James Fowler of University of California, San Diego, and Nicholas Christakis of Harvard Medical School said happiness spreads far and wide through a social network -- traveling not just the well-known path from one person to another but even to people up to three degrees removed.

Christakis and Fowler used data from the Framingham Heart Study to recreate a social network of 4,739 people whose happiness was measured from 1983 to 2003. To assess the participants' emotional well being, they relied on answers to four items: "I felt hopeful about the future"; "I was happy"; "I enjoyed life"; and "I felt that I was just as good as other people."

The study, published in the British Medical Journal, showed that happiness loves company, that happy people tend to cluster together, and, on the surface, people with more social contacts seem generally happier.

Fowler and Christakis observed, on average, every happy friend increases one's chance of being happy by 9 percent. Each unhappy friend decreases it by 7 percent.

"Consider $5,000 extra dollars, in 1984, was associated with just a 2 percent increase in happiness," Fowler said in a statement. "But, a friend of a friend of a friend -- can have a greater influence than hundreds of bills in your pocket."