New research from Harvard Medical School and the University of California, San Diego suggests that happiness is influenced not only by the people you know, but by the people they know.
The study showed that happiness spreads through social networks, sort of like a virus, meaning that your happiness could influence the happiness of someone you've never even met.
Sadness spreads too, but much less efficiently, says study co-author James H. Fowler, PhD, of the University of California-San Diego.
"We have known for a long time that there is a direct relationship between one person's happiness and another's," Fowler tells WebMD.
"But this study shows that indirect relationships also affect happiness. We found a statistical relationship not just between your happiness and your friends' happiness, but between your happiness and your friends' friends' friends' happiness."
Last year the pair made headlines when they reported that obesity seems to spread through social groups, so that your chances of becoming overweight are greater when your friends and their friends gain weight.
A related study, published earlier this year, found that smokers were more likely to give up cigarettes when their family, friends, and other social contacts stopped smoking.
The researchers were able to recreate the social networks of 4,739 Framingham participants whose happiness was measured from 1983 to 2003. A standard test for assessing happiness was used, which included questions like "I felt hopeful about the future," and "I was happy."
Important family changes for each participant -- such as a birth, death, marriage, or divorce -- were also recorded. The participants were also asked to name family members, close friends, co-workers, and neighbors.
Because many of these contacts were also study participants, the researchers were able to identify more than 50,000 social and family ties and analyze the spread of happiness through the group.
Happy Friends Make You Happy
They concluded that the happiness of an immediate social contact increased an individual's chances of becoming happy by 15%, Fowler says.
The happiness of a second-degree contact, such as the spouse of a friend, increases the likeliness of becoming happy by 10%, and the happiness of a third-degree contact -- or the friend of a friend of a friend -- increases the likelihood of becoming happy by 6%.
The association was not seen in fourth-degree contacts (the friends of friends of friends of friends).
Having more friends also increased happiness, but having friends who were happy was a much bigger influence on happiness.
Fowler says the findings do not mean you should avoid unhappy people, but that you should make an effort whenever you can to spread happiness.
"We need to think of happiness as a collective phenomenon," he says. "If I come home in a bad mood, I may be missing an opportunity to make not just my wife and son happy, but their friends."
Richard Suzman, PhD, who directs the behavioral and social research division of the National Institutes of Health, which funded the study, calls the research "pioneering."
"These findings are very strong," Suzman tells WebMD. "From a public policy perspective, this research means we need to consider the societal impact on happiness, or obesity, or smoking. We are only just beginning to understand how social networks influence these things for good or for bad."