In the study, the researchers, writing in the British Medical Journal, found that clusters of happy and unhappy people were visible in the networks and the effect lasted for three degrees of separation - meaning one person benefited from the happiness of their friends' friends.
To reach the conclusion, Professor Nicholas Christakis from Harvard Medical School and Professor James Fowler from the University of California, San Diego, analysed data collected in the Framingham Heart Study recruited 5,124 adults aged 21-70 and followed them between 1971 and 2003 to examine various aspects of their life and health.
n the study, participants were asked to identify their relatives, "close friends," place of residence, and place of work to ensure they could be contacted every two to four years for follow-up.
From the analyses, the experts found 53,228 social ties between the 5,124 participants and a total of 12,067 people. They focused on 4,739 people followed from 1983 to 2003.
Additional data on mental health, collected using a depression rating scale during the original study, recorded agreement or disagreement with four statements "I felt hopeful about the future," "I was happy," "I enjoyed life," "I felt that I was just as good as other people."
According to the researchers, happiness was defined as a perfect score for all four statements.
Using statistical analysis the researchers measured how social networks were correlated with reported happiness. They found that live-in partners who become happy increase the likelihood of their partner being happy by 8 percent, similar effects were seen for siblings who live close by (14 percent) and neighbours (34 percent). Work colleagues did not affect happiness levels suggesting that social context may curtail the spread of emotional states.
In the study, researchers reported that close physical proximity is essential for happiness to spread. A person is 42percent more likely to be happy if a friend who lives less than half a mile away becomes happy, the effect is only 22 percent for friends who live less than two miles away, and this effect declines and becomes insignificant at greater distances.
"Changes in individual happiness can ripple through social networks and generate large scale structure in the network, giving rise to clusters of happy and unhappy individuals," the researchers said.
They conclude: "Most important from our perspective is the recognition that people are embedded in social networks and that the health and wellbeing of one person affects the health and wellbeing of others. This fundamental fact of existence provides a fundamental conceptual justification for the specialty of public health. Human happiness is not merely the province of isolated individuals."