The effect was particularly strong if young people were exposed to a parent's tobacco use before their teen years, Dr Stephen E Gilman of the Harvard School of Public Health in Boston and his colleagues found. But they also found that in children of ex-smokers "that risk goes away if parents quit," Gilman explained in an interview.

While there is mounting evidence that children of smokers are more likely to be smokers themselves, less is known about whether one parent has a stronger effect than the other, and whether the influence of parents on their offspring's smoking behaviour is the same throughout childhood and adolescence, Gilman and his team note.

To investigate, they looked at 559 boys and girls ages 12 to 17. The researchers also spoke with one parent of each adolescent participant.

Among parents, 62.4 per cent had ever smoked in their lives, while 46 per cent had met criteria for nicotine dependence during their lifetime.

Overall, 27.8 per cent of the adolescents reported having used cigarettes, with the prevalence of use increasing with age; 7.2 per cent of 12-year-olds said they had smoked, while 61.3 per cent of 17-year-olds did.

Each parent independently influenced the likelihood that a young person would start smoking, the researchers found. A mother's smoking affected sons and daughters' risk equally, but a father's smoking had a stronger effect on boys than girls, and the smoking habits of fathers who did not live with their families had no affect on offspring's smoking risk. The longer a parent smoked, the greater an adolescent's likelihood of starting smoking. Whether or not the parent was actually dependent on nicotine didn't affect the strength of the relationship.

"What was striking to us is that the effects were strongest at younger ages," Gilman said. Children who were 12 or younger when their parents were actively smoking were about 3.6 times as likely to smoke as children of non-smokers. But the adolescents who were 13 and older when their parents smoked were only about 1.7 times more likely to use tobacco.