A particular set of genes can turn a teenager who experiments with tobacco into a life-long addicted smoker, research has shown.
Around 40% of people with European origins have "high risk" versions of the genes, which affect the brain's sensitivity to nicotine.
They are in danger of getting hooked if they start smoking before the age of 17, scientists found.
Other variations of the same genes work the opposite way and help prevent addiction.
Scientists in the US at the University of Utah studied 2,827 European American smokers, recording their level of nicotine dependence and smoking history.
Participants were asked what age they were when they started smoking, the number of years they had smoked, and the average number of cigarettes they got through per day.
DNA samples were also taken. The researchers looked for single-letter changes in the genetic code, known as single nucleotide polymorphisms (SNPs), linked to nicotine addiction.
Individuals were placed into one of four groups or "haplotypes" according to what gene variations they had.
Those who began smoking before the age of 17 and had two copies of the "high risk" genes were between 1.6 and five times more likely than normal to become an adult heavy smoker.
For people who began smoking at 17 or older, the high-risk haplotype had no significant influence on future addiction.
