Facebook, MySpace and other Web sites have unleashed a potent new phenomenon of social networking in cyberspace. But at the same time, a growing body of evidence is suggesting that traditional social networks play a surprisingly powerful and underrecognized role in influencing how people behave.

The latest research comes from Nicholas A. Christakis, a medical sociologist at the Harvard Medical School, and James H. Fowler, a political scientist at the University of California at San Diego. The pair reported last summer that obesity appeared to spread from one person to another through social networks, almost like a virus or a fad.

In a follow-up to that provocative research, the team has produced similar findings about another major health issue: smoking. In a study published last week in the New England Journal of Medicine, the team found that a person's decision to kick the habit is strongly affected by whether other people in their social network quit -- even people they do not know. And, surprisingly, entire networks of smokers appear to quit virtually simultaneously.

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