(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.