Happiness is contagious, and the more people you know who are full of good cheer, the more likely it is that you're also happy, a study of our social connections suggests.

In Thursday's online issue of the British Medical Journal, researchers analyzed data from more than 4,700 participants in Framingham, Mass., who took part in a heart study from 1983 to 2003.

The research trove included information on births, marriages, death, divorces and contact information of close friends, co-workers and neighbours of the volunteers, allowing Nicholas Christakis, a professor of medical sociology at Harvard Medical School in Boston, and his colleagues to assess happiness.

"What we are dealing with is an emotional stampede," Christakis said.

"Whether you're happy depends not just on your own actions and behaviours and thoughts, but on those of people you don't even know."

Happiness was assessed based on scores for four statements:

• "I feel hopeful about the future."
• "I am happy."
• "I enjoy life."
• "I feel that I am just as good as other people."

The 60 per cent of people who gave high scores to all four statements were rated as happy, and the rest as unhappy.

Happy and unhappy clusters were found that were significantly bigger than would be expected by chance alone, said the researchers, who previously reported that obesity and smoking habits are also spread socially as well.

Joy clusters

Happiness extended to three degrees of separation, from friends to friends of friends, said study co-author James Fowler, a political scientist at the University of California, San Diego.

If a social contact such as spouse, sibling or friend was happy, it increased the likelihood that you were happy by 15 per cent, Fowler said.

"The pursuit of happiness is not a solitary goal," Fowler said. "We are connected, and so is our joy."

Christakis likened it to sitting on a quilt of humanity, with each person sitting on a different-coloured patch that is connected to the next one.

"Imagine that these patches are happy and unhappy patches. Your happiness depends on what is going on in the patch around you," Christakis said.

"It is not just happy people connecting with happy people, which they do. Above and beyond, there is this contagious process going on."

Each happy friend boosts your own happiness by nine per cent, the researchers suggest. On the other hand, having grumpy friends decreased the chances by about seven per cent.

As for why, happiness is known to produce fewer stress hormones, said Andrew Steptoe, a professor of psychology at University College London who was not involved in the study.

The study was conducted before the rise of online social networking sites such as Facebook and MySpace, but Christakis speculated the sites might support emotional contagion.

The researchers are also looking at the spread of depression, loneliness and drinking behaviour.

With files from Associated Press, Reuters