If there are any men left who still believe that women are the weaker sex, it is long past time for them to think again. With respect to that most essential proof of robustness—the power to stay alive—women are tougher than men from birth through to extreme old age. The average man may run a 100-meter race faster than the average woman and lift heavier weights. But nowadays women outlive men by about five to six years. By age 85 there are roughly six women to every four men. At age 100 the ratio is more than two to one. And by age 122—the current world record for human longevity—the score stands at one-nil in favor of women.

So why do women live longer than men? One idea is that men drive themselves to an early grave with all the hardship and stress of their working lives. If this were so, however, then in these days of greater gender equality, you might expect the mortality gap would vanish or at least diminish. Yet there is little evidence that this is happening. Women today still outlive men by about as much as their stay-at-home mothers outlived their office-going fathers a generation ago. Furthermore, who truly believes that men’s work lives back then were so much more damaging to their health than women’s home lives? Just think about the stresses and strains that have always existed in the traditional roles of women: a woman’s life in a typical household can be just as hard as a man’s. Indeed, statistically speaking, men get a much better deal out of marriage than their wives—married men tend to live many years longer than single men, whereas married women live only a little bit longer than single women. So who actually has the easier life?

It might be that women live longer because they develop healthier habits than men—for example, smoking and drinking less and choosing a better diet. But the number of women who smoke is growing and plenty of others drink and eat unhealthy foods. In any case, if women are so healthy, why is it that despite their longer lives, women spend more years of old age in poor health than men do? The lifestyle argument therefore does not answer the question either.

As an experimental gerontologist, I approach this issue from a wider biological perspective, by looking at other animals. It turns out that the females of most species live longer than the males. This phenomenon suggests that the explanation for the difference within humans might lie deep in our biology.

Many scientists believe that the aging process is caused by the gradual buildup of a huge number of individually tiny faults—some damage to a DNA strand here, a deranged protein molecule there, and so on. This degenerative buildup means that the length of our lives is regulated by the balance between how fast new damage strikes our cells and how efficiently this damage is corrected. The body’s mechanisms to maintain and repair our cells are wonderfully effective—which is why we live as long as we do—but these mechanisms are not perfect. Some of the damage passes unrepaired and accumulates as the days, months and years pass by. We age because our bodies keep making mistakes.

We might well ask why our bodies do not repair themselves better. Actually we probably could fix damage better than we do already. In theory at least, we might even do it well enough to live forever. The reason we do not, I believe, is because it would have cost more energy than it was worth when our aging process evolved long ago, when our hunter-gatherer ancestors faced a constant struggle against hunger. Under the pressure of natural selection to make the best use of scarce energy supplies, our species gave higher priority to growing and reproducing than to living forever. Our genes treated the body as a short-term vehicle, to be maintained well enough to grow and reproduce, but not worth a greater investment in durability when the chance of dying an accidental death was so great. In other words, genes are immortal, but the body—what the Greeks called soma—is disposable.

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Or at least that’s the idea I proposed in the late 1970s. Since then, the evidence to support this disposable soma theory has grown significantly—something I wrote about in Scientific American in September [“Why Can’t We Live Forever?”]. In my own laboratory some years ago we showed that longer-lived animals have better maintenance and repair systems than short-lived animals do. The longer-lived animals are also the smarter ones, or the bigger ones, or the ones like birds and bats that evolved adaptations such as wings to make their lives safer. If you can avoid the hazards of the environment for a bit longer by flying away from danger or being cleverer or bigger, then the body is correspondingly a bit less disposable, and it pays to spend more energy on repair.

Could it be that women live longer because they are less disposable than men? This notion, in fact, makes excellent biological sense. In humans, as in most animal species, the state of the female body is very important for the success of reproduction. The fetus needs to grow inside the mother’s womb, and the infant needs to suckle at her breast. So if the female animal’s body is too much weakened by damage, there is a real threat to her chances of making healthy offspring. The man’s reproductive role, on the other hand, is less directly dependent on his continued good health.

It is too extreme to say that for all biology cares, males need only to attract a mate and then can pretty much die. A study of children in Tanzania, for example, showed that children who lost a father before the age of 15 tended to be a little shorter than their peers, and height is a reasonably good proxy for health. But children who lost a mother fared even worse—they were shorter, poorer and did not live as long as fatherless orphans. From an evolutionary point of view, however, the drivers of mating success for males are generally not the drivers of longevity. In fact, high levels of testosterone, which boost male fertility, are quite bad for long-term survival.

Women may still struggle to achieve equality in many spheres of life. To be less disposable, however, is a blessing that offers some compensation. There is evidence from studies in rodents that cells in a female body do repair damage better than in the body of a male and that surgical removal of the ovaries eliminates this difference. As many dog and cat owners can attest, neutered male animals often live longer than their intact counterparts. Indeed, the evidence supports the notion that male castration might be the ticket to a longer life.

Might the same be true of humans? Eunuchs were once members of the elite in many societies. In China, boys were castrated to enable them to serve the emperor without the risk of impregnating his concubines. In Europe, such extreme practices were used to retain the singing qualities of boys as they moved into adolescence.

The historical record is not good enough to determine if eunuchs tend to outlive normal healthy men, but some sad records suggest that they do. A number of years ago castration of men in institutions for the mentally disturbed was surprisingly commonplace. In one study of several hundred men at an unnamed institution in Kansas, the castrated men were found to live on average 14 years longer than their uncastrated fellows. Nevertheless, I doubt that many men—myself included—would choose such a drastic remedy to buy a few extra years.