Meet "network man." He has basic desires of his own, but has many arbitrary preferences, such as in music or clothes, that have been influenced by the people he knows.

Network man's likes and dislikes, in turn, affect the behavior of his friends, and their friends, and their friends. For example, when he gets into an obscure indie rock band, he shares an album with his friend, who likes it so much that he recommends it to his cousin, who spreads the word to her friends.

This is the view of human behavior put forth in "Connected," a new book by Dr. Nicholas Christakis, professor at Harvard University, and James Fowler, associate professor at the University of California, San Diego.

Examining years of research of their own and from others, the authors conclude that social networks, both offline and online, are crucial in understanding everything from voting patterns to the spread of disease.

People have profound influences on each other's behavior within three degrees of separation, the authors find. That means that your friends, your friends' friends, and your friends' friends' friends may all affect your eating habits, voting preferences, happiness, and more. At the fourth degree, however, the influence substantially weakens. Read about their research on how happiness is contagious

These are not small effects. "If a mutual friend becomes obese, it nearly triples a person's risk of becoming obese," the book said. Even geography doesn't matter; you're still at risk for gaining weight if a friend 1,000 miles away gets bigger. Listen to James Fowler talk about social networks

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