Last week I published a post intended to show that the profound sexual dimorphism for human size (and musculature) reflected sexual selection in our ancestors, a form of selection that can be explained only by an evolved difference in behavior: in humans, as in many other species in which females invest more in reproduction than do males, males (who often make little reproductive investment, sometimes only sperm) must compete with each other for access to females. The behavioral difference is a marked tendency to be promiscuous, compared to the greater choosiness of females. That behavioral difference is in turn a direct result of an evolved difference in gamete size and reproductive investment between males and females.
Competition among males for females can involve either direct male-male “battles” (as in elephant seals, gorillas, and chimps, as well as stag beetles and deer), or female choice of males based on of their ornamentation (as in African widowbirds, peacocks, and lions). I think the size dimorphism of humans is more likely a result of male “battling” for dominance and access to females than simply female preference for large males, though of course both factors can be involved. But regardless of whether the sexual selection involves inter-male competition—what Darwin called “the law of battle”—or female preference, it implies a behavioral difference between the sexes, and one involving the traits most crucial for evolution: those directly involved in sexual reproduction.
I also adduced four other bits of evidence predicted by the sexual selection hypothesis, which you can see at my earlier post. Those predictions were made before the data were collected, and they were confirmed. There are many other data supporting the sexual selection theory, and I’ll discuss them tomorrow. One I’ll mention now is that the measured variance in reproductive success among human males is higher than among human females, particularly in hunter-gatherer tribes. That is, in such groups some males leave a lot of kids (and thus their genes) and many others leave none, while, in contrast, the variation in offspring number among females is much lower. The difference in variance between males and females, by the way, is directly correlated with the degree of sexual dimorphism in those groups: the greater the difference in variance among males than among females—and thus the more polygynous the society—the greater the sexual dimorphism for body size. That relationship is a prediction made by the sexual selection theory.
Now, however, Holly Dunsworth, a biocultural anthropologist at The University of Rhode Island, has taken issue with the long-accepted theory of dimorphism (it’s not mine; Darwin was the first to suggest it!), and goes after me in a blog post called “In man’s evolution, woman [sic?] evolve too.” (That post was also picked up and supported by Jesse Singal in a column in New York Magazine, which makes the same errors as Dunsworth).
Dunsworth offers her own thesis, which, she says, puts more emphasis on female evolution. I suspect her own hypothesis is in fact ideologically driven, and also neglects the possibility, which I did indeed raise, that female preference has evolved. Apparently anthropologists bridle when what evolves in females during sexual selection is psychology rather than morphology! I also see that Dunsworth has emitted a very long string of tweets about my piece, which suggests some obsessiveness about the sexual-selection hypothesis and male-male competition. I don’t engage in Twitter battles, which are unproductive, but will make my positions clear on this site.
At any rate, here is Dunsworth’s own theory of why human males are bigger than females (I’ve put her theory in bold).
It’s not that Jerry Coyne’s facts aren’t necessarily facts, or whatever. It’s that this point of view is too simple and is obviously biased toward some stories, ignoring others. And this particular one he shares in this post has been the same old story for a long long time. [JAC: Yes, because it’s supported by lots of diverse evidence and makes predictions that have been met!]
What about the other side of the body size sexual dimorphism story?
What about the women?
Selection could well be the reason they stop growing before men and why they end up having smaller bodies than men, on average.
Perhaps men can make babies while growing, but perhaps women can’t. Energetically, metabolically. So reproduction wins over growth. We reach sexual maturity and stop growing. Is that just a coincidence?
Why doesn’t this (and other tales) fit alongside the big-aggressive-males-take-all explanation for sexual dimorphism? #evolution
Not only is it absent, but selection on women’s bodies be the driving force (if such a thing could be identified) and, yet, it’s as if women don’t exist at all in these tales except as objects for males to fight over or to fuck (but *thankfully* there’s that female choice!).
Knowledgeable people aren’t objecting to facts, as Coyne suggests. They’re objecting to biased story-telling and its annoying and harmful consequences, which Coyne doesn’t acknowledge or grapple with in his piece. [JAC: I do indeed acknowledge that we must be mindful of the misuse of biological facts, and not use what we deem “natural” to make social policy. Did Dunsworth even read what I wrote?]
I’ll respond to her hypothesis tomorrow (she calls mine a “story,” a snarky way of denigrating it since there’s ample evidence supporting the sexual selection hypothesis), but right now I want to make three points:
- Dunsworth, who says that I am a sucker for unsupported just-so stories in evolutionary psychology, doesn’t seem to realize that I have a long history of criticizing adaptive evolutionary-psychology stories unsupported by evidence (go here, for instance). In fact, evolutionary psychologists used to be mad at me, considering me overly critical. But there are some aspects of evolutionary psychology, like that of human sexual behavior mirrored by sexual dimorphism in body size, that are more scientific, for they make testable predictions that have been met. It would be churlish and intellectually blinkered to ignore both this hypothesis and the evidence that supports it, equating this to more speculative adaptive hypotheses that I’m warier of.
- Dunsworth’s own “story” really is closer to a story, as it’s contradicted by the known facts about human reproduction. I’ll let the readers figure out what those facts are.
- Finally data on the nature and traits that are sexually dimorphic in humans have, as noted above, been predicted by the sexual-selection hypothesis but not by Dunsworth’s “growth and reproduction tradeoff hypothesis.” So not only is her hypothesis contradicted by data already known, but is countered by many facts about sexual dimorphism in body size, not only in humans, but also in our primate relatives and other animals. Comparing the sexual selection theory with the tradeoff story, it’s clear that the former is the best explanation for the facts.
I conclude that Dunsworth knows nothing of my history of writing on evolutionary psychology and, further, is remiss in her own scientific approach, offering a story that’s amenable to her ideology because it allows females to evolve (mine does too!), and also a story that not only fails the empirical tests, but can’t predict the observations that sexual-selection theory can. Too, there seems to be more than a touch of intellectual mendacity in the way both she and Singal blithely ignore the supported predictions of the sexual-selection theory. Believe me, it’s more than “just a story.”
I’ll have more to say on this tomorrow, but am throwing it out here now for the readers to chew on.