BOSTON, May 22 (UPI) -- People who quit smoking in tandem and those who still smoke form separate "cliques" that shift to the periphery of their social network, U.S. researchers said.

Researchers at Harvard University and the University of California, San Diego, said smoking is bad not only for physical health but for social health, too.

The study, published in the New England Journal of Medicine, found the closer the relationship between contacts, the greater the influence when one person quit smoking. When one spouse quit, the other spouse's chances of continuing to smoke decreased by 67 percent. Among friends, the effect was 36 percent. Among co-workers in small firms, 34 percent. Among siblings, the effect was 25 percent but neighbors didn't seem to be influence smoking habits.

Dr. Nicholas Christakis of Harvard Medical School and James Fowler of University of California, San Diego, analyzed changes in smoking behavior from 1971 to 2003 in a large social network of 12,067 densely interconnected people.

Those who continued to smoke, formed their own groups on the periphery of their former social network but once a person quit, they tend to move back to a more central position, Fowler said.