People may be being let down by their genes when they fail to give up smoking. Scientists have identified genetic variants that increase a person's likelihood of becoming a lifelong heavy smoker.
Those with the variants are more easily hooked as teenagers and quickly progress to smoking 20 or more cigarettes a day.
As adults, they find it harder to quit the habit than individuals with a different genetic make-up.
Researchers studied almost 1,000 New Zealanders from birth to the age of 38 to identify those at a greater genetic risk of smoking.
"Genetic risk accelerated the development of smoking behaviour," said study leader Dr Daniel Belsky, from Duke University in Durham, US. "Teens at a high genetic risk transitioned quickly from trying cigarettes to becoming regular, heavy smokers."