December 8, 2008 — People's happiness is largely influenced by the happiness of those they are connected to — whether they know them or not — new findings from the Framingham Heart Study suggest.

"We've found that your emotional state may depend on the emotional experiences of people you don't even know, who are 2 to 3 degrees removed from you. And the effect isn't just fleeting," study investigator Nicholas Christakis, MD, PhD, from Harvard Medical School, in Boston, Massachusetts, said in a statement.

The study is published online December 4 in BMJ.

According to the authors, happiness is a fundamental component of human health determined by a complex set of voluntary and involuntary factors. While previous studies have identified a broad range of stimuli identified with happiness or unhappiness, none has examined the happiness of others as a key determinant of human happiness.

Furthermore, the authors point out there is evidence that emotional states can be transferred directly from 1 individual to another by mimicry. "People can 'catch' emotional states they observe in others over time frames ranging from seconds to weeks," they write.

Can Happiness Spread?

However, they note, despite this evidence, little is known about the role of social networks in happiness or whether happiness has the potential to spread.

To examine whether happiness can spread through social networks that include direct as well as indirect relationships, the investigators used data from 5124 participants from the Framingham Offspring Study to reconstruct the social fabric in which individuals are enmeshed and analyze the relationship between social networks and health.

The data included all family changes for each study participant, such as birth, marriage, death, and divorce. In addition, there was also extensive information on participants' closest friends, coworkers, and neighbors. Coincidentally, many of these friends were also study participants.

The final analysis included 4739 individuals and more than 50,000 social and family ties. Using the Center for Epidemiological Studies depression index, the investigators found that when an individual becomes happy, a friend living within a mile experiences a 25% increased chance of becoming happy. A coresident spouse experiences an 8% increased chance, siblings living within 1 mile have a 14% increased chance, and next-door neighbors, 34%.

Popularity Leads to Happiness

However, the most surprising finding, say the researchers, was with indirect relationships. While an individual becoming happy increases his or her friends' chances, a friend of those friends experiences a nearly 10% chance of increased happiness and a friend of that twice-removed friend has a 5.6% increased chance — a 3-degree cascade.

"We found that while all people are roughly 6 degrees separated from each other, our ability to influence others appears to stretch to only 3 degrees. It's the difference between the structure and function of social networks," said Dr. Christakis.

With coauthor James Fowler, PhD, from the University of California, San Diego, the investigators also found that popularity leads to happiness and that individuals in the center of their social network clusters are the most likely to become happy.

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.

The study was supported by the National Institutes of Health and the Robert Wood Johnson Foundation. The authors report no financial disclosures.

BMJ. Published online December 4, 2008. Abstract