Every year TIME magazine releases the TIME 100, its list of the World's Most Influential People. This year, one of the honourees was Nicholas Christakis, a physician and sociologist at Harvard University who has done extensive research into how to become happy. The crux of his decades of exhaustive study: if you want to be happy, you should surround yourself with happy people.

Dan Ariely, professor of behavioral science at Duke University and the author of the best seller Predictably Irrational, profiled Christakis for the magazine. Ariely points out that social scientists have often depended on a certain kind of relativity as an explanation for happiness: if you surround yourself with people who are less attractive, who have less money, who are unhappily married and who are more miserable, then the contrast will make you appreciate what you have and subsequently make you happier.

But Christakis challenges this idea, suggesting that happiness is more contagious than previously thought. Exploring data that tracked 5,000 for 20 years, Christakis found that when people close to us, both emotionally and physically, become happier, we do, too. When a person living within a mile of a good friend becomes happier, for example, there is a 15 percent probability that the good friend will also become happier.

Writing about social networks and happiness on edge.org, Christakis says that happiness can spread from person to person - just as being around someone who makes negative or depressing statements is more likely to make you unhappier, being around people who smile and offer positive sentiments is likely to make you happier.

"We found that happy people tend to be located in the center of their social networks and to be located in large clusters of other happy people," he writes. "And we found that each additional happy friend increases a person's probability of being happy by about 9%. For comparison, having an extra $5,000 in income (in 1984 dollars) increased the probability of being happy by about 2%. Happiness, in short, is not merely a function of personal experience, but also is a property of groups. Emotions are a collective phenomenon."