“Wash your hands regularly.” “Cover your mouth when you sneeze.” “Throw away your used tissues.” These are some of the exhortations currently posted around London in an attempt to reduce the spread of flu. But one day, perhaps we’ll have public health campaigns of a different kind. “Be jolly: it’s catching.” Or, “Eat less: do it for your friends.”
Why? Because “traditional” infectious diseases — those, like flu and tuberculosis, that are caused by viruses or bacteria — are not the only aspects of health that can spread from one person to another. Taking up smoking is contagious; so is quitting. Obesity is contagious. So is happiness.
At least, these are the results coming in from long-term studies of social networks — the networks of friends and families, neighbors and colleagues that we all belong to. Such studies have found that one person’s change in behavior ripples through his or her friends, family and acquaintances. If one of your friends becomes happy, for example, you’re more likely to become happy too. If you’re great friends with someone who becomes obese, you’re much more likely to become obese as well.
And the effect doesn’t stop there. If your friend’s friend becomes happy, that increases the chance your friend will become happy — and that you will too. Conversely, if you become obese or depressed, you may inadvertently help your friends, and your friend’s friends, to become fat or gloomy. (Intriguingly, happiness and obesity seem to spread in different ways. Obesity spreads most easily between friends of the same sex who are emotionally close. Happiness spreads most readily between friends who live near each other: a happy friend on the same block makes more difference than a happy friend three miles away.)
I should say that doing long-term studies of social networks is difficult — it means interviewing and measuring thousands of people repeatedly over many years. After all, if I want to know whether you and your friends will change weight over the next five years, I have to measure all of you now and again in five years’ time. Moreover, I have to keep track of how friendships come and go, of who moves house and so on. In short, it’s a massive task just to collect the data.
