By Markham Heid
February 27, 2019

The numbers don’t lie: women tend to live longer than men. The average American man will live to age 76, according to the latest CDC figures, while the average woman in America will live to age 81.

And a woman’s extra years tend to be healthy ones. The World Health Organization’s HALE index, which calculates the number of years a man or woman can expect to live without a major disease or injury, finds that American men can look forward to 67 healthy years, while American women will enjoy 70 years of “full heath.”

This male-female lifespan gap is not a new phenomenon; experts have known about it for decades. It’s also not restricted to Americans. “This gender gap in life expectancy is true for all societies, and it is also true for the great apes,” says Dr. Perminder Sachdev, a professor of neuropsychiatry at the University of New South Wales in Australia who has studied human longevity.

Why do women tend to outlive men? Sachdev says there are a few popular theories—some to do with biology, and some to do with behavior.

“Men are more likely to smoke, drink excessively and be overweight,” he says. “They are also less likely to seek medical help early, and, if diagnosed with a disease, they are more likely to be non-adherent to treatment.” On top of all that, he says, men are more likely to take life-threatening risks and to die in car accidents, brawls or gun fights.

There’s evidence that a man’s biology—namely, his elevated levels of the male sex hormone testosterone—may lead him into the kind of trouble that could shorten his life. Research from Duke University has found that elevated testosterone levels are associated with risky behaviors.

Experts say testosterone may abbreviate a man’s lifespan in other ways. “Male sex hormones decrease immune function and increase the risk for cardiovascular diseases,” says Kyung-Jin Min, a professor of biological sciences at Inha University in South Korea.

In a 2012 study published in the journal Current Biology, Min and his colleagues examined the historical health records of 81 Korean eunuchs: men who were castrated as children, and who therefore stopped producing much testosterone. They found the eunuchs tended to live about 14 to 19 years longer than uncastrated men who shared their same socio-economic status.

While the links between testosterone and immune function aren’t clear, Min’s study points to lab research showing that testosterone may block the release of some disease-fighting immune cells. On the other hand, there’s also a good amount of research linking low levels of testosterone to heart disease and poor health outcomes in men, so the relationships between testosterone and a man’s health are complex.

It may well be that a man’s hormones aren’t to blame; instead, a woman’s hormones may offer her some added lifespan benefits.

“Estrogen appears to be protective—it has been shown to have an antioxidant role,” says Sachdev. A 2013 review in the International Journal of Endocrinology found evidence that estrogen can prevent the kind of DNA damage that leads to disease. That review also turned up evidence that estrogen can help maintain normal, healthy cell function.

These sorts of findings help explain the male-female longevity gap. But why would evolution and natural selection instill women, but not men, with these life-extending attributes?

“All this is entirely speculative,” Sachdev says, but it may have to do with a female’s historical role as child-rearer. “Once children are born, men are disposable,” he says. “But the robust body of the mother is important for the survival of the offspring.” A woman’s body has evolved to withstand and bounce back from the physical trauma of pregnancy and childbirth, as well as the demands of breastfeeding—challenges to which a male’s body is never exposed.

As the saying goes, what doesn’t kill us makes us stronger. And for women, that strength may translate to a longer, healthier life.

Contact us at editors@time.com.

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