From now on, whenever you go for the next smoke, don't blame your urge. Your gene decides whether you smoke five cigarettes a day or 15. A new study, conducted at the biological psychology department, Vrije Universiteit, the Netherlands, says the level of nicotine addiction is grossly dependent on a person's genetic trait.

After scrutinising the data of smoking behaviour of more than 16,000 twins and their relatives, researcher Jacqueline Vink discovered that in most cases the number of cigarettes smoked per day is settled by the four genes, which normally carry the instructions for making enzymes that detoxify a range of destructive cancer-causing chemicals.

The study, published in the Pharmacogenomics Journal, also examined whether an environment indulges a person to start smoking. Smoking friends and family members encourage one's chance to take it up, Vink and her colleagues say.

While investigating the DNA collected from the participants in the study, they found that regions of chromosomes 6 and 14 contain the elements that provoke one to get addicted. The cues for the numbers of cigarettes that a person smokes per day reside on a region at chromosome 3. And chromosome 10 contains a segment that incites both.

Vink and her colleagues think they now understand why giving up smoking is more difficult for some people than for others. A genetic inclination towards nicotine addiction does not mean that those with it will necessarily become addicted or remain so for ever, they say.

The researchers, working for the de-addiction programme of the Netherlands' Organisation for Health Research and Development, has now embarked on further studies to identify the other causes of nicotine-dependence.