ANSWERING the question of why people smoke tobacco is reasonably easy. Tobacco plants have evolved a chemical called nicotine that locks into particular molecular receptors in the outer membranes of certain animal nerve cells. Once there, it stimulates those cells in ways that they were never intended to be stimulated. If the animal in question is an insect, the result is lethal—which, from the plant's point of view, is a good outcome. But in a big, bulky animal such as a human, a small amount of nicotine produces a pleasant sensation (though enough of the stuff can kill a human, too).

Nicotine has a second effect, though. It induces semi-permanent changes in the ways the nerves it stimulates talk to each other. The result is that those nerves are uncomfortable without it, and the owners of those nerves become addicted to smoking dried tobacco leaves.

Just how many of those leaves an addict needs seems to vary from person to person, and here the reason is not well known. Or, rather, it wasn't. For Hidetoshi Nakamura of the Tokyo Electric Power Company Hospital, in Japan, and his colleagues have just thrown some light on the issue. In a paper published in the European Respiratory Journal they suggest that the number of packs a smoker smokes is a consequence of which versions of a particular enzyme he has in his body—in other words, of his genetics.

The enzyme in question is called, rather inelegantly, CYP2A6. It is part of a family of toxin-destroying enzymes known as the cytochrome P450s, and one of its jobs is to convert nicotine into a less harmful chemical called cotinine that can then be excreted. The gene that encodes CYP2A6, however, comes in three varieties, each resulting in a different form of the enzyme. On top of that, some people lack the gene altogether.

Dr Nakamura and his team looked at 200 regular smokers over the age of 50, to see if the particular varieties of CYP2A6 that those people had could be correlated with their smoking habits. They could. Those with two copies of the commonest form of the gene (one copy inherited from each parent) smoked most. Those individuals with rarer forms smoked less, and those completely without the enzyme smoked least.

That, paradoxically, is because the commonest form of the enzyme is also the most effective at detoxifying nicotine while, of course, an absence of CYP2A6 means that the drug must be detoxified by other, slower, routes. In people with effective enzymes, nicotine vanishes rapidly, so they need another cigarette soon. The less effective the enzyme, the fewer cigarettes you need to smoke to keep your drug levels up. So what natural selection has favoured as healthy may end up killing you faster. But then, there were not many tobacconists in the African savannah where humanity evolved.