If you live in the northern hemisphere, this is probably not your favourite month. January tends to dispirit people more than any other. We all know why: foul weather, post-Christmas debt, the long wait before your next holiday, quarterly bills, dark evenings and dark mornings. At least, that is the way it seems. 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. ...

(Ful article at New Scientist).