By PAM BELLUCK for the NYTimes published December 5, 2008

How happy you are may depend on how happy your friends’ friends’ friends are, even if you don’t know them at all.

And a cheery next-door neighbor has more effect on your happiness than your spouse’s mood.

So says a new study that followed a large group of people for 20 years — happiness is more contagious than previously thought.

“Your happiness depends not just on your choices and actions, but also on the choices and actions of people you don’t even know who are one, two and three degrees removed from you,” said Dr. Nicholas A. Christakis, a physician and social scientist at Harvard Medical School and an author of the study, to be published Friday in BMJ, a British journal. “There’s kind of an emotional quiet riot that occurs and takes on a life of its own, that people themselves may be unaware of. Emotions have a collective existence — they are not just an individual phenomenon.”

In fact, said his co-author, James H. Fowler, an associate professor of political science at University of California, San Diego, their research found that “if your friend’s friend’s friend becomes happy, that has a bigger impact on you being happy than putting an extra $5,000 in your pocket.”

The researchers analyzed information on the happiness of 4,739 people and their connections with several thousand others — spouses, relatives, close friends, neighbors and co-workers — from 1983 to 2003.

“It’s extremely important and interesting work,” said Daniel Kahneman, an emeritus psychologist and Nobel laureate at Princeton, who was not involved in the study. Several social scientists and economists praised the data and analysis, but raised possible limitations.

Steven Durlauf, an economist at the University of Wisconsin, Madison, questioned whether the study proved that people became happy because of their social contacts or some unrelated reason.

Dr. Kahneman said unless the findings were replicated, he could not accept that a spouse’s happiness had less impact than a next-door neighbor. Dr. Christakis believes that indicates that people take emotional cues from their own gender.

A study also to be published Friday in BMJ, by Ethan Cohen-Cole, an economist at the Federal Reserve Bank of Boston, and Jason M. Fletcher, an assistant professor at the Yale School of Public Health, criticizes the methodology of the Christakis-Fowler team, saying that it is possible to find what look like social contagion effects with conditions like acne, headaches and height, but that contagion effects go away when researchers factor in environmental factors that friends or neighbors have in common.

“Researchers should be cautious in attributing correlations in health outcomes of close friends to social network effects,” the authors say.

An accompanying BMJ editorial about the two studies called the Christakis-Fowler study “groundbreaking,” but said “future work is needed to verify the presence and strength of these associations.”

The team previously published studies concluding that obesity and quitting smoking are socially contagious.

But the happiness study, financed by the National Institute on Aging, is unusual in several ways. Happiness would seem to be “the epitome of an individualistic state,” said John T. Cacioppo, director of the University of Chicago’s Center for Cognitive and Social Neuroscience, who was not involved in the study.

And what about schadenfreude – pleasure in someone’s misery – or good old-fashioned envy when a friend lands a promotion or wins the marathon? “There may be some people who become unhappy when their friends become happy, but we found that more people become happy over all,” Dr. Christakis said.

Professor Cacioppo said that suggested that unconscious signals of well-being packed more zing than conscious feelings of resentment. “I might be jealous of the fact that they won the lottery, but they’re in such a good mood that I walk away feeling happier without even being aware that they were the site for my happiness,” he said.

The subtle transmission of emotion may explain other findings, too. In the obesity and smoking cessation studies, friends were influential even if they lived far away. But the effect on happiness was much greater from friends, siblings or neighbors who lived nearby.

A next-door neighbor’s joy increased one’s chance of being happy by 34 percent, but a neighbor down the block had no effect. A friend living half a mile away was good for a 42 percent bounce, but the effect was almost half that for a friend two miles away. A friend in a different community altogether can win an Oscar without making you feel better.

“You have to see them and be in physical and temporal proximity,” Dr. Christakis said.

Body language and emotional signals must matter, said Professor Fowler, adding, “Everybody thought when they came out with videoconferencing that people would stop flying across the country to have meetings, but that didn’t happen. Part of developing trust with another person is being able to take their hand in yours.”

Still, they said, it is not clear if increased communication via e-mail messages and Webcams may eventually lessen the distance effect. In a separate study of 1,700 Facebook profiles, they found that people smiling in their photographs had more Facebook friends and that more of those friends were smiling. “That shows that some of our findings are generalizable to the online world,” Dr. Christakis said.

The BMJ study used data from the federal Framingham Heart Study, which began following people in Framingham, Mass., after World War II and ultimately followed their children and grandchildren. Beginning in 1983, participants periodically completed questionnaires on their emotional well-being.

They also listed family members, close friends and workplaces, so researchers could track them over time. Many of those associates were Framingham participants who also completed questionnaires, giving Dr. Christakis and Professor Fowler about 50,000 social ties to analyze. They found that when people changed from unhappy to happy in self-reported responses on a widely used measure of well-being, other people in their social network became happy too.

Sadness was transmitted the same way, but not as reliably as happiness. Professor Cacioppo believes that reflects an evolutionary tendency to “select into circumstances that allow us to stay in a good mood.”

Still, happiness has a shelf life, the researchers found.

“Your happiness affects my happiness only if you’ve become happy in the last year — it’s almost like what have you done for me lately,” Dr. Christakis said. Plus, the bounce you get lasts a year tops. Better if your friends can spread out their happy news, and not, say, all get married the same year.

Another surprising finding was that a joyful coworker did not lift the spirits of colleagues, unless they were friends. Professor Fowler believes inherent competition at work might cancel out a happy colleague’s positive vibes.

The researchers cautioned that social contacts were less important to happiness than someone’s personal circumstances. But the effect of social contacts even three degrees removed — friends of friends of friends — was clear, and also occurred with obesity and quitting smoking. More distant contacts exerted no influence.

And people in the center of social networks were happier than those on the fringes. Being popular was good, especially if friends were popular too.

So should you dump melancholy friends? The authors say no. Better to spread happiness by improving life for people you know.

“This now makes me feel so much more responsible that I know that if I come home in a bad mood I’m not only affecting my wife and son but my son’s best friend or my wife’s mother,” Professor Fowler said. When heading home, “I now intentionally put on my favorite song.”

Still, he said, “We are not giving you the advice to start smiling at everyone you meet in New York. That would be dangerous.”