Nicholas Christakis, a physician and social scientist at Harvard Medical School and James Fowler, an associate professor of political science at the University of California, San Diego, examined the way happiness spreads through a social network and learned that it travels not only from person to person, but also to people up to three degrees removed -- friends of friends of friends, in other words.

The scientists took data from the well-known Framingham Heart Study, and were able to recreate a social network of almost 5,000 people who had answered questions about their subjective feelings of happiness over a 20-year period -- whether they felt hopeful about the future, for example.

Not surprisingly, people closest to each other had the greatest impact on happiness levels. But people who were so far removed they might never even have met also had an observable effect.

The more happy connections a person had, the happier the individual was likely to be, Fowler and Christakis observed.

The takeaway from this work? We're not totally in control of our own happiness. It doesn't depend solely on our own choices, actions and experience. Emotions have what Christakis calls "a collective existence."

Unhappiness is also "contagious," the researchers noted, but it appears to have a far weaker effect on members of a social network.

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