A woman smokes in a restaurant in Mexico City, Wednesday, April 2, 2008. A law banning smoking in virtually all public spaces in Mexico City , exceptions include parks, soccer stadiums and outdoor seating areas, goes into effect Thursday. (AP Photo/Alexandre Meneghini)

Washington ? Scientists say they have pinpointed a genetic link that makes people more likely to get hooked on tobacco, causing them to smoke more cigarettes, making it harder to quit, and leading more often to deadly lung cancer.

The discovery by three separate teams of scientists makes the strongest case so far for the biological underpinnings of the addiction of smoking and sheds light on how genetics and cigarettes join forces to cause cancer, experts said. The findings also lay the groundwork for more tailored quit-smoking treatments.

“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 this genetic variation 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 three studies, funded by governments in the U.S. and Europe, is being published today in the journals Nature and Nature Genetics.

The scientists surveyed genetic markers in more than 35,000 people in Europe, Canada and the United States, zeroing in on the same set of genetic differences. They aren’t quite sure if what they found is a set of variations in one gene or in three closely connected genes. But they said the result is the same: These genetic quirks increase the risk of addiction and lung cancer.

The studies’ authors disagreed on whether the set of variants directly increased the risk of lung cancer or did so indirectly by causing more smoking that led to the cancer.

The genetic variations, which encode nicotine receptors on cells, could eventually help explain some of the mysteries of chain smoking, nicotine addiction and lung cancer that can’t be chalked up to environmental factors, brain biology and statistics, experts said. These oddities include why there are 100-year-old smokers who don’t get cancer and people who light up an occasional cigarette and don’t get hooked.

In the last 40 years, the rate of adult Americans smoking has been cut from 42 percent in 1965 to less than 21 percent now.

The new studies point to surprising areas of the genes not associated with pleasure and addiction rewards. That may help explain why people have trouble quitting, said Dr. Nora Volkow, director of the National Institute of Drug Abuse in Bethesda, Md., which funded one of the studies. Eventual testing for the genetic variants could lead to custom treatments for quitting smoking.

“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, and a genetics and smoking expert who did not take part in the studies. She praised the research as “very intriguing.”