The life expectancy for boys born in the United States—about 75 years—is five years less than girls. At nearly every age, males die at higher rates than females, and this vulnerability holds true across the world, from industrialized nations to isolated tribes of hunter-gatherers.
But that isn't the half of it, according to Portland scientists, who have proposed a deeper explanation for the male survival disadvantage.
Soon after conception, as embryos in their mother's wombs, males adopt a risky, fast-growth strategy. And that appears to set a trajectory that makes them more vulnerable than females during fetal development and for the rest of their lives, say Dr. David Barker and Dr. Kent Thornburg, both professors at Oregon Health & Science University.
"It's a core question in human biology why women live longer than men," Barker says. "There have to be some core answers."
Scientists have known for years that the survival edge begins in the womb. About 120 males are conceived per 100 females, probably because sperm carrying the male Y chromosome swim faster than those carrying the female X chromosome and are more likely to win the race to fertilize an egg. But male fetuses succumb to miscarriage at a higher rate than females, cutting the sex ratio at birth to about 105 boys per 100 girls.
The cause of this dramatic culling remains unclear. Barker and Thornburg believe it is rooted in the male growth strategy. Their reasoning goes like this: Male embryos grow faster than females and that entails critical trade-offs, such as investing less in the growth of their placenta, the organ that connects them with their mother's blood supply. At the same time, males need more energy to support their fast growth, which puts them at greater risk of malnourishment if the mother suffers a food shortage, injury or sickness.
Previous researchers have shown that the number of male births drops compared to female births during times of severe deprivation, for example, in the famines in Holland during World War II.
Barker and Thornburg say it's likely that the male rapid-growth strategy also sets the stage for male susceptibility to many diseases in later life. "If the mother, or the placenta, is inadequate, babies must compromise the structural quality of their organs as they develop," Thornburg says. "This makes them vulnerable as adults."
This supports the 'fetal origins hypothesis' Barker has championed for years, which asserts that subtle deprivation of nutrients during fetal development can produce long-lasting changes in organs and metabolism. In adulthood, the changes can trigger high blood pressure, heart disease and other diseases.
Other researchers say many factors probably contribute to the female advantage. Estrogen, the female sex hormone, for example, keeps the immune system running strong, while the testosterone can suppress immunity. Estrogen helps balance cholesterol and may protect arteries from hardening and clogging. Female cells, at least in some animal studies, also pump out more antioxidant enzymes than male cells to better disarm DNA-damaging chemicals.
Nevertheless, says Vicki Clifton, a professor at the University of Adelaide in Australia, the ideas put forward by Barker and Thornburg seem plausible.
"Our research [in fetal development] shows that males keep growing and females reduce growth if there is a problem during the pregnancy," she says. "Females adjust in response to the first problem and if there are any reductions in oxygen or nutrients subsequently due to another problem, they survive while males don't do so well."
On average, men die earlier than women. Why male embryos commit to such a risky fast-growth strategy remains a mystery. Clifton says it may have evolved long ago in human evolution as a way for males to produce more offspring.
"It's important to be big if you are a male because in most animal populations you fertilize more females and therefore you spread your genes further and ensure your gene pool survives," she says.
Evolutionary theory predicts such differences in species in which female reproduction is limited by how often they can give birth and how much they can put into rearing babies, while males can maximize their offspring by having sex with as many females as possible. These circumstances create pressure for males to adopt a "live hard, die young" strategy, says Marlene Zuk, an evolutionary biologist at the University of California, Riverside.
Zuk says the longevity gap between men and women is rooted in such a complex and ancient series of steps in evolution that no one will ever be able to point to a single cause for women living longer -- and it may never be possible to close the longevity gap.
"None of this is to say that we should give up and let males smoke, drink, or infect themselves to death," she says in a recent essay. It's just that there is nothing unnatural about a sex difference in longevity.