Quitting smoking appears to be contagious, according to a study published May 22 in the New England Journal of Medicine.

Smoking kills about 400,000 people in the U.S. alone every year, according to the Centers for Disease Control and Prevention. About 45 million US adults are smokers, though the prevalence has fallen dramatically since the 1960s, when 42 percent of the population was smoking.

Dr. Nicholas Christakis of Harvard Medical School and James Fowler of the University of California tracked 12,067 people who participated in the Framingham Heart Study from 1971 until 2003, analyzing them as part of a large network of relatives, co-workers, neighbors, friends and friends of friends. The Framingham study, launched in 1948, has provided the strongest evidence of the links between diet, lifestyle, and heart disease.
According to results, spouses have the most influence on another person’s smoking behavior. If a partner gave up smoking, there was a 67 percent chance their significant other would, as well.

The study also found that a friend quitting decreased the chance of smoking by 36 percent among their friends, while when a sibling quit, it reduced the chance of smoking by 25 percent among their brothers and sisters.

Even co-workers are influential, the results show, as a quitter could decrease smoking among colleagues by 34 percent.

Overall, closeness was found to be of big importance in someone’s decision to give up smoking. The better you know someone quitting the habit, the higher are your chances to quit too.

“Interestingly, geography did not appear to play a role because smoking behaviors spread between contacts living miles (km) apart and in separate households. Rather, the closeness of the relationship in the network was key to the spread of smoking behaviors,” Christakis said according to Reuters.

Also, education played an important role as “we are more influenced by others if we ourselves are more educated.”

In an editorial accompanying the study, Steven Schroeder, a professor of health and health care at the University of California, San Francisco, warned that those continuing on their smoking habit were more likely to be marginalized, regardless of their education and income level.

“A risk of the marginalization of smoking is that it further isolates the group of people with the highest rates of smoking—persons with mental illness, problems with substance abuse or both. These people are already stigmatized by their underlying psychiatric condition. Adding the further burden of the stigma associated with smoking makes it even harder for them to achieve the wellness that they and their families seek,” Prof. Schroeder wrote.