Christakis also found that a person's happiness is dependent not only on the happiness of an immediate friend but - to a lesser degree - on the happiness of their friend's friend, and their friend's friend's friend. Furthermore, someone's chances of being happy increase the better connected they are to happy people, and for that matter the better connected their friends and family. "Most people will not be surprised that people with more friends are happier, but what really matters is whether those friends are happy," says Christakis.Data came from the Framington Heart Study, ongoing since 1948. The limit of social influence seems to be three degrees--beyond a friend of a friend, influence is minimal.
They also discovered that the effect is not the same with everyone you know. How susceptible you are to someone else's happiness depends on the nature of your relationship with them. For example, if a good friend who lives within a couple of kilometres of you suddenly becomes happy, that increases the chances of you becoming happy by more than 60 per cent. In contrast, for a next-door neighbour the figure drops to about half that, and for a nearby sibling about half again. Surprisingly, a cohabiting partner makes a difference of less than 10 per cent, which coincides with another peculiar observation about some social epidemics: that they spread far more effectively via friends of the same gender.
Specific behaviors are also altered by social dynamics.
"Obesity appears to spread through social ties," Christakis says. Again, how likely you are to catch it depends on who you are interacting with: after controlling for factors such as difference in socioeconomic status, the researchers found that an individual's chances of becoming obese increased by 57 per cent if one of their friends became obese, 40 per cent if a sibling did and 37 per cent if their spouse did, irrespective of age (The New England Journal of Medicine, vol 357, p 370).The next question, then, is to what degree MySpace and Facebook amplify these phenomena.
However, neighbours have no influence, and how far away you live from a friend counts for little, which implies that obesity spreads via a different mechanism to happiness. Rather than behavioural mimicry, the key appears to be the adoption of social norms. In other words, as I see my friends gain weight, this changes my idea of what an acceptable weight is. One similarity with happiness is that friends and relatives have a far greater influence if they are of the same gender. While it is not evident why that should matter for emotional contagion, norms of body size are clearly gender-specific: "Women look at other women, men look at other men," says Christakis. This could also help explain the epidemics of eating disorders reported among groups of schoolgirls in recent decades.
The spread of a social norm appears to account for another of Christakis's findings: that when people stop smoking, they usually do so along with whole clusters of friends, relatives and social contacts. As more people quit, it becomes the socially acceptable thing to do, and those who choose to continue smoking are pushed to the periphery of the network. In this case, people are most strongly influenced by those closest to them - if your spouse quits, it is 67 per cent more likely that you will too.