That may help explain why some people can quit and others fail, said Dr. Nora Volkow, director of the National Institute of Drug Abuse in Bethesda, Md., which funded one of the studies.

"It opens our eyes," Volkow said Wednesday. "Not everyone takes drugs for the same reason. Not everyone smokes cigarettes for the same reasons."

One clue is in the location of the just-discovered variants, on the long arm of chromosome 15, Volkow said. It is in an area that, when damaged during tests on animals, makes them depressed and anxious. While some people smoke because it helps them focus or gives them a physiological reward, others do it to stave off depression.

That suggests that adding antidepressants to some smokers' treatment could help them kick the habit.

Bierut said a simple, inexpensive test could be developed to screen people for the variants. Kari Stefansson, lead author of the largest of the three studies, agreed. He is chief executive of deCode Genetics of Iceland, which already does prostate cancer genetic tests.

Such testing could carry risks all its own, bioethicist Arthur Caplan of the University of Pennsylvania warned. People who have been found to have a genetic predisposition to addiction and lung cancer could find it harder to get health or life insurance, or their employer might drop their coverage, he said.

"The good news is that getting these risk estimates will help focus anti-smoking campaigns, and some people will want to voluntarily get into anti-addiction programs early, where they will probably work better," Caplan said in an e-mail. But if such testing is done, it should be voluntary, and the results should be kept private, he said.

Smoking-related diseases worldwide kill about one in 10 adults, according to the World Health Organization.

Among the findings:

— Smokers who get the set of variants from only one parent see a risk of lung cancer that is about one-third higher than that of people without the variants. They also smoke about one more cigarette a day on average than other smokers. This group makes up about 45 percent of the population studied.

— Smokers who inherit the variants from both parents have nearly a 1-in-4 chance of developing lung cancer. Their cancer risk is 70 to 80 percent higher than that of smokers without the genetic variants. They smoke on average two extra cigarettes a day. This group accounts for about one in nine people of European descent.