I have just returned from Disneyland Paris, where everyone smiles whether or not they are having a good time. It was enough to make you think that smiling, like yawning, could be catching. And so it is. In 1963 an “epidemic” of laughter among girls in a boarding school in Tanganyika (now Tanzania) forced it to close – although the outbreak was possibly due to mass hysteria rather than genuine mirth – while a study on the effects of a “laugh box” showed that most people laughed or smiled in response to canned guffaws.

Laughter is almost always social – occurring 30 times more often in company than in solitary situations. A study published this week in the Proceedings of the Royal Society has taken the idea of contagious smiling one step further, by examining the effect on our own happiness of having friends who have a “healthy mood”. The researchers used American data from the National Longitudinal Study of Adolescent Health to do complex mathematical modelling, which the study’s author, Ed Hill, did a good job of trying to explain to me.

Essentially, they had data on mood (self-assessed at specific points in time using a validated questionnaire) and on friendship networks, and they modelled the spread of moods among individuals using methods similar to those used to track the spread of infectious diseases. They found that adolescents with five or more mentally healthy friends had half the probability of developing – and double the likelihood of recovering from – depression over a six- to 12-month period. So if you can catch happiness from other people, shouldn’t you hang around them more?

The solution

Yes, you definitely should: there is other evidence that happiness spreads. A study published in the BMJ in 2008 of 4,739 members of the Framingham Heart Study social network found that the relationship between people’s happiness extends as far as three degrees of separation – to the friends of friends’ friends. The effects were stronger with geographical proximity – a happy nextdoor neighbour increases the likelihood that you will be happy by just over a third. Happy people, say the authors, may be nicer to others, share their good fortune by being more helpful, or just exude an emotion that makes other people feel better about life. However they do it, they make those around them happy, too.

Hill, an applied mathematician, believes we can help teenagers avoid depression by encouraging their friendships – and his study found no evidence for depression being contagious, so there is no risk from having a mate with a low mood. The BMJ study, too, found that misery did not spread through social networks as effectively as happiness. “When you’re smiling the Whole World Smiles With You,” as the old song has it, appears to be scientifically accurate.