In a good mood? Your neighbor, her friends and even her friends' friends should thank you – you're likely infecting them with your cheer. Happiness spreads through social networks about as easily as the flu, according to a new study.

The researchers analyzed data compiled from nearly 5,000 interconnected people over a 20-year period. After establishing a baseline mood for each participant, the team found that when one person became happier, it rippled through the network, increasing the likelihood that others would become happier too.

Sadness, thankfully, is not nearly as infectious. An attack of the blues creates a much smaller ripple than a case of giddiness, said head researcher James Fowler of the University of California, San Diego.

Cascades of happiness

A happy infection lasts an average of 12 months, Fowler said; that is, if your neighbor wins the lotto it could give you a mood boost for about a year. And a joy virus can spread to people three degrees removed from the original mood shifter. So someone experiencing bliss makes his friends happier, his friends' neighbors happier and even his friends' neighbors' friends happier.

The ripple of joy continues diffusing over all of society, Fowler theorized, but is undetectable past the third degree of separation "because it is part of a whole sea of different cascades of happiness and unhappiness."

The infectiousness of certain behaviors, such as overeating, smoking and innovating, began making headlines over a year ago. (Fowler and his colleague Nicholas Christakis of Harvard Medical Center are responsible for several of these findings.)

The study in the current online issue of the British Medical Journal is the first to show the contagiousness of an emotion.

Proximity counts

Emotional "contagions" are more dependent on proximity than behavioral ones, the research shows. A faraway friend or sibling may influence our perception of social norms, such as what is an acceptable weight gain, but they have surprisingly little effect on our mood. Conversely, our neighbors do not influence our weight, but they do impact our happiness, said Fowler.

But only if they live next door. The mood of the neighbor on the next corner has little to no effect — a result, said Fowler, that likely rules out explanations of mood synchronizing based on class (people in the same neighborhood tend to have similar houses) and suggests we are more likely to catch joy from people we often see.

Exactly how the happy bug is transmitted remains a mystery, but not for lack of theories. Cheerful people may simply be friendlier or more helpful, spreading joy germs in the process. Or it could be psycho-immunological; being around happy people may be good for our health — both physical and mental.

Fowler credits natural selection and the social function of emotions. "When we are happy, we smile. What is the purpose of that unless it is to spread it?" he said, explaining that happiness helps us cooperate and achieve larger goals.

"Even little things that we do for ourselves can pay off in a really big way," Fowler said, not only influencing the moods of those we know and love, but even those we don't.

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