How your friends’ friends can affect your mood – life – 30 December 2008 – New Scientist

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.