Children whose mothers smoked at least a pack of cigarettes a day while pregnant are more susceptible to nicotine addiction, according to a study by a Harvard scientist.

The study, by Stephen L. Buka of the Harvard School of Public Health, was published in the November issue of The American Journal of Psychiatry.

Buka said he intended to find out “why it is that so many people try cigarettes and why only some of them become addicted.”

Other scientists have hypothesized that the distinction may be genetic—that there is a gene that predisposes some to addiction in general or a gene that determines a person’s response to nicotine.

But Buka and his two co-authors thought the explanation might not be solely genetic.

They set out to show that exposure to nicotine in the womb might actually physiologically change the development of the brain’s nicotine receptors, resulting in an increased risk of nicotine dependence in adulthood.

The study reviewed about 1,200 interviews asking pregnant women about their smoking habits. The scientists then followed up with the women’s children—now adults with an average age of 29—to determine whether there was any association between their exposure to nicotine in the womb and their current smoking habits.

They found that a “significant” number met the criteria for nicotine dependency.

“It’s a neat sort of study,” Buka said in an interview.

He called the study “unique” in its size and scope, and said the public might find it interesting because it addresses “the question of how we become who we become.”

The study highlights its findings’ important public health implications in the United States, where nearly half of all woman smokers continue to smoke through their pregnancy. Half a million babies who were exposed to nicotine in the womb—and are thus predisposed to the habit themselves—are born each year.

The study also examined the association between marijuana use among the mothers and their children, but found no significant correlation. Buka said this aspect of the study tested whether the pattern was specific to nicotine dependence and not to all substance addiction.

Buka added that since the brain continues to develop for two to three years after birth, there’s no reason to think that exposing young children to second-hand smoke wouldn’t have the same effect on their potential for nicotine dependency.

Previous studies have linked smoking while pregnant with a number of other adverse effects, including low birth weight, higher incidence of miscarriage and Sudden Infant Death Syndrome.