesis time. Happy people, being superficial and self-absorbed and delusional, can’t stand being around unhappy people, and so won’t accept any as friends. Therefore the correlation between the number of happy people you’re connected to and your own happiness is just coincidental, not causal.

Christakiss argues instead that “the spread of emotion has a fundamental psychobiological aspect.” “Physical personal interaction is necessary, so the effect decays with distance”—which is why a friend who lives within a mile of you and who becomes happy (or happier) increases the chance that you, too, will feel happy, but a friend who lives farther away has almost no effect.

All you unhappy people out there can now obsess on yet another reason for feeling miserable: you’re not doing your part to increase your social network’s level of happiness. At least sadness does not spread through social networks they way happiness does, the researchers conclude—but while you may not be infecting people with your glumness you are still failing in your responsibility to increase humanity’s sum total of joy. For isn't that our paramount goal these days?