A study published Friday in the British journal BMJ, authored by Dr. Nicholas A. Christakis, a physician and social scientist at Harvard Medical School, has found that a person’s happiness depended not only on their friends’ or close relatives’ mood, but even on the actions of people they didn’t know.

The study monitored a large group of people over a period of 20 years and concluded that, as co-author James H. Fowler, an associate professor of political science at University of California, San Diego, put it, if one’s friend’s friend’s friend was happy, the impact on that person’s happiness was even larger than coming into an extra $5,000.

Researchers looked into 4,739 people from 1983 to 2003 and performed analysis on their levels of happiness and their connections with friends, family, neighbours and co-workers and found that happiness was actually contagious and that people one, two and three degrees removed from a person could significantly contribute to the latter's happy mood.

The study, which was financed by the National Institute on Aging, further discovered that a next-door neighbor’s joy made the chances of a person being happy go up by 34 percent, while a down the block neighbour’s good spirits had no such effect.

Moreover, a friend living one mile away could increase one’s happiness by 42 percent, but one living two miles away could only give rise to a 21 percent bounce. As for a friend from a different community, no impact on the level of happiness was registered.

Dr. Nicholas A. Christakis revealed that in order for one’s friends or neighbours to play a role in their mood, physical and temporal proximity were necessary.

Nevertheless, a study conducted on 1,700 Facebook profiles revealed that users who smiled in the pictures they had posted on the social networking website had more friends than those who didn’t, so maybe distance is becoming a less important factor in how people’s moods are interlinked.

The recent research used data from the federal Framingham Heart Study, which started monitoring people in Framingham, Massachusetts after the end of World War II and eventually came to study their children and grandchildren. As of 1983, the ones who took part in the BMJ study were asked to complete questionnaires concerning  their emotional well-being on a regular basis.

Drawing on the questionnaires, Dr. Christakis and Professor Fowler analyzed approximately 50,000 social connections.

Researchers said that the impact other people had on one’s happiness was not long-standing, since friends’ good mood only boosted a person’s level of happiness if the former had joyful news to share within a year’s time.

In addition, a co-worker’s good news increased one’s happiness only when the two were also friends, so it seems that competition at the work place prevails over the contagious joy.

The Christakis- Fowler team has also previously published studied that revealed obesity and quitting smoking were socially contagious, as well, but in those cases, even the friends who lived far away had an impact on a certain person.