On average, women live about five years longer than men globally. According to the United Nations Population Division, women live an average of 76.4 years while men live 71.2 years. This gap is biological (e.g., protective effects of estrogen, two X chromosomes) and behavioral/social (e.g., higher rates of smoking, risk-taking behavior, and occupational hazards among men).
Does that explain more than a five-year difference in lifespans between the sexes? Partially, yes. But longevity has much more to do with how the two sexes evolved over the last two million years than with simple biology.
Specifically, it has to do with sex: who we do it with, our strategies for finding a suitable mate, and how we unconsciously train our bodies to be more desirable to the opposite sex.
There's a reason that mammals, and specifically humans, are so wildly successful in populating the planet. We've got a sex drive that won't quit and endures into middle and even old age.
Why the sex drive in overdrive? For men, it's testosterone. The hormone many men take today as a supplement promotes muscle growth (specifically sex-specific upper-body muscle mass), increases metabolic rate, burns fat tissue, and drives reproductive behavior and libido.
It's no secret that many women prefer well-developed upper-body muscle mass in a mate. Many women also prefer men who are aggressive (to a point). But with that testosterone-fueled sexual attraction, there's a price to be paid.
Building and maintaining extra muscle mass requires substantial energy — metabolically comparable to the lifetime energy expenditure women undergo during pregnancy and breastfeeding. And unlike estrogen, which bolsters immune function, testosterone suppresses immune responses in many species. In wild bird, reptile, and mammal populations, higher testosterone levels correlate with higher infection severity and mortality.
"Sex trumps candles." The drive to reproduce results in many evolutionary trade-offs that prevent men from living as long as women.
The key is that natural selection favors traits that increase lifetime reproductive opportunities over traits that extend lifespan. Yale evolutionary biologist Stephen Stearns: “Macho makes you sick.” Indeed, women are better at fighting off infections than men because of estrogen and the immuno-compromising effects of testosterone.
Males tolerate the biological hazards of high testosterone, such as suppressed immunity, higher cardiovascular risks, and increased energy expenditures, because the potential evolutionary payoff in mating success outweighs the cost of a shorter lifespan.
Testosterone and other sex hormones are also associated with greater cancer risk, particularly when it comes to prostate cancer. Populations with higher testosterone levels, for example, tend to also exhibit higher incidence of prostate cancer.9 Once again, sex trumps candles.
So why do males tolerate the negative effects of testosterone? The Darwinian explanation is that the potential reproductive payoffs are huge in mammalian males compared to females. Mating opportunities are an important constraint for male fitness. Hypothetically, a male mating with 100 different females in a year could potentially father 100 offspring or more. The same is not true for females. The prevalence of polygyny in mammals, other primates, and many human societies is evidence of the influence of this difference in fitness constraints between males and females. Women can also increase their fitness by obtaining more mating opportunities, but not through bearing more offspring. In essence, mammalian males are willing to deploy costly hormones such as testosterone, invest in expensive tissue, and engage in risky behavior because the potential fitness payoffs are so high.
This makes sense if you’re hominid living in the Pleistocene a couple million years ago. But is this relevant for men today? Perhaps. While humans are tremendously influenced by culture, the conditions of natural selection—trait variation, trait heritability, and differential reproductive success—are difficult to escape.
"At our core, humans are incredibly malleable," writes Yale University professor Richard G. Bribiescas. "The physiology that supports this malleability is probably why our species has evolved the traits that define us: big, expensive brains; long lives; extended childhood; offspring that require lots of care. It might also help explain why there are over 7 billion of us."
That, and a God-given enjoyment of the sexual act, especially when performed with someone you love.
