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 Thursday 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…