Suddenly feeling really happy? It could be that happiness is spreading through your social network just like a virus right now. Or are you putting on weight? That could be the fault of your friend's husband's work colleague—someone you have never met. These are just two of the dramatic findings in a new book by Dr Nicholas Christakis and Professor James Fowler.

Their research shows that each person’s social influence stretches three degrees before it fades out. And moods and behaviour can spread through social networks like a virus, they say.  Smoking, obesity and depression are socially infectious – but so are quitting smoking, staying thin and happiness.

 ‘You may not know him personally, but your friend’s husband’s co-worker can make you fat. And your sister’s friend’s boyfriend can make you thin,’ say Dr Christakis and Professor Fowler in their book Connected.

 ‘We are tied not just to those around us, but to others in a web that stretches farther than we know,’ they say. ‘A person's happiness is related to the happiness of their friends, their friends' friends, and their friends' friends' friends—that is, to people well beyond their social horizon.’

Some behaviour spreads through subconscious social signals which tell us what is considered normal behavior. For example, we eat more if others around us are eating more. With emotional states, Christakis and Fowler argue that the contagion may be deeply subconscious: the spread of good or bad feelings, they say, might be driven partly by “mirror neurons” in the brain that mimic what we see in the facial expressions of those around us.

‘This makes me feel so much more responsible now that I know that if I come home in a bad mood I’m not only affecting my wife and son but my son’s best friend or my wife’s mother,’ says Professor Fowler, who now plays upbeat music on his way home.

The good news is that happiness is more contagious than unhappiness. So get out there and spread a little happiness around!

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