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The theory that everyone on the planet is only a half dozen people away from knowing everyone else was popularized by John Guare's 1993 movie Six Degrees of Separation. Now research by a pair of social scientists might have Hollywood thinking of a sequel: Three Degrees of Connection.

Using statistical analyses of thousands of subjects, a study in The British Medical Journal has shown that happiness actually spreads from person to person, up to three connections away. "So if your friend's friend's friend becomes happier, it ripples through the network and affects you, even if you don't know that person," says author Nicholas Christakis, MD, a medical sociology professor at Harvard Medical School. Proximity plays a part: A happy sibling who is a mile away can increase your probability of happiness by up to 14 percent; a nearby friend, by 25 percent; and a next-door neighbor, by 34 percent. Interestingly, the effect also applies to smoking and obesity, Christakis has shown. "If people around you gain weight, it changes your expectations about what an acceptable body size is," he explains. "Our work strongly suggests that when one person quits smoking, loses weight, or becomes happy, others around her follow suit. I am reluctant to suggest you pick your friends solely on this basis, but one could say that helping a friend do better is a roundabout way of helping yourself."

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