Perhaps the brain's makeup and chemistry during human adolescence might make youths more susceptible to the effects of nicotine, said Hanson, former acting director of the National Institute on Drug Abuse.

Studies there and replicated elsewhere on human brain development show that the brain isn't physically mature or fully capable of reason until the late-20s.

"Adolescents have different cognitive abilities — risk-taking and how they see the world," Hanson said. "Among those who smoke, nicotine could also be playing a very critical role in their choices. Perhaps the biochemical and structural makeup makes them more vulnerable to the effects of nicotine. That's just one of the questions raised by the study and where the follow-up research is going to have to go."

The study results also suggest strong behavioral correlations between nicotine and mental health. Doctors and therapists working with patients who are schizophrenic, for example, report that nearly 100 percent are heavy smokers.

They or anyone who smokes or uses any substance is going for some kind of desirable effect or feeling, Hanson said.

Research at NIDA and other studies report that tobacco users say smoking helps them feel more alert or less stressed by daily activities.

Smokers aren't going after cancer or heart disease associated with it, just like alcoholics aren't going after the domestic abuse linked to drinking, he said.

Among the participants in the new study with the gene variation and who started as teens, the metabolism of nicotine likely does heighten awareness and cognition, he said.

"But they appear much more vulnerable to biological changes that will ultimately create a lifelong dependence," he said. "It's finding these little pieces of understanding that will help us ultimately understand why some people use substances, why some use and can quit, and why some can't seem to no matter what."

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E-mail: jthalman@desnews.com