THURSDAY, Dec. 4 (HealthDay News) -- Happy friends can make you happy -- and so can happy friends of your friends.

That's the unusual conclusion of a new study that suggests you and people you've never met can have an impact on each other's feelings.

"Our own personal happiness spreads beyond people we're directly connected to," said study co-author James Fowler, an associate professor of political science at the University of California, San Diego.

Researchers already know that a person's attitudes affect those of other people, Fowler said. "If I smile, it increases the chances that you'll smile. We know that waiters and waitresses who smile get better tips," he added.

But what about other people further down the line? As Fowler put it, "Is there a person-to-person effect that can spread to the whole social network?"

The new study, one in a series examining the potential contagiousness of things like loneliness and smoking, aimed to answer that question.

Fowler and Harvard Medical School professor Nicholas Christakis looked at findings from the Framingham Heart Study, which began following 5,209 people in Framingham, Mass., in 1948. The study continues to this day and now includes descendents of those in the original group.

The researchers focused on 4,739 children of the original participants and tracked their friendship ties with other people for 20 years, from 1983 to 2003.

The scientists found that a person's happiness is most likely to boost the happiness levels in people closest to him -- spouses, relatives, neighbors, and friends.

But, if one person is happy, that increases the chances of happiness in a friend living within a mile by 25 percent. The "cascade" effect, as the researchers put it, continues: a friend of the friend has almost a 10 percent higher likelihood of being happy, and a friend of that friend has a 5.6 percent increased chance.

In the other words, one person's happiness can spread outward through three degrees of separation. Those at the center of such circles may be people that "you have never met. But their mood can have a profound effect on your own mood," Fowler said.

The researchers tried to adjust their statistics to account for factors that could possibly affect the trends, such as people being more likely to choose friends who are like them.

The findings were published online Dec. 5 in the British Medical Journal.

Fowler said the study has "totally changed the way I see the world."

"To think about the way we're connected to one another has caused me to take more responsibility for my own actions," he said. "If I head home in a happy mood, I'm not just making my son happy, I'm potentially making my son's friend happy. I'm not just making my wife happy, I'm making my wife's mother happy."

Rosemary Blieszner, a professor of human development at Virginia Tech, called the study findings impressive, because they relied on a large sample of participants who were followed for a long period of time.

Overall, the study reveals the importance of the company people keep, Blieszner said.

"People who are surrounded by many happy people are more likely to be happy in the future than those who are surrounded by unhappy people," she said.

More information

To learn more about happiness, check with the Pew Research Center.