Happiness ripples well beyond a person's inner circle of friends and family, lifting the mood of an extended network of social contacts who might even be strangers, according to a provocative study published today.
Psychologists have long known that feelings can be contagious over short time frames or that people reflexively return smiles. But the new social network analysis showed that that contagious effect extends three "degrees" - as far as a friend of a friend of a friend - and drops off with time and distance.
The effects can last a year, researchers said in British Medical Journal.
"Your happiness is not just about your own choices and actions and behaviors and thoughts," said Dr. Nicholas Christakis, a coauthor of the study and a professor of medical sociology at Harvard Medical School. "It's like there are emotional stampedes that ripple across this infinite fabric of humanity."
Scientists have looked at factors such as lottery windfalls, genes, exercise, church attendance, political elections, and divorce in the quest to understand happiness. But this new study merely looks around - at spouses, coworkers, siblings, neighbors, and friends. Using comprehensive data collected on 4,739 people over two decades, the researchers studied individuals' levels of happiness and their social relationships.
A happy friend who lives within a mile, for example, boosts your odds of being happy by 25 percent, researchers found. A happy sibling within the same distance increases your probability of happiness by 14 percent.
It seems obvious that your closest friends might influence your mood, but the study found that even the happiness of a friend's friend boosts your chance of being happy by 9.8 percent. Even more surprising, the happiness of a friend of a friend of a friend boosts your chance of being happy by 5.6 percent.
Unhappiness, on the other hand, did not spread as much.
"This is a stunning paper," Harvard University psychologist Daniel Gilbert wrote in an e-mail. "It is sometimes said that you can't be happier than your least happy child. It is truly amazing to discover that when you replace the word 'child' with 'best friend's neighbor's uncle,' the sentence is still true."
Martin Seligman, director of the Positive Psychology Center at the University of Pennsylvania, called the study "path-breaking." After reading it, he said, he decided to switch his lecture topic for the week.
"There's been quite a lot of research on individual happiness and its determinants, but there's really been nothing rigorous until this came along about the contagion of happiness," Seligman said. "People have asked in evolutionary psychology about what does positive emotion do, and one very important notion is that it tunes the group . . . if you want to hunt mastodons or gather rice, it's good to be tuned."
The happiness researchers used data from the Framingham Heart Study, a long-running program initiated in 1948 to examine factors that contribute to heart disease among residents of the Boston suburb and their offspring. The happiness researchers looked at responses to a depression test that was administered several times between 1983 and 2001, in which people were asked how often in the previous week they had experienced positive feelings such as "I felt hopeful about the future," "I was happy," "I felt that I was just as good as other people," and "I enjoyed life."
Those data were overlaid with information about the subjects' family members, close friends, jobs, and home addresses, and then used to reconstruct and analyze their social networks.
Some findings seem made for a sitcom about family life. While a cheerful next-door neighbor increases one's likelihood of happiness by 34 percent, a happy spouse who lives in the same house contributes just 8 percent. The authors said happiness seemed to spread more through same-sex relationships, and suggested that might help explain the neighbor effect.
James Fowler, associate professor of political science at the University of California, San Diego, and coauthor of the study, said he was already using the findings in his daily life, making a conscious effort to be cheerful when he came home at night, because he knows it could cheer his wife and his wife's mother or his son and his son's friends.
"These small things I didn't think to do for myself suddenly take on great significance - I feel empowered to have an impact," Fowler said.
Previous studies by the authors using Framingham Heart Study data found the network effect also manifested itself on obesity and smoking rates.
This form of network analysis is not without controversy, and another study published today in the same journal presents a cautionary tale. The study argues that using social network analysis without controlling for relevant environmental factors - such as the presence of a fast-food restaurant in a neighborhood study of obesity rates - can produce the illusion of a network effect where there is none.
"We're very interested in this area, and we do think peer effects or social network effects do exist," said Jason Fletcher, coauthor of the paper and assistant professor of public health at Yale University. "We also think it's just difficult to show it's true."
The next step for the happiness research will be to design experiments that test the effect and try to understand how the emotion moves through groups. Researchers speculate that a similar pattern might hold true for drinking habits, loneliness, depression, and exercise.
"We think that while there might be six degrees of separation between any two people," Christakis said, when it comes to the impact on your social network, there are "three degrees of influence."
Carolyn Y. Johnson can be reached at email@example.com.