Recently I wrote a short summary of Christakis's research for Time magazine 100 people of 2009. Here is my summary of the research:

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.