Smoking may be addictive but quitting is contagious, according to a provocative study of why people give up the weed.

The act of quitting passes through a social network like an outbreak of flu. Husbands are likely to pass it on to their wives, or vice versa, workers to colleagues, brothers to sisters, friends to friends.

But neighbours seldom catch it over the garden fence. Mere proximity is not enough. You have to know somebody who is quitting, and the better you know them, the more likely you are to give up yourself.

The findings, published in The New England Journal of Medicine, come from 32 years of data from a network of more than 12,000 people who participated in the Framingham Heart Study. The study, launched in 1948 in Framingham, Massachusetts, has provided the strongest evidence of the links between diet, lifestyle, and heart disease.

The data included smoking habits and, over the years, many participants gave up. The information was mined by Nicholas Christakis, of Harvard Medical School, and James Fowler, of the University of California in San Diego.

The doctors set out to find patterns in the way people quit, using the links between them that were recorded.

At regular intervals since 1971, participants recorded births, marriages, divorces and deaths, as well as listing contact information for their closest friends, co-workers, and neighbours.

The doctors’ analysis of the data showed that:

— When a husband or wife quit, the chance that their spouse would smoke fell by 67 per cent;

— When a friend quit, the chances of smoking among their other friends fell by 36 per cent;

— In small firms, a worker quitting would reduce smoking by his or her colleagues by 34 per cent. But in big firms the effect was insignificant;

— When a brother or sister quit, the other sibling was 25 per cent less likely to smoke.

“Interestingly, geography did not appear to play a role because smoking behaviours spread between contacts living miles apart and in separate households,” Dr Christakis said.

“Rather, the closeness of the relationship in the network was the key to the spread of smoking behaviours.” The quitting behaviour is not literally contagious, but a response to the changing social climate that is passed on through people who are linked.

“If there’s a change in the Zeitgeist of this social netwrok, like a cultural shift, a whole group of people who are connected but who might not know each other all quit together,” Dr Christakis said.

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