Is happiness contagious?

Since World War II, the residents of Framingham, Mass., and their descendants have been tracked as part of a project on cardiovascular disease. For their paper, Nicholas Christakis and James H. Fowler followed almost 5,000 subjects of the Framingham study for 20 years. Participants were asked to provide a list of their relatives, close friends, neighbors and co-workers, which gave Christakis and Fowler about 50,000 social ties to work with. The subjects were asked to fill out a form on how often they experienced certain feelings, such as happiness or hope for the future.

Once the researchers established an emotional baseline for participants, they were able to measure emotional fluctuations. Then they compared emotional fluctuations within each participant's social network, finding that our emotional state can be affected by that of people around us. Though sadness is somewhat spreadable, the transmission of happiness is far more potent.

Christakis and Fowler found that people we don't even know have an impact on our emotional state. If a friend of a friend is happy, then our chances of happiness leap 10 percent, while an extra $5,000 is only good for a possible 2 percent increase in happiness [source: Park]. The influence seems to stop after three degrees, however, and you're far more likely to experience a bump in happiness when a dear friend in close proximity becomes happy.

A good friend that lives half a mile away had the potential to boost happiness by 42 percent, but if the friend lived two miles away, the effect was halved [source: Belluck]. A cheerful next-door neighbor, however, might provide a 34 percent increase in happiness [source: Callaway]. Clearly, distance is a key factor, but gender may be as well. A joyful spouse only increased happiness by about 8 percent, seeming to indicate that we take our emotional cues from people of the same gender. The happiness doesn't stick around forever, though; it seems to have a shelf life of one year [source: Harvard Medical School]. The one place where happiness doesn't seem to spread, however, is in the workplace, perhaps because one person's promotion means that another person was passed over.

Not everyone buys into these results, however. On the same day that Christakis and Fowler's work was made available, Ethan Cohen-Cole and Jason Fletcher published a paper that used the same data set to demonstrate that height, headaches and acne were also contagious. This second paper warns against confusing correlation with causation. Still, it's worth considering the people you allow in your social network -- they may influence you in ways you're not even aware of.

As the people of Framingham continue to fight outbreaks of happiness and headaches, you can read more about happiness on the next page.