The gender gap, as academics call this difference, is very new, a 20th-century phenomenon. Yet strangely it is now a worldwide universal. The difference is biggest in the West and smallest in the Middle East and south Asia, but right across the globe women are living longer.

What is the cause? Despite its newness, some observers happily postulate a biological root to the gender gap, suggesting that there is something intrinsically duff about the male sex. They have a point, for it's not only the human male that has trouble staying alive. In mammals, birds, reptiles, fish and even the most primitive life forms, males come off worse. Quite why is something of a mystery. Moreover, an examination of the causes of death in humans reveals that males figure disproportionately in each of the 12 most common causes, heart disease and cancer being the principal killers.

Now step back to the beginnings of human life - in the womb - and note that about 1.2 males are conceived for every female. Yet the ratio at birth is almost l:l. Far more male embryos are spontaneously aborted or die during pregnancy. If human life expectancy were measured from conception, rather than birth, we would be looking at a gender gap not of 6 years, but more like 16.

One possible biological problem with males focuses on genetics, and in particular the sex chromosomes. Mammalian females possess two X chromosomes, males only one. Quite simply, when something goes wrong with a woman's X chromosome she has a spare one in reserve. A man has nothing. Disorders like haemophilia that are caused by faults on the X chromosome are therefore more common in males. Such diseases are rare and cannot alone explain the gender gap, but it may be that X chromosome faults contribute to disease more subtly.

As if dodgy sex chromosomes are not enough, there are the testicles. Recent scientific concern with the male genitalia has centred around the woefully declining sperm count of the human male. Little attention has been paid to the potential benefits of having no testicles at all. Yet an extraordinary 'natural experiment' in an American mental hospital early last century found that patients who were castrated (in the dubious hope of controlling their violent behaviour) lived far longer - on average to 69 years of age whereas intact males managed a mere 56 (at the time). Perhaps thankfully, these results have never been replicated but it is well known that castration increases the life span of other male mammals.

Two American medics, William Hazzard and Deborah Applebaum-Bowden of Wake Forest University in North Carolina, believe the killing agent is not so much the testicles as the male sex hormones they produce. They argue that male sex hormones contribute to heart disease by increasing the levels of LDL (low-density lipoprotein) or 'bad cholesterol' in the blood. Conversely, female sex hormones, the oestrogens, raise the levels of HDL (high-density lipoprotein) or 'good cholesterol'. Stephen Seely, a Manchester cardiologist, notes that high HDL levels in women also aid nutrient transfer from mother to fetus. The demands of pregnancy make women good at storing nutrients, such as cholesterol, in the body. He suggests that this means that they are better than men at coping with the excesses of a modern Western diet. In short, gluttony, in the shape of heart disease, kills more men than women.

Others are less convinced that biology is at the heart of men's earlier demise. They point instead to the reckless male lifestyle. Men overeat more than women, drink more, visit the doctor less and used to smoke more. Men are less health-conscious. Throw in the odd war, murder, suicide and motor accident (in all of which men figure more prominently) and one can begin to see what is going wrong.

Sociobiology neatly explains all this male recklessness as a kind of display to attract a mate. 'I'm such a fit male,' goes the story, 'that I can afford to eat, smoke and drink too much and generally toy with my good health.' That may seem a bit fantastic, but it's harder to dispute that, in the cause of finding a mate, the male mammal has evolved to become bigger and stronger than his female contemporary. And being big and strong isn't easy: it costs time and energy that could be spent fighting disease and staying alive. For the young male mammal, such as a deer or baboon, successful breeding is paramount and his lifespan suffers accordingly in fighting off competitors for mates. Modern humans do not have to mate like that so the relevance of this to 20th-century man, however, is problematic. The question we should ask is, what has been the biggest change affecting male or female health, especially in the West, during the past century?

The best candidate is the dramatic decline in female fertility. Look at it this way. Women have evolved, bearing seven, eight or more children. One presumes this demanded an extremely efficient and resilient physiology. Suddenly they are having fewer children and finding life far easier. Are they benefiting in terms of increased life span? Proof is impossible but the evidence is quite strong. Compared with the death rates of 60 or 70 years ago, today's figures show a sharp decline in female deaths during the child-bearing years. Nowadays female death rates are lower than males in every single age group.

The male campaign, for an equal life, is only just beginning.