Los Angeles ? The urge to smoke is contagious, but quitting apparently is, too.

A team of researchers who showed that obesity can spread person-to-person has found a similar pattern with smoking cessation: A smoker is more likely to kick the habit if a spouse, friend, co-worker or sibling did.

What’s more, smokers tend to quit in groups and those who don’t stop puffing increasingly find themselves pushed to the edge of their social circles, the researchers found.

“Your smoking behavior depends upon not just the smoking behavior of the people you know, but also the people who they know” and so on, said Dr. Nicholas Christakis, a medical sociologist at Harvard Medical School and lead author of the new report.

The findings back up previous studies showing that peer influence plays a key role in people’s decision to stop lighting up and provide evidence that the “buddy system” used by smoking cessation, weight loss and alcoholism programs to change addictive behavior works.

“Anecdotally, we hear people say they quit smoking because their spouse or friend quit,” said Jennifer Unger, a smoking prevention expert at the University of Southern California who had no role in the study. “If you influence a few people, those people might go on to help others to quit.”

Last year, Christakis and his colleague James Fowler of the University of California, San Diego, published a study suggesting that obesity can spread among friends, much like an infectious disease. The duo mined data from a large social network of people who had been followed for three decades and found that when one person gained weight, close friends tended to pack on the pounds, too.

Their latest study, which appears in Thursday’s New England Journal of Medicine and is funded by the National Institute on Aging, focused on people’s smoking habits in the same social network.

The researchers examined the social lives of 12,067 people in the Framingham Heart Study, which has been tracking the health of residents of that Boston suburb from 1971 to 2003. They were able to reconstruct people’s ties to one another since participants had to list contact information for their family, friends, co-workers and neighbors so researchers would not lose track of them over the years. The prevalence of smokers in the Framingham study over the years mirrored national trends.

Not surprisingly, the greatest influence was seen in close relationships. When a spouse stops smoking, the other partner is 67 percent less likely to smoke. Similarly, when a friend quits, the odds of the other continuing drops by 36 percent. The odds are similar among co-workers and siblings.