WASHINGTON — Why do some 90-year-old chain smokers avoid lung cancer, while other people who smoke far less wind up dying of the disease? How can some people light up now and then without getting hooked, while others are addicted practically from their first puffs?

The answer, at least in part, may be in your genes.

Scientists have identified certain genetic variations that appear to make people more likely to get hooked on cigarettes and more prone to develop lung cancer.

The findings could someday lead to screening tests and customized treatments for smokers trying to kick the habit.

The discovery by three separate teams of scientists makes the strongest case so far for the biological underpinnings of nicotine addiction and sheds more light on how genetics and lifestyle habits join forces to cause cancer.

"This is kind of a double whammy gene," said Christopher Amos, a professor of epidemiology at the M.D. Anderson Cancer Center in Houston and author of one of the studies. "It also makes you more likely to be dependent on smoking and less likely to quit smoking."

A smoker who inherits these genetic variations from both parents has an 80 percent greater chance of lung cancer than a smoker without the variants, the researchers reported. And that same smoker on average lights up two extra cigarettes a day and has a much harder time quitting than smokers who don't have these genetic differences.

The researchers disagreed on whether the variants directly increased the risk of lung cancer or did so indirectly, by causing more smoking.

The three studies, funded by governments in the U.S. and Europe, were published Thursday in the journals Nature and Nature Genetics.

The scientists studied the genes of more than 35,000 white people of European descent in Europe, Canada and the United States. Blacks and Asians will be studied soon and may yield different results, scientists said.

They aren't quite sure if what they found is a set of variations in one gene or in three closely connected genes. But the gene variations govern nicotine receptors on cells.

"This is really telling us that the vulnerability to smoking and how much you smoke is clearly biologically based," said psychiatry professor Dr. Laura Bierut of Washington University in St. Louis, a genetics and smoking expert who did not take part in the studies. She praised the research as "very intriguing."

The smoking rate among U.S. adults has dropped from 42 percent in 1965 to less than 21 percent now.

The new studies are surprising in that they point to areas of the genetic code that are not associated with pleasure and the rewards of addiction.

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."