REG DEAN, who died on January 5th at the ripe age of 110, was unusual. Centenarians are rare in themselves, of course, but male centenarians particularly so. In Britain, where Mr Dean lived, five times as many women as men receive the famed card of congratulation from the queen when they celebrate their 100th birthdays. That may, however, cease to be the case in the future, for the fact that women tend to live longer than men, though still true, is less true than it was, and the gap is shrinking—in rich countries, at least—every year.

In England and Wales, the biggest peacetime difference between the life expectancies at birth of the two sexes was 6.3 years. That was in 1967. It is now 4.1 years, and falling. In the early 1980s women who made it to 65, the traditional age of retirement for British men, could expect to live four years longer than their male counterparts. The gap now is less than three years (see chart), though there is still some way to go before things return to the nine-month gap that prevailed in the 1840s, when records began. Other industrialised countries, except Japan and Russia, show something similar. This trend is superimposed on another: that life expectancy in most developed countries has been improving for both sexes. But of late it has been improving more for men than for women. A report about to be published by the Longevity Science Advisory Panel (a group of scientists and actuaries set up by Legal & General, an insurance company) examines the factors behind these trends. The biggest by far is changes in the use of tobacco. All are equal

Around half the difference in the longevity of the sexes can be explained by smoking. One reason why Russia bucks the trend towards equal life expectancies for the two sexes (women there live 12 years longer than men) is that its men have not followed their Western confrères and cut down on the cancer sticks.

In Britain in the 1960s, when the habit was commonplace, men were much more likely to be smokers than women. But they have also been more likely than women to give up cigarettes over the past half-century. As a result, between 1979 and 2009 male smoking-related deaths fell by 64% and the male-to-female ratio of such deaths fell from 2.1 to 1.7. Deaths from cancers of the lung, trachea and bronchus in particular fell by 39% between 1991 and 2005 among Englishmen over 49. For women the comparable figure was 3%.

A further fifth of the longevity gap between the sexes is explained by alcohol. In this case, however, the gap is widening. In 1979 two men died from alcohol-related causes for every woman who succumbed. In 2009 it was 2.4.

A third important factor is obesity—or, rather, the physiological complications obesity brings, such as high blood pressure and type-2 diabetes. On the face of things, there is little difference between the sexes in this area: 15.6% of women in the EU are obese, compared with 15.4% of men. But obesity may have a greater impact on women because it increases the risks of both hypertension and diabetes in their sex more than it does in men. And men are also closing the gap in another area related to obesity and high blood pressure: coronary heart disease. In England, deaths from this fell more than 50% between 1991 and 2005 for both men and women. But, because heart disease kills twice as many men as it does women, the reduction in the male mortality rate has been greater.

All of which is good news if you are male. Men do, nevertheless, have the deck stacked against them by biology. One way the cards are marked is that female mammals (women included) have two X chromosomes, whereas males have an X and a Y—the latter being a runty little thing with only a small complement of genes. Females’ “spare” X chromosome protects them from genetic mutations on the other one. Males have no such protection. Women are thus carriers of, but rarely suffer from, diseases like haemophilia which are caused by the mutation of X-chromosome genes. In birds, by contrast, it is the males who have matched chromosomes while females sport the runt. As a result, male birds tend to outlive their mates.

A further biological difference between the sexes is in the lengths of their telomeres. These are sections of DNA that protect the ends of chromosomes from decay. Men’s telomeres are shorter than those of women, and also degrade more quickly. Both of these attributes have been linked to reduced lifespans.

But some are more equal than others

The biggest biological difference between health of the sexes, however, can be summed up in a single word: testosterone. Testosterone is the hormone that more or less defines maleness (though women have it too, in lesser quantities). It promotes both aggression and risky behaviour. It also suppresses the immune system, which is why castrated tomcats and rams live longer than those that have not been neutered. The same applies to people. A study on eunuchs found they live 13.5 years longer than men who are intact.

Testosterone-driven behaviour means that men are more likely than women to die in accidents, and more likely to die from the violence of others. They are also more likely to kill themselves. These things are particularly true of young adults. Men are two-and-a-half times more likely to die in their 20s than women are. Testosterone may also explain the differences between the sexes in risky behaviours like smoking and drinking.

But blaming testosterone for male risk-taking explains only the “how”, not the “why”. For that, you must turn to evolutionary biology. It is no coincidence that the gap between the sexes’ mortality is widest in people’s 20s. This is the peak period for reproduction. Men are fighting each other, and showing off to the girls, in a competition whose prize is, in an evolutionary sense, immortality itself—the passage of their genes to the next generation.

To stake a claim in the afterlife, as any religion will tell you, you must make sacrifices in the present one. In actuarial terms, therefore, the Longevity Science Advisory Panel reckons that even if men adopt healthy lifestyles, women will continue to outlive them. A gap of between one and two years of life expectancy (at age 65) will persist indefinitely. That, if you are a man, might seem unfair. But if it does, then think of it as the price of eternity.