According to a large study, happiness is contagious and how happy you are could depend on people you do not even know. In the study, researchers analyzed 4739 people for 20 years along with their association with many others from spouses, relatives, close friends, co-workers and neighbors. They found that people who are happy boost the chances of someone they know being happy and can even elevate the mood of the persons spouse, friend neighbor etc.

The author of the study Dr. Nicholas A. Christakis, a physician and social scientist at Harvard Medical School said, "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. 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." Researchers in the study, to be published Friday in BMJ, a British journal said, "You would think that your emotional state would depend on your own choices and actions and experience," said Christakis. "But it also depends on the choices and actions and experiences of other people, including people to whom you are not directly connected. Happiness is contagious." They also found that the effect can last for as much as a year and although unhappiness can also spread in a similar manner, the "infectiousness" of that emotion appears to be far weaker.

The chances of a person becoming happy when a friend, sibling, neighbor became happy increased from 8-34 % the researchers reported that the effect continued through three degrees of separation although it progressively dropped from 15 % to 10% on to 6% before disappearing. The study's co-author, James H. Fowler, an associate professor of political science at University of California, San Diego said the research showed 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."

Another study also to be published 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 said, "Researchers should be cautious in attributing correlations in health outcomes of close friends to social network effects." "You have to see them and be in physical and temporal proximity," Dr. Christakis said explaining that a next-door neighbor's joy increased one's chance of being happy by 34% , but a neighbor down the block had no effect. Sadness was transmitted in a similar fashion but not as reliably as happiness. Professor Fowler added, "We are not giving you the advice to start smiling at everyone you meet in New York. That would be dangerous." The happiness study was financed by the National Institute on Aging.