THURSDAY, Dec. 1 (HealthDay News) -- Despite decades of public health efforts aimed at snuffing out cigarette smoking, 20 percent of Americans still light up. New research suggests it might be because of their genes.

While anti-smoking campaigns are credited with slicing cigarette use drastically over the past 40 years -- from 42 percent of all Americans in 1965 to just under 20 percent in 2010 -- the number of people who haven't been able to nix their nicotine habit has flatlined in recent years, according to the U.S. Centers for Disease Control and Prevention.

Two out of three adults who smoke want to quit, a CDC report out earlier this month said, and more than half (52 percent) had attempted to quit in the past year.

The authors of the new study, released online in advance of publication in an upcoming print issue of Demography, say new tactics may be needed to help the remaining smokers.

"Federal and social policies may be somewhat less effective now because maybe the composition of those at risk [those who smoke] has changed," said study co-author Fred Pampel, a professor of sociology at the University of Colorado at Boulder and a research associate at the Institute of Behavioral Science there. Those who can quit easily have probably done so, the authors said.

Study lead author Jason Boardman, an associate professor of sociology, said anti-smoking messages, higher taxes and restrictions on smoking have made a difference. "But for hard-core smokers, there may be something else going on," he said. That "something else" is likely genetics, he added.

The researchers drew this conclusion after analyzing the smoking habits between 1960 and 1980 of nearly 600 pairs of twins who answered an extensive health questionnaire -- 363 were identical sets of twins and 233 were fraternal twins. Identical twins come from the same fertilized egg before it splits into two embryos and they share the same genes or DNA, while fraternal twins come from two separately fertilized egg cells and only share some genetic similarities.

In the identical twin group, 65 percent of both individuals quit within a two-year period of each other, while only 55 percent of the fraternal twins quit within that same stretch of time.

"The logic here is that the identical twins share genes, so if they act alike it probably reflects a genetic component," said Pampel.

The new research adds to a growing body of literature suggesting there is probably a substantial genetic influence when it comes to nicotine addiction, said Dr. Aditi Satti, an assistant professor of medicine and director of the smoking cessation program at Temple University Hospital in Philadelphia. But scientists are still trying to pinpoint the gene or genes involved, she said.

"There has not been one specific gene linked to nicotine addiction," said Satti.

She noted that while numbers of smokers have decreased over the years, smoking is up among women and black Americans, possibly another clue as to who is genetically at risk. Inner city, low socioeconomic and less-educated populations are more likely to be smokers, too, Satti added.

"I think the combo of finding better medications and educating people, even if doctors just spend five minutes talking with your patient about smoking, will lead to higher quit rates," Satti said.

Boardman said a policy shift might be in line. Instead of government anti-smoking campaigns focusing on high taxes and splashy advertisements, he said current smokers may discover more success using medication aimed at nicotine addiction, as well as counseling.

"I'd argue that nicotine replacement therapies may be far more effective with existing smokers still trying to quit than the posters showing images of smokers that are not cool. Behavior-changing efforts -- I don't think that's going to help the two-pack a day smokers," said Boardman, who is also a research associate at the Institute of Behavioral Science.

Smoking-related diseases lead to approximately 443,000 deaths a year in the United States, including babies born prematurely to women who smoke while pregnant and those harmed by secondhand smoke, according to the American Lung Association.

Satti is concerned that cigarette addiction isn't taken seriously enough by some.

"Maybe we don't see smoking as being as important as alcohol and drug addiction, but I'm a pulmonologist and I see chronic obstructive pulmonary disease (COPD), emphysema and lung cancer every day," she said. "And we know tobacco smoke is linked to cardiovascular disease and stroke. It's one of the most preventable things you can do. If you quit smoking, you'll see a huge impact on health."

More information

The American Cancer Society can help you determine if you need help to quit smoking.