Olivia Judson on the influence of science and biology on modern life.

“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.

So, while there have been plenty of studies of how pairs of people, especially spouses, affect each others’ health, there have been far fewer studies of how health reverberates through large social networks. The results I’m referring to here all come from the so-called “Framingham Heart Study,” which began in Massachusetts in 1948 and has continued to the present day. (Up to now, the study has involved two cohorts of several thousand people each; a third cohort has just been enrolled.)

It’s possible, therefore, that the results I’m talking about are specific to this group of people. However, I think that’s unlikely. The details of how an attribute spreads may differ from one group to another: perhaps in some places, friends have less influence and siblings have more. But the general result — that healthy (or unhealthy) behaviors, habits and outlooks are infectious — is, I think, likely to prove robust.

But then, how does something like obesity get “caught”? That’s not clear. One idea is that people judge their own weight by that of their friends — you think of yourself as thin if you are thinner than the people you know — and eat accordingly. Another is that friends mirror one another’s eating habits. Many studies have found that people tend to eat less when they are eating with someone who is not eating much. Also, people tend to eat more when they eat with friends rather than with strangers. Perhaps, too, a habit of eating, say, dessert when you are with your friends makes you more likely to eat it when you are alone.

(And happiness? Again, it’s not clear how it spreads, though there are plenty of ideas. For example, since we tend to unconsciously mimic the postures and facial expressions of those around us, perhaps we begin to take on the moods of others by adopting postures consistent with those moods. Or perhaps talking to people who are cheerful imbues us with cheerful thoughts.)

Whatever the mechanisms, the implications are profound. Obesity, smoking and happiness all have substantial effects on health. Obesity is associated with a host of troubles, from diabetes to many kinds of cancer.

Happiness, meanwhile, appears to improve health. Although this idea is still controversial, several recent studies have found that happy people tend to have more robust immune systems. For example, in one experiment volunteers were scored for happiness then had cold viruses squirted up their noses; those who were happier were much less likely to fall sick. (I’m not sure I’d be volunteering for that experiment, regardless of my emotional state — hats off to those who did.) In another, volunteers were vaccinated against the virus that causes hepatitis B; several months later, the happy volunteers showed a stronger response to the vaccine than the unhappy volunteers.

The discovery that the health of one person affects the health of those around them isn’t new. Depressed mothers, for example, have a suite of unhappy effects on their children: their children are less likely to be vaccinated, less likely to do well at school and more likely to have emotional problems of their own. Nor is it news that social lives affect health: it’s long been known that people with more friends tend to be healthier. But the finding that particular attributes of health — or lack of it — can ripple through social networks shows that one person’s health can have far reaching effects.

I draw a couple of conclusions from this. The first is that unless you are a hermit living entirely alone, your choices and wellbeing do not affect just you. The second, and more important, conclusion is that medicine isn’t simply about improving the health of an individual here and an individual there. It’s about the health of the whole society.

Notes:

For the spread of quitting smoking, see Christakis, N. A. and Fowler, J. H. 2008. “The collective dynamics of smoking in a large social network.” New England Journal of Medicine 358: 2249-2258. For obesity spreading, see Christakis, N. A. and Fowler, J. H. 2007. “The spread of obesity in a large social network over 32 years.” New England Journal of Medicine 357: 370-379. For happiness spreading, see Fowler, J. H. and Christakis, N. A. 2008. “Dynamic spread of happiness in a large social network: longitudinal analysis over 20 years in the Framingham Heart Study.” BMJ 337: a2338. Note that these authors have recently published a book about human connectedness; I haven’t read it as I didn’t find out about it until too late, but if you find this subject interesting, my guess is that the book would be worth a look.

For an overview of social effects on health — both in networks and between pairs of individuals — see Smith, K. P. and Christakis, N. A. 2008. “Social networks and health.” Annual Review of Sociology 34: 405-429. For a classic review of how social activity affects health, see House, J. S., Landis, K. R. and Umberson, D. 1988. “Social relationships and health.” Science 241: 540-545.

For patterns of eating with friends versus strangers, see Salvy, S.-J. et al. 2007. “Effects of social influence on eating in couples, friends and strangers.” Appetite 49: 92-99; and Salvy, S.-J. et al. 2009. “The presence of friends increases food intake in youth.” American Journal of Clinical Nutrition 90: 282-287. For emotions spreading through mimicking the postures of others, see, for example, Niedenthal, P. M. 2007. “Embodying emotion.” Science 316: 1002-1005. For an interesting look at emotional convergence, with some consideration of other mechanisms, see Anderson, C., Keltner, D. and John, O. P. 2003. “Emotional convergence between people over time.” Journal of Personality and Social Psychology 84: 1054-1068.

For the relationship between obesity and cancer, go here.

For an overview of happiness and health, see Steptoe, A. et al. 2008. “Positive affect and psychosocial processes related to health.” British Journal of Psychology 99: 211-227. For happiness and resistance to colds, see Cohen, S. et al. 2006. “Positive emotional style predicts resistance to illness after experimental exposure to rhinovirus or influenza A virus.” Psychosomatic Medicine 68: 809-815. For happiness and hepatitis B vaccination, see Marsland, A. L. et al. 2006. “Trait positive affect and antibody response to hepatitis B vaccination.” Brain, Behavior, and Immunity 20: 261-269. For depression in mothers and the effects it can have on infant health see, for example, Zajicek-Farber, M. L. 2009. “Postnatal depression and infant health practices among high-risk women.” Journal of Child and Family Studies 18: 236-245.

Many thanks to Dan Haydon for insights, comments and suggestions.