U.S. researchers studied changes in smoking habits over three decades
Giving up smoking is contagious - people are more likely to quit in groups than alone, a study suggests.
The research reveals one person's decision to stop smoking often spreads through friends, family and work mates, leading them to quit as well.
The closer the relationship, the greater the influence on the person giving up.
The U.S research, in the New England Journal of Medicine, looked at the habits of more than 12,000 people.
Studying changes over a 30 year period, the researchers found social relationships had a crucial role in deciding whether someone started smoking - and then whether they quit.
The report also claimed that smoking was changing the nature of society.
Where once smokers and non-smokers mixed freely, it said the two groups now tended to form "separate clusters", and suggested smokers were being pushed to the margins of society.
"We've found that when you analyse large social networks, entire pockets of people who might not know each other all quit smoking at once," said researcher Professor Nicholas Christakis, based at the Harvard Medical School.
"So if there's a change in the zeitgeist of this social network, like a cultural shift, a whole group of people who are connected but who might not know each other all quit together."
Professor Christakis said geography did not appear to play a role because smoking behaviours spread between contacts living miles apart and separate households.
"Rather, the closeness of the relationship in the network was key to the spread of smoking behaviours, " he said.
For instance when a husband or wife quit, it decreased the chance of their spouse smoking by 67%.
When a sibling quit, it reduced the chance of smoking by 25% among their brothers and sisters.
A friend quitting decreased the chance of smoking by 36% among their friends.
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Education was also a factor. The higher the educational levels among the contacts, the greater the influence on smoking behaviour.
Professor Christakis said: "We are more influenced by the quitting behaviour of others if those people are highly educated.
"To add a further twist, we are also more influenced by others if we ourselves are more educated."
Dr Martin Hagger, a health psychologist from the University of Nottingham said social support is a key factor in giving up smoking.
"People like to be in the same boat. People like to empathise with each other. It happens with other health behaviours such as getting fit - support from people close to you really helps."
Dr Hagger said government initiatives such as putting tax on cigarettes or banning smoking in public places can only go so far.
He said these need to be part of a coordinated approach to change behaviour, but he advises those wanting to quit smoking to do it in a group.
"If you are trying to give up don't keep it a secret, try and recruit people from your own social network. It gives you a sense of group identity and you are more likely to go along with what is expected in the group.
"It's not so important in the early stages of giving up smoking but if the social network is consistent in supporting the attempt to quit then it is much more likely a person will not relapse back," he added.
The study, funded by the US National Institute on Aging, based the research on a community based study of 12,067 people between 1971 and 2003. The group ranged in age from 21 to 70 and people smoking one or more cigarettes a day were deemed smokers.