Children with mothers who smoke cigarettes are more likely to be regular marijuana users by early adulthood, a new study suggests.

Part of the link seems to be explained by the fact that children of smokers were more likely to have been rebellious and aggressive as teenagers, the Australian researchers note.

Past studies have found that children of smokers are more likely than their peers to take up the habit themselves; less is known about whether parents’ smoking and drinking habits are related to their children’s marijuana use.

However, many people who use the drug first try it as a teenager, the authors note, and family environment is an important influence on teenagers’ behavior.

To study the question, Dr. Mohammad Reza Hayatbakhsh of the University of Queensland in Brisbane and associates used data from a project that began following a group of pregnant women in Brisbane between 1981 and 1983. The women had completed questionnaires on their health and lifestyle habits - including smoking and drinking - while they were pregnant, and at several other points as their children grew up.

The researcher then evaluated nearly 3,200 of these women’s offspring who were 21 years old, and had been followed since birth. The findings are published in the American Journal of Epidemiology.

In general, the study found that children who were exposed to their mothers’ smoking as teenagers were twice as likely as their peers to be frequent marijuana users at age 21.

The children of smokers were also more likely to start smoking cigarettes by age 14. Further analysis found a relationship between maternal drinking and child marijuana, but further analysis indicated this relationship was not statistically significant.

Early smoking has been linked to a higher likelihood of marijuana use, explained lead study author Hayatbakhsh told Reuters Health. A “simple message” from these results is that young people’s substance abuse is often a “consequence of the learning process.”

“Children who are exposed to parents’ smoking cigarettes may learn this behavior.”

“In other words,” Hayatbakhsh said, “parents…who continue to smoke cigarettes during the development of the child not only put themselves at risk of health problems, but also may play as a role model for the children who live with them.”

SOURCE: American Journal of Epidemiology, September 1, 2007.