Not yet mainstream, non-traditional, logical and reasonable evolutionary explanation for the average greater strength of human males

Common question I hear (including common "truth" about the evolution of men's strength): 

From an evolutionary standpoint, it doesn't really make sense that men are stronger than women. Of course I know men evolved that way to fight off other males to get mating opportunities, but shouldn't women also evolve strength to protect their kids? Wouldn't it make more sense for women to evolve a greater muscle mass to protect their kids from danger?

Answer: 

You’ve hit on one of the most important questions about human evolution, so I hope you’ll forgive me if this goes a little long. I’ve learned that it’s better to say too much than too little when you’re asking people to question (and maybe reject entirely) some of Charles Darwin’s ideas. One hundred and fifty years ago, he couldn’t have possibly gotten everything right. In 2024, science is still just a baby.

Though, it’s only taken humanity 150 years to know that evolution (=change over deep time throughout all lineages which are all related to, and interconnected with, one another) is true.

But there are the facts of evolution and then there are the stories we tell about those facts.

It’s a fact that males are on average stronger than females, and it’s a fact that humans (and every organism) evolved from earlier forms/ancestors, but the story we tell about why male humans evolved to be on average stronger than females is very far from fact.  

Unfortunately, that story is taken for granted as such a fact-of-life in our culture right now that questioning its truth can earn you some serious eyerolls or can make people leap to the conclusion that you’re doubting the truth of evolution itself. But enough about my personal problems. Let’s answer your wonderful question.

We can start by considering the fact that female humans are actually, already, totally evolved to be totally capable of protecting their children—wits-wise, strength-wise, throwing-projectiles-wise, fire-wise, booby-trap-wise, and otherwise, and probably have been for millions of years (considering we’re primates and knowing what female primates are capable of: surviving and helping others to too). And remember, few of our ancestors were ever isolated and alone. Group-living with complex culture and highly social coalitional behavior is rampant among primates and the human primate takes sociality, cumulative culture, and cooperation to the extreme. So, from the perspective where female humans are perfectly strong enough to care for themselves, their kids, and their loved ones (just as any female primate is), then it remains a mystery why males evolved to be even stronger. But because we started from a different assumption about the evolution of females compared to tradition/pop culture, then our scientific imagination can be open to non-traditional explanations for the evolution of male-female differences in strength.

Ever since Darwin, it’s become common sense to explain male-female differences in human strength to be about males fighting other males, the winners getting to mate more, and therefore putting their winning/strong genes into more sons than the other guys. Eventually that common sense story included males fighting other males to protect their wives and babies from imaginary Stone Age marauders. In our cultural tradition, male-female differences in strength have become evidence for a belief about human nature that has us loving only our close family, all others we battle to the death. But (combined with our initial understanding that females are perfectly physically strong enough to survive and to help others’ do the same) the fact that daughters would clearly benefit from evolving the same level of strength as sons, as you point out, is exactly why we should question that common sense story.

There is obviously a male-female difference in average strength, with males being stronger and having more total muscle mass, on average—though it is less pronounced in people who have been active since childhood and throughout their lives. And there is obviously a male-female difference in physical violence, with males fighting more than females. However, fighting (and the winners of the fights passing down more winning genes, ratcheting up strength in only sons in the future) need not be the reason physical differences exist between the sexes.

After all, fighting is not the reason for another major difference in male and female biology. Fighting’s not the reason that gonads and genitals are different between the sexes. And, while all bodies count on both testosterone and estrogen to develop and to function (and while estrogen in all bodies is synthesized from androgens), testes in male bodies produce more testosterone than is found in female bodies, while ovaries in female bodies produce more estrogen than is found in male bodies.  (And both of those hormones are also produced elsewhere all bodies, like the brain.) A delicate balance of both of those hormones (and many others), not too little and not too much, is required of all bodies to reproduce (to produce gametes and to facilitate in their meeting gametes of the other sex). And so, evolutionarily changing the levels of those hormones risks preventing that change from existing into the future. That is, evolving to have more or less testosterone or estrogen is a risky evolutionary process, given it could prevent evolution/life from continuing. Greater testosterone is the best explanation we have for greater male strength, by its promotion of the development of muscle mass. But males’ greater testosterone is fundamentally linked to their reproductive biology in the first place. Messing with testosterone levels, up or down, in males risks lowering male reproductive success. That’s true for females, too.

Bear with me because there is not a whole lot of well-understood biological science here, but it’s not likely that a female with male levels of circulating testosterone, which could make her stronger, would actually be able to reproduce. Remember, we all need a certain level of estrogen and testosterone to do so and producing so much testosterone would probably affect female reproductive ability (it may also cause a reduction in estrogen which would add to the problems, but that’s just an educated guess).

The point is, given what we know about how estrogen and testosterone contribute to reproductive biology (and if we also assume that female bodies are good and strong enough to do what humans need to do to survive and to help others to too), then it’s easy to imagine why daughters of strong men are not as strong as strong men’s sons. 

We could say that males' higher testosterone and the resulting increased muscle mass is just a consequence of being not-female. When bodies are so intricately linked through hormones to the complexities of sexual reproduction, it’s probably wise to appreciate those hormones' crucial roles in reproduction. When we do, we can see how sex differences in bodies, like muscle mass and height, could be by-products of the evolution of the reproductive system.

That is, since the reproductive system differs between males and females, then why would we expect everything else that’s developmentally linked to the reproductive system to be identical? The old common-sense story about males being stronger because of selection for fighting ability assumes that all humans would be the same if not for male-specific selection for strength. But we’re not the same in ovaries and testes. We’re not the same in hormone levels. All that difference in our gonads is doing the fundamental work with eggs and sperm to sustain our lineage.

One by-product of those hormone levels caused by our gonads is our height difference. Because males don’t have the higher levels of estrogen that ovaries produce for the ovulatory/menstrual cycle, male bodies keep growing for longer than females’. That’s because, in all bodies, estrogen drives long bone growth and an even higher level of it is needed for growth plates to fuse and for height to stop increasing. Females stop gaining height early in puberty because of the high levels of estrogen. Males keep growing for up to a few more years because there is nothing stopping them so soon like there is in females. Male-female differences in height are therefore not because of selection for big, tall males (despite that being common sense story since Darwin); they are because of reproductive biology’s side-effects on bone growth duration resulting in patterned variation where on average females are shorter than males.

I suspect that the same side effects are happening with muscle mass and strength. There’s no reason to expect us all to be the same and then, from there, to require a male-specific adaptive tale for the difference. Selection is already building ovaries and testes and from there viable/successful human variation forms (height, strength) and we’re clearly doing fine with all this variation because our ovaries and testes keep putting all this variation into the future. (There is probably something about the increased fat mass in female bodies compared to males, because it is linked to estrogen and to successful reproduction. And so, there may be something about building fat in bodies with more estrogen and, in the absence of lots of estrogen, building more muscle instead.)

That leaves us wondering why human males fight so much more than females (and so much more than other primates) if they have not necessarily evolved to do so. It may have lots more to do with culture than old evolutionary stories have led us to believe. It may even have a little bit to do with telling boys and men that they have evolved to be fighters.

As you point out implicitly in your question, what I’ve written here isn’t mainstream (yet), but, like I said, science is still a baby. Keeping an open mind about human biological variation and remaining skeptical of the stories we tell to explain human variation is, I think, the strongest way to face human evolution with science.