"How your friends' friends can affect your mood"

.......For while all these things might contribute to the way you feel, there is one crucial factor you probably have not accounted for: the state of mind of your friends and relatives. Recent research shows that our moods are far more strongly influenced by those around us than we tend to think. Not only that, we are also beholden to the moods of friends of friends, and of friends of friends of friends - people three degrees of separation away from us who we have never met, but whose disposition can pass through our social network like a virus.

Indeed, it is becoming clear that a whole range of phenomena are transmitted through networks of friends in ways that are not entirely understood: happiness and depression, obesity, drinking and smoking habits, ill-health, the inclination to turn out and vote in elections, a taste for certain music or food, a preference for online privacy, even the tendency to attempt or think about suicide. They ripple through networks "like pebbles thrown into a pond", says Nicholas Christakis, a medical sociologist at Harvard Medical School in Boston, who has pioneered much of the new work......

......To get an idea of what is going on, take Christakis's findings on the spread of happiness, which were published last month. His team looked at a network of several thousand friends, relatives, neighbours and work colleagues who form part of the Framingham Heart Study, an ongoing multi-generational epidemiological survey that has tracked risk factors in cardiovascular disease among residents of Framingham, Massachusetts, since 1948. They found that happy people tend to be clustered together, not because they naturally orientate towards each other, but because of the way happiness spreads through social contact over time, regardless of people's conscious choice of friends (BMJ, DOI: 10.1136/bmj.a2338).

.......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.

All this poses a key question: how can something like happiness be contagious? Some researchers think one of the most likely mechanisms is empathetic mimicry. Psychologists have shown that people unconsciously copy the facial expressions, manner of speech, posture, body language and other behaviours of those around them, often with remarkable speed and accuracy. This then causes them, through a kind of neural feedback, to actually experience the emotions associated with the particular behaviour they are mimicking......

.......The power of strangers

Two factors appear crucial: the frequency of social contact, and the strength of the relationship. This is not too surprising: we know that emotional contagion requires physical proximity. It is also likely that the closer we feel to someone, the more empathetic we are towards them, and the more likely we are to catch their emotional state. However, how these two factors play out in day-to-day interactions is uncertain. What is also unclear - because it has never been properly tested - is the extent to which emotions can propagate through virtual networks, where the opportunity for physiological mimicry is much reduced.

So much for emotions - what about other phenomena that we unwittingly pick up, and pass on, through our social networks? In 2007, Christakis's team, again tracking members of the Framingham Heart Study, found that obesity is transmitted in a similar way to happiness. Your risk of gaining weight increases significantly when your friends gain weight, and it is also affected by the weight of people beyond your social horizon. "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).

......Your work colleagues can also have an effect, particularly if you are in a small, close-knit workplace; and more highly educated friends influence one another more than less educated (The New England Journal of Medicine, vol 358, p 2249).

Happiness, obesity, smoking habits - activities that we traditionally think of as shaped by individual circumstances, turn out to be ruled to a large degree by social forces. Many other day-to-day phenomena fit a similar pattern, often counter-intuitively. Take autism: Peter Bearman at the Institute for Social and Economic Research and Policy at Columbia University who in 2004 uncovered a link between suicidal behaviour and certain friendship patterns (American Journal of Public Health, vol 94, p 89), is looking at whether the recent rise in the diagnosis of autism is in any way socially determined. His study is ongoing, but he says his findings could be "explosive". "It is likely that if you have an autistic child in your community the probability of your child being diagnosed with autism is significantly higher."......