问答题 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. At first sight, the idea that we can catch the moods, habits and state of health not only of those around us, but also those we do not even know seems alarming. It implies that rather than being in charge of where we are going in life, we are little more than back-seat drivers, since most social influence operates at a subconscious level. But we need not be alarmed, says Duncan Watts, a sociologist at Columbia University, New York. "Social influence is mostly a good thing. We should embrace the fact that we're inherently social creatures and that much of who we are and what we do is determined by forces that are outside the little circle we draw around ourselves." What's more, by being aware of the effects of social contagion we may be able to find ways to counter it, or use it to our benefit. "There's no doubt people can have some control over their networks and that this in turn can affect their lives," says Christakis. To get an idea of what is going on, take Christakis' findings on the spread of happiness, which were published last month. His team looked at a network of several thousand friends, relatives, neighbors and work colleagues. They found that happy people tend to be clustered together, not because they naturally orientate towards each other, lint because of the way happiness spreads through social contact over time, regardless of people's conscious choice of friends. (337 words)