The brain in my skull is quite a different organ than the one in my mate’s head. In fact, as I investigate this, I’m increasingly convinced that the human animal has two brains, each built, wired, and operated according to its sex.
For starters, males have ~15% bigger brains. But females have more neurons:
Some women even have as many as 12 percent more neurons than men do. Those neurons are most densely crowded on certain layers of the cortex, namely the ones responsible for signals coming in and out of the brain. This may be one reason why women tend to score higher on tests that involve language and communication, and these differences are probably present from birth.
Females have 9x more white matter; males have 6x more gray matter(!):
In those regions of the brain that support “general intelligence,” females have nine times more white matter than males. That’s nine times more of the stuff that ferries data around the brain. As for gray matter related to general intelligence, males have six times more of the data-gathering stuff. These are huge disparities.
“Smarts” are concentrated in different parts of the brain:
The bulk of the human animal’s IQ is based in a handful of areas that govern memory, attention, and language, and there are disparities in where males and females store these crucial brain segments: Females hold “smart matters,” both gray and white, largely in the frontal lobes, where emotions, speech, reasoning, judgment, and movement dwell. Males, by contrast, strew their wits all over the brain. He has crucial patches of gray matter in his frontal lobes and nearby parietal lobes, where reading and math reside. His “smart” white matter resides in a completely different nation, the temporal lobes, home of sound perception and memory processing.
For context, while many other mammal brains show a girl-brain/boy-brain dichotomy, the human differences are much more extreme and puzzling:
Granted, some degree of divergence should be expected. For instance, many mammal brains contain a “sexually dimorphic nucleus,” a buried clump of cells that’s much bigger in the male brain than in the female brain. (Its function is unknown.) Many also host a sex-specific clump that dictates breeding behavior, as do my brain and my mate’s. But what’s so puzzling about the human head is that the sex differences spill into regions of the brain that have nothing to do with sex-specific tasks—the parts governing speech, reasoning, and movement. The human brains are different in the sophisticated areas, where highfalutin thoughts are constructed. Why should that be? Why isn’t the same thinking gear good enough for both sexes?
The kicker is that these huge differences in brain structure matter almost nilch:
Very little of the human behavioral repertoire can be traced to sex differences in the brain. And that’s weird. How can two brains be so different in their layout yet produce such similar behavior? How can we be wired so differently—yet both build bookshelves and write thank-you notes? With brains this different, I’m impressed that we can communicate at all, let alone agree what color to paint the den.
