Now, researchers have discovered one reason why: It's in the genes.
A new study published Friday by researchers in Utah, Minnesota and Wisconsin found a particular genetic variation puts smokers at a higher risk of being heavy smokers as adults. But the variation only seems to matter if the smoker started at age 16 or younger. People with the same variation who started at age 17 or older didn't have the same risk of dependence later on.
The researchers and others see the results as further proof that teens shouldn't smoke.
"Did we already want to keep kids from smoking? Of course," said Wilson Compton, director of the National Institute on Drug Abuse's division of epidemiology services and prevention research. "This gives [us] extra reasons to understand why particularly eliminating early use of tobacco is important."
The federal agency helped fund the study, which was published in the Public Library of Science's online journal PLoS Genetics.
Glen Hanson, director of the Utah Addiction Center, lauded the study as "incredible" and said it could lead to new prevention methods and treatments, including medications targeted to people with the variation.
That will take further research to discover the effect on the brain from variations in a gene cluster called nAChR, which governs nicotine receptors and binds nicotine in the nervous system.
Hanson, who wasn't involved in the study, gave an example of what new research could find: For example, if the variation alters the way people respond to stress, a known trigger for smoking, counselors could help those susceptible teens deal with anxiety.
"Now we can get beyond the message: 'Don't smoke. It's bad for you,' " Hanson said at a news conference. "If we can protect them until they're young adults, the chances they're going to become addicted are very small."
The genetic variations in the study are common among the Americans of European ancestry, the population that was studied. About 38 percent of that loosely-defined ethnic group have the variation that makes them more susceptible to heavy smoking. By comparison, less than 10 percent of Americans with Asian and African roots have it.
On the other hand, 20 percent of the European-American population have a variant that actually reduces their risk of becoming heavy adult smokers.
Researchers don't know why. Nor can they say why starting smoking after age 16 seems to neutralize the genetic variation that puts younger smokers at risk. Studies in rats suggest smoking may be more addictive when it's started in adolescence.
Previous research has identified how having versions of certain genes makes people more or less likely to smoke, quit or have lung cancer. The Utah study, authored by several University of Utah researchers from various fields, was the first to show the relationship between age, genetics and smoking, said Robert Weiss, lead author and human genetics professor.
Brandon Smart, of Ogden, may have a genetic basis for half-a-pack-day-habit. The 32-year-old started at age 15. He's tried to quit, but the longest he has lasted without a cigarette is about five days. He said had he known he might be more susceptible to addiction because of his genes, he may have avoided it. Knowing alcoholism runs in his family, Smart generally steers clear of drinking, he said.
But he questions whether teens will care if they learn their genetic risk. "When you're a teenager, you tend to not believe what adults are telling you," he said.
And some doctors saying knowing your genetic risk may even cause problems.
Writing in The Journal of the American Medical Association in 2006, Seattle doctors Chris Carlsten and Wylie Burke point out several potential problems: Patients blaming their genes for smoking and refusing to try to quit; nonsmokers starting after finding they don't have an increased risk of cancer from smoking; doctors unmotivated to help smokers who appear to have little chance of success; cigarettes marketed to those with a low genetic risk.
"In the case of smoking, the appropriate public health stance seems clear: Smoking cessation should be encouraged uniformly. From this perspective, the potential harms of genetic testing could be significant," the doctors wrote.
