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Forget six degrees of separation. How about three degrees of happiness? Researchers from Harvard University and the University of California, San Diego have mapped the relationships of happy people and found that happiness is a collective phenomenon that spreads like a virus through social networks - affecting even strangers three times removed from each other.

The theory builds on the notion of emotional contagion, the process at work when a person smiles back at someone who smiles at him. Human emotions appear in clusters, behaving like stampeding animals, says study co-author Nicholas Christakis.

"You would never think to ask a particular buffalo in a herd, 'Why are you running to the left?' " says the Harvard Medical School sociology professor. "The whole herd is running to the left."

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Misery, on the other hand, does not love company as much as happiness does. "Unhappiness doesn't spread as intensely or as consistently as happiness," he says.

The research, being published today in the British Medical Journal, is the latest analysis of data gleaned from the Framingham Heart Study, a longitudinal U.S. survey begun in 1948. The researchers, who have previously published similar findings on the spread of obesity and smoking from the data, focused on 4,739 individuals over 20 years, accounting for 50,000 social and family ties. As the mantra goes in real estate, the top factor in happiness is location, location, location.

Using a standard measure of well-being, the Center for Epidemiological Studies Depression scale, they found that when an individual becomes happy, a friend who lives nearby experiences a 25-per-cent increased chance of becoming happy. And the more centrally located you are in your social cluster of happy people, the more likely you are to become happy.

"Popularity leads to happiness, not happiness to popularity," Dr. Christakis says.

Next-door neighbours who became happy increase a person's happiness by 34 per cent, whereas a neighbour who lives merely on the same block has little impact.

"For emotions to spread you have to see the other person or interact with the other person," he says.

Even your proximity to happy people you don't know is a factor. While a person becoming happy affects his closest friends, a friend of that friend also experiences a nearly 10-per-cent chance of increased happiness. Take one more step, and a friend of that friend enjoys a 5.6-per-cent chance of a happiness boost.

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How does this work among strangers? Dr. Christakis uses the analogy of the adoption of fashion trends.

"You start changing your fashion in clothes because a person four degrees removed from you adopts a different habit."

While previous work in the area has shown that a person's emotions are highly influenced by her spouse, this research found an 8-per-cent boost - small, compared to a next-door neighbour's whopping 34-per-cent influence.

Dr. Christakis attributes the discrepancy to the fact that his research found happiness appears to spread more through same-sex relationships than opposite-sex relationships.

They also found the happiness ripple effect doesn't happen at work. Our happiness has no effect on that of our co-workers. While Dr. Christakis says he's not sure why this is, he speculates that it may be due to inherent competition in the workplace, or even schadenfreude.

Ulrich Schimmack, a happiness researcher and associate professor of psychology at the University of Toronto (Mississauga) who did not work on the paper, says he is impressed by the study's method, which measured multiple points of view about both relationships and happiness.

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And while he says tracking the increase in happiness in a social group over time is an important finding, he says the study may not have taken into account the effects of environmental factors, such as changes in employment or income, that may have had a blanket effect on people within the same social network.

"That becomes very tricky to separate," he says. "Contagion is possible, but they may just be responding to similar changes in their environment."

Dr. Christakis says he controlled for this possibility, as well as for the tendency of happy people to befriend other happy people. Even when these factors are taken into account, he says, there is an additional "flow of happiness."

Shared happiness has probably affected human evolution over time, he says.

"It would make sense that happiness is more socially productive," he says. "There's a survival advantage to emotional contagion ... it increases group cohesion."

*****

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Location, location, location

This visual snapshot of "happiness clusters" documents happiness and sadness within social networks in the

Framingham Heart Study in 2000. The lighter shades

represent people who reported feeling happy, whereas the darker shades represent those who felt sad (circles are female, squares are male). Researchers found that happiness spread more intensely in social networks among those who were happy.

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