The 2009 TIME 100

Social scientists used to have a straightforward, if tongue-in-cheek, answer to the question of how to become happy: Surround yourself with people who are uglier, poorer and shorter than you are — and who are unhappily married and have annoying kids. You will compare yourself with these people, and the contrast will cheer you up.

Nicholas Christakis, 47, a physician and sociologist at Harvard University, challenges this idea. Using data from a study that tracked about 5,000 people over 20 years, he suggests that happiness, like the flu, can spread from person to person. When people who are close to us, both in terms of social ties (friends or relatives) and physical proximity, become happier, we do too. For example, when a person who lives within a mile of a good friend becomes happier, the probability that this person's good friend will also become happier increases 15%. More surprising is that the effect can transcend direct links and reach a third degree of separation: when a friend of a friend becomes happier, we become happier, even when we don't know that third person directly.

This means that surrounding ourselves with happier people will make us happier, make the people close to us happier — and make the people close to them happier. But social networks don't transmit only the good things in life.

Christakis found that smoking and obesity can be socially infectious too. If his thesis proves out, then the saying that you can judge a person by his or her friends might carry more weight than we thought.

Ariely is the James B. Duke professor of behavioral science at Duke University and the author of the best seller Predictably Irrational

Fast Fact: Smokers have been pushed from the center of their social webs, Christakis found

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