Why do women live longer? Biologists and sociologists have come up with a string of partial answers but they are still undecided whether the natural lifespans of the two sexes are truly different.
Women have been living longer ever since reliable records have been kept: in 1850 women in the United States lived on average 40.5 years and men 38.5. Deaths in childbirth held back female life expectancy until the 1930s, when the introduction of blood transfusions, antibacterial drugs and safe anaesthetics combined to reduce mortality.
The main reason that women live longer is that men do things that are likely to kill them. From the teens to the thirties, men are three times more likely to die than women - from accidents on the roads and in sport and violent crime. More young men commit suicide, and in Britain the numbers are rising, possibly because of a link with unemployment. Men wear seat belts less often than women and the few remaining jobs with high mortality rates - deep sea fishing, for example - are virtually confined to men.
For most of this century more men than women have taken up the self-destructive behaviours of smoking and drinking, and some experts argue that as much as half the current sex difference in longevity is due to differences in smoking in the past. Smoking takes about 40 years to kill people so differences in mortality now are linked with smoking habits from a time when virtually all men smoked. Just as many young women as young men smoke nowadays so this explanation for sex differences in mortality will gradually disappear.
These behavioural differences have - at least in part - a hormonal basis. Male sex hormones seem to encourage risk-taking. Female sex hormones have other advantages, too. The effect of oestrogen on the transport of cholesterol in the bloodstream makes women less likely to have heart disease in middle age, though they catch up on men after the menopause. As more women take hormone replacement treatment in their fifties and sixties, the sex difference in rates of heart disease may be expected to widen.
There may be some differences in survival potential at the most fundamental level. The fact that women have two X-chromosomes may affect life expectancy, according to a recent book (Human Longevity, Oxford University Press, pounds 27.50) by the American gerontologist David Smith. He also observes that men die younger but women are sicker - they have higher rates of arthritis, anaemia and depression. Most of their survival advantage, however, is due to known factors which men could avoid if they chose.