A 20 year study by American scientists suggests that happiness may spread from person to person because they found that people surrounded by happy people in their friends and family network were more likely to remain happy in the future.

The study was published online in the British Medical Journal, BMJ on 4 December by authors James H Fowler, associate professor in the Department of Political Science at the University of California in San Diego, and Nicholas A Christakis, associate professor at the Department of Health Care Policy, Harvard Medical School, and Department of Sociology, Harvard University, Cambridge, Massachusetts.

The authors suggest that a person’s happiness depends on the happiness of those with whom they are in regular direct and indirect contact – it really does rub off and is not just a matter of personal choice or experience. They also suggest that how close you live to a happy person in your network also has an effect: a friend who becomes happy and lives within a mile of you increases the likelihood of you being happy by 25 per cent.

For the study, Christakis and Fowler analysed data collected from the Framingham Heart Study (FHS) between 1971 and 2003. The FHS, a long term cohort study of cardiovascular disease that started in 1948 with about 5,000 participants and is now providing data about their grandchildren. In this third generation, there are just over 5,000 adults aged 21 to 70 who joined the study in 1971 and were followed until 2003. During this time each participant identified their close friends, relatives, and where they lived and worked, so they could be contacted every 2 to 5 years.

From these initial 5,000 or so participants the researchers found over 53,000 social ties connecting over 12,000 people. They selected 4,739 people who were followed from 1983 to 2003. These participants had filled in questionnaires about their mental health, including a four-point depression scale that asked them the extent to which they agreed or disagreed with the statements “I felt hopeful about the future”, “I enjoyed life”, “I was happy”, and “I felt that I was just as good as other people”.

Using this four-point scale, the authors defined happiness as a perfect score on all four items and then used statistical tools to measure how the social ties correlated with the various scores.

The results showed that:

  • There were clusters of happy and unhappy people in the network.
  • The relationship between people’s happiness extended up to three degrees of separation (eg to a friend of one’s friend’s friend).
  • People who have many happy people around them, and those who are at the centre of the network, are more likely to become happy in the future.
  • Longitudinal statistical methods suggest that clusters of happiness happen because happiness spreads and is not just a tendency for people to link up with other people who are similar.
  • A friend living up to one mile away (about 1.6 km) and who becomes happy increases the chance of a person being happy by 25 per cent (95 per cent confidence interval 1 to 57 per cent).
  • Similar effects appeared in spouses who lived with their partner (8 per cent), brothers and sisters who lived within 1 mile of each other (14 per cent), and next door neighbours (34 per cent).
  • Coworkers did not show this effect, suggesting the workplace context may limit the spread of emotional states.
  • The effects decayed with time and geographical distance.

Christakis and Fowler concluded that:

“People’s happiness depends on the happiness of others with whom they are connected. This provides further justification for seeing happiness, like health, as a collective phenomenon.”

They wrote that: “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.”

Commenting in an accompanying editorial, Professor Andrew Steptoe from University College London and Professor Ana Diez Roux from University of Michigan School of Public Health, called the study “groundbreaking”:

“If, [as these findings suggest] happiness is indeed transmitted through social connections, it could indirectly contribute to the social transmission of health, and has serious implications for the design of policies and interventions,” they said.

But writing in another study, Jason Fletcher from Yale University and Ethan Cohen-Cole from the Federal Reserve Bank of Boston, said we should be careful when interpreting results from studies that use the methods used in Christakis and Fowler’s study because they are subject to “potentially large biases…that might produce effects where none exist.”

Fletcher and Cohen-Cole investigated the potential network effects of headaches, skin problems, and height. They found, for example, that a person’s acne problems were higher if their friend had acne problems, and the likelihood of headaches also went up in people who had friends with headaches. But when they took potential environmental confounders into account, such effects disappeared. They concluded that:

“These methods might produce premature claims of social network effects in health outcomes.”

“Dynamic spread of happiness in a large social network: longitudinal analysis over 20 years in the Framingham Heart Study.”
James H Fowler, Nicholas A Christakis.
BMJ 2008;337:a2338 , published online 4 December 2008.

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Sources: BMJ.

Written by: Catharine Paddock, PhD