(Note to non-biologists: “sexual dimorphsim” refers to any trait or behavior that differs between the sexes, like the ornamented tail of the male peacock, the brighter color of the male painted bunting—and of many birds—and the bower-building behavior of male but not female bowerbirds.)

There are some science-friendly folk (including atheists) who simply dismiss the entire field of evolutionary psychology in humans, saying that its theoretical foundations are weak or nonexistent. I’ve always replied that that claim is bunk, for its “theoretical foundations” are simply the claim that our brains and behaviors, like our bodies, show features reflecting evolution in our ancestors. While some evolutionary psychology studies are weak, and I’ve been a critic of them, the discipline as a whole is growing in rigor and should certainly not be dismissed in toto.

Those who still do, though, should answer this question:

Why are human males, on average, bigger and stronger than females?

This is true not only in our species, but in our three closest relatives: chimpanzees, orangutans, and gorillas, as well as in most primates (there are a few exceptions, like gibbons).

The most obvious answer is male-male competition: our male ancestors competed with each other for females, and bigger bodies made for more successful competitors. That size difference can be useful in both direct physical competition (as in mule deer!) or simply in dominance, like establishing territories, or even in showing you have “better genes”.  (In fact, mate choice based on size may still operate in humans if females prefer bigger or taller males as opposed to smaller ones like me.) And if males competed for females, that reflects a difference between the sexes in reproductive strategy—that is, in behavior.  Finally, if the physical result of this behavioral difference remains in our species, why would the behavioral difference itself not remain as well, with males competing for female attention? Various psychological and sociological studies in fact show this to be the case in modern humans.

The theoretical underpinnings of the behavioral difference have long been understood and supported with data: the difference in parental investment that usually makes males less discriminatory in choosing mates than are females (sperm is cheap; pregnancy and suckling expensive). The theory also makes substantiated predictions. One of them is this: in those species (both primate and nonprimate) in which males have a larger variation among individuals in reproductive success (i.e., those having “harems” versus those that are more monogamous), the species having more variation (more “polygynous”) should show a bigger size difference between males and females. For in those species in which a male can garner lots of females, leaving a lot of males as chaste bachelors, there will be stronger selection for males be larger. And that is what we find.

So those who dismiss evolutionary psychology wholesale must still explain why in every human society males are on average larger than females. (The answer probably doesn’t involve an ecological difference between the sexes in our ancestors, as such ecological differences don’t seem to exist in our closest primate relatives.) And if you admit that those differences in body size reflect ancient evolution, why do opponents of evo-psych claim that the differences in behavior that produced the physical dimorphism are no longer with us?

This is not to justify any sex differences in behavior as “right” or “moral.” That is the naturalistic fallacy.  But the left-wing opposition to evolutionary psychology as a valid discipline in principle, especially when it involves differences in sexual behavior, seems to me based more on ideology than on biology. Ideologues cannot allow any possibility that males and females behave differently because of their evolution. Such people think that this would buttress the view that one sex would be “better” than the other.

But what evolved does not mean what’s right or what’s inevitable; and everybody with two neurons to rub together knows that. Humans may have evolved to be xenophobic and even violent towards members of “outgroups,” but we have the ability through culture and learning to overcome such a tendency. And, in fact, overcoming xenophobia happens to be both more useful and more ethical in a world of wide interactions between people and nations—interactions much different from those experienced by the small social groups of our African ancestors.

Biology is not ideology, but neither should ideology dictate biology.