David Bosworth in The Hedgehog Review:
The research was conducted by Nicholas Christakis and James Fowler, and was derived from data collected by the Framingham Heart Study, which has been tracking the vital statistics and psychological states of the residents of one Massachusetts town for over five decades. The researchers were initially interested in the impact of social contacts on health habits, and the richness of the Framingham data allowed them to track the long-term behavior of more than 12,000 individuals. The results, as reported in Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives, were startling, and have further undermined modernity’s presumptions about the individual as a rational and self-reliant decision maker. As clearly tracked on the researchers’ graphs, health habits spread rapidly through the separate social networks of the Framingham population: Whom one knew strongly affected what one chose to do—overeat or not, smoke or not—and highlighted the power of emulation in human behavior. Further study showed that the influence of these social networks was not limited to health decisions, leading the authors to conclude that
our connections affect every aspect of our daily lives…. How we feel, what we know, whom we marry, whether we fall ill, how much money we make, and whether we vote all depend on the ties that bind us. Social networks spread happiness, generosity, and love. They are always there, exerting both subtle and dramatic influence over our choices, actions, thoughts, feelings, even our desires.
More startling still, according to the authors,
our connections do not end with the people we know. Beyond our own social horizons, friends of friends of friends can start chain reactions that eventually reach us, like waves from distant lands that wash up on our shores.
Our misery or happiness, our good or bad health, and our indifference or commitment to political participation, are not only contagious; according to Christakis and Fowler, they are mysteriously influenced at a distance by the decisions of people we never meet. The persistence of this influence within the social networks could be traced through “three degrees of separation,” so that the habits of a man’s sister’s neighbor’s wife had a statistically significant effect on his own behavior. If she quit smoking, though out of sight and out of mind, his chances of doing the same were increased by nearly a third.