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NEWS ALERT
Harvard Medical School Office of Public Affairs

Happiness is a collective - not just individual - phenomenon

FINDINGS:
Happiness spreads through social networks like an emotional contagion, according to a study that looked at nearly 5,000 individuals over a period of 20 years. When an individual becomes happy, the network effect can be measured up to three degrees. One person’s happiness triggers a chain reaction that benefits not only his friends, but his friends’ friends, and his friends’ friends’ friends. The effect lasts for up to one year. Conversely, sadness does not spread through social networks as robustly as happiness.

RELEVANCE:
This is the first study to demonstrate the indirect spread of happiness, showing that one person’s emotional state can be affected by someone he doesn’t directly know. Previous research has shown that certain behavior-based phenomena such as obesity and smoking cessation spread through networks like a social contagion, but this is the first to demonstrate that emotions can as well. This research adds further proof to the idea that because we are all interconnected in social networks, our health is interconnected.

BOSTON, Mass. (Dec. 4, 2008) —If you’re happy and you know it, thank your friends—and their friends. And while you’re at it, their friends’ friends. But if you’re sad, hold the blame. Researchers from Harvard Medical School and the University of California, San Diego have found that “happiness” is not the result solely of a cloistered journey filled with individually tailored self-help techniques. Happiness is also a collective phenomenon that spreads through social networks like an emotional contagion.

In a study that looked at the happiness of nearly 5000 individuals over a period of twenty years, researchers found that when an individual becomes happy, the network effect can be measured up to three degrees. One person’s happiness triggers a chain reaction that benefits not only their friends, but their friends’ friends, and their friends’ friends’ friends. The effect lasts for up to one year.

Happiness loves company

Entosis: An internalized cell begins to die
Entosis: Two cells lie within another cellEver wonder what happiness looks like? Each node in the diagrams (1181 people for 1996 at the top; 1020 people for 2000 at the bottom) represents one person (circles are female, squares are male). The lines indicate relationships (black = siblings; red = friends, spouces). Color represents degree of happiness, with blue indicating “the blues,” and yellow indicating sheer joy. Green is somewhere in between.

Images by Nicholas Christakis and James Fowler, courtesy of BMJ

The flip side, interestingly, is not the case: Sadness does not spread through social networks as robustly as happiness. Happiness appears to love company more so than misery.

“We’ve found that your emotional state may depend on the emotional experiences of people you don’t even know, who are two to three degrees removed from you,” says Harvard Medical School professor Nicholas Christakis, who, along with James Fowler from the University of California, San Diego co-authored this study. “And the effect isn’t just fleeting.”

These findings will be published online Dec. 4 in the BMJ.

For over two years now, Christakis and Fowler have been mining data from the Framingham Heart Study (an ongoing cardiovascular study begun in 1948), reconstructing the social fabric in which individuals are enmeshed and analyzing the relationship between social networks and health. The researchers uncovered a treasure trove of data from archived, handwritten administrative tracking sheets dating back to 1971. All family changes for each study participant, such as birth, marriage, death, and divorce, were recorded. In addition, participants had also listed contact information for their closest friends, coworkers, and neighbors. Coincidentally, many of these friends were also study participants. Focusing on 4,739 individuals, Christakis and Fowler observed over 50,000 social and family ties and analyzed the spread of happiness throughout this group.

Using the Center for Epidemiological Studies Depression Index (a standard metric) that study participants completed, the researchers found that when an individual becomes happy, a friend living within a mile experiences a 25 percent increased chance of becoming happy. A co-resident spouse experiences an 8 percent increased chance, siblings living within one mile have a 14 percent increased chance, and for next door neighbors, 34 percent.

But the real surprise came with indirect relationships. Again, while an individual becoming happy increases his friend’s chances, a friend of that friend experiences a nearly 10 percent chance of increased happiness, and a friend of *that* friend has a 5.6 percent increased chance—a three-degree cascade.

“We’ve found that while all people are roughly six degrees separated from each other, our ability to influence others appears to stretch to only three degrees,” says Christakis. “It’s the difference between the structure and function of social networks.”

These effects are limited by both time and space. The closer a friend lives to you, the stronger the emotional contagion. But as distance increases, the effect dissipates. This explains why next door neighbors have an effect, but not neighbors who live around the block. In addition, the happiness effect appears to wear off after roughly one year. “So the spread of happiness is constrained by time and geography,” observes Christakis, who is also a professor of sociology in the Harvard Faculty of Arts and Sciences. “It can’t just happen at any time, any place.”

They also found that, contrary to what your parents taught you, popularity does lead to happiness. People in the center of their network clusters are the most likely people to become happy, odds that increase to the extent that the people surrounding them also have lots of friends. However, becoming happy does not help migrate a person from the network fringe to the center. Happiness spreads through the network without altering its structure.

“Imagine an aerial view of a backyard party,” Fowler explains. “You’ll see people in clusters at the center, and others on the outskirts. The happiest people tend to be the ones in the center. But someone on the fringe who suddenly becomes happy, say through a particular exchange, doesn’t suddenly move into the center of the group. He simply stays where he is—only now he has a far more satisfying sense of well-being. Happiness works not by changing where you’re located in the network; it simply spreads through the network.”

Fowler also points out that these findings give us an interesting perspective for this holiday season, which arrives smack in the middle of some pretty gloomy economic times. Examination of this dataset shows that having $5,000 extra increased a person’s chances of becoming happier by about 2 percent. But that the same data also show, as Fowler notes, that “Someone you don’t know and have never met—the friend of a friend of a friend—can have a greater influence than hundreds of bills in your pocket.”

This is the third major network analysis by Christakis and Fowler that shows how our health is affected by our social context. The two previous studies, both published in the New England Journal of Medicine, described the social network effects in obesity and smoking cessation.

The research was funded by the National Institutes of Health/National Institute on Aging, a Pioneer Grant from the Robert Wood Johnson Foundation, and a contract from the National Heart, Lung, and Blood Institute to the Framingham Heart Study.

Written by David Cameron

PRINCIPAL INVESTIGATORS:
Nicholas Christakis, Professor of Medical Sociology, Harvard Medical School; Professor of Sociology, Harvard Faculty of Arts and Sciences
James Fowler, Associate Professor of Political Science, University of California, San Diego

CITATION:
BMJ, early online publication, December 4, 2008

"Dynamic spread of happiness in a large social network: longitudinal analysis over 20 years in the Framingham Heart Study"

James H Fowler(1), Nicholas A Christakis(2)

(1) University of California, San Diego, San Diego CA
(2) Harvard Medical School, Boston, MA

RELATED PRESS RELEASES:
Smoking is addictive, but quitting is contagious (5/21/2008)
Obesity spreads through social networks (7/23/2007)

CONTACTS:

David Cameron
public_affairs@hms.harvard.edu
617.432.0442

Harvard Medical School has more than 7,500 full-time faculty working in 11 academic departments located at the School's Boston campus or in one of 47 hospital-based clinical departments at 17 Harvard-affiliated teaching hospitals and research institutes. Those affiliates include Beth Israel Deaconess Medical Center, Brigham and Women's Hospital, Cambridge Health Alliance, Children's Hospital Boston, Dana-Farber Cancer Institute, Forsyth Institute, Harvard Pilgrim Health Care, Joslin Diabetes Center, Judge Baker Children's Center, Immune Disease Institute, Massachusetts Eye and Ear Infirmary, Massachusetts General Hospital, McLean Hospital, Mount Auburn Hospital, Schepens Eye Research Institute, Spaulding Rehabilitation Hospital, and VA Boston Healthcare System.