Genetics likely play a role in nicotine addiction, researchers say, although no single gene has yet been identified.

Anti-smoking campaigns have help reduce the number of smokers in the U.S. from 42 percent in 1965 to less than 20 percent as of last year. But the number of people smoking has remained about the same in recent years, despite two out of three wanting to quit, says the Centers for Disease Control and Prevention.

Researchers at the University of Colorado believe the difficulty may lie in a person's genes.

They analyzed data from about 600 pairs of twins about smoking habits between 1960 and 1980. More than half were identical sets of twins; the rest were fraternal twins. Among the identical twins, 65 percent quit within two years of each other; 55 percent of the fraternal twins did so.

"The logic here is that the identical twins share genes, so if they act alike it probably reflects a genetic component," said study co-author Fred Pampei, a professor of sociology.

"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," said lead author Jason Boardman, an associate professor of sociology.

Behavior-changing efforts won't help the two-pack-a-day smokers, he added.

An estimated 450,000 people die each year from smoking-related causes, says the American Lung Association.