"What spreads is an idea," says Christakis.
In one of his best known studies he took data from the famous Framingham Heart Study to demonstrate how weight gain in one person might ripple through a social network. In looking at the relationships of 12,000 people Christakis and his team of researchers found that a person's chances of becoming obese increase by 57% when a friend becomes obese; by 40% if a sibling becomes obese; and by 37% if a spouse becomes obese.
"When people around you gain weight, your attitude about what constitutes an acceptable body size changes, and you might gain weight too," Christakis explains.
He sees online social networks like Twitter and Facebook as powerful for disseminating information on a large scale, but questions whether their ability to influence matches that of an off-line social network. "To be effective an online network must be real or at least feel real and something must be at stake," he said.