You have probably heard that a trans woman is a male with a female brain. As Magnus Hirschfeld put it: “a woman’s soul in a man’s body.” This is why we often hear of people being born in the wrong bodies. People are assigned gender at birth, but the assignment is sometimes wrong. Therefore, a man can really be a woman and those around him are supposed to affirm his womanhood. Although the man who believes he is a woman is indistinguishable from the man who portrays himself as a woman (deceit or fetish), we are to accept his claim. This argument rests on a false premise.
Humans are sexually dimorphic—there are clear biological differences between males and females. But those differences taken as attributes aren’t one or the other; they lie along a spectrum with plenty of overlap. We often refer to categorical variations in the gender binary as overlapping distributions. Whether we’re talking attributes like hand size or something more complex like brain structure, traits vary considerably within each sex without jumping the boundary into the other sex’s territory.
The brain shows signs of sexual dimorphism—size, wiring, or specific regions can differ between men and women. But those are averages, not strict types. Daphna Joel’s work on brain mosaics suggests that most of us have a mix of “male-typical” and “female-typical” brain traits, not a pure “male” or “female” brain. A man might have some characteristics more common in women, and a woman might have some that lean toward what’s typical for men. This doesn’t mean their biological sex flips—it’s just how people vary.
Joel’s 2015 article “Sex beyond the genitalia: The human brain mosaic,” published in the Proceedings of the National Academy of Sciences (PNAS), analyzes brain scans from over 1,400 individuals, looking at features like size, connectivity, and volume in regions known to show sex differences. She found that while there are average differences between male and female brains, individual brains rarely align entirely with one “sex typical” pattern. Instead, most show mosaic of traits, some more common in men, others more common in women.
Joel expands her argument in her 2019 book Gender Mosaic: Beyond the Myth of the Male and Female Brain. Here she explains how every brain is a unique blend of traits that don’t neatly split into “male” or “female” categories. She argues against the old-school idea of gendered brains, using the mosaic concept to challenge stereotypes. Marco Del Giudice and associates, in a 2016 PNAS letter, argues that Joel’s methods miss large, consistent sex differences. But this only counters the trans gender argument more stridently, namely the notion that there are female brains born in male bodies.
Consider the range of variability in hands. If a man has slender, delicate fingers, we don’t say he has “female hands.” We recognize that’s how his hands turned out—they’re still male hands—because they are attached to a male body. (Pick most any body part: hips, feet, even genitalia.) Brains work the same way. So, to be sure, there’s variation, but slapping on a label like “female brain in a male body” (or the reverse) is ideology not science. Gender is binary, even if traits sprawl across messy, overlapping ranges.
It seems an intentional oversight by the trans activists. If we don’t call a man with dainty hands “part female”—rather we just say his hands are an expectation of the male spectrum—why not treat brains the same? If someone is male, then their brain is a male brain—even if it’s got traits that show up more frequently in women. The idea of a “female brain” appearing in a male body leans on this assumption that there’s some perfect “female brain” blueprint ready to be mismatched. But brains don’t come with a gender sticker apart from the body they are in. They’re molded by genetics, hormones, and life experiences—all tied to the body’s gender, even if the result looks different from person to person.
Say a man’s brain leans toward higher emotional sensitivity or less aggression—traits that might line up with female patterns. It’s still a male brain, just wired in its own way. The hand comparison is instructive here (podcaster Andrew Gold suggested it): we’re fine with physical traits varying without saying they belong to the other sex. Brains shouldn’t be any different.
Determining gender comes down to basics: gametes, chromosomes, and reproductive anatomy. If we stick to that—sperm or eggs, XX or XY, testes or ovaries—then brain wiring or hand shape can vary without negating the binary. It’s just diversity within the framework. A man can have the daintiest hands or a brain that skews “feminine” and still be a man, because gender isn’t about variation—it’s about the reproductive biology.
People err when they use variation in attributes to argue that the binary itself isn’t valid. This is what we call the continuum fallacy: thinking that because there’s no sharp line between two groups, the groups aren’t real.
Variation doesn’t wipe out categories. Sex reduces to binary reproductive roles—males make sperm, females make eggs—tied to gametes, chromosomes, and anatomy. That’s a clear either/or. It’s a binary. You’ve either got ovaries or testes. You are either XX or XY—and extra or missing chromosomes are anomalies. Traits like brain organization or hand size overlap between men and women, but that doesn’t make male and female fuzzy concepts. A man with some “womanly” brain, whatever that’s supposed to mean, isn’t negating the binary—it just showing how males can differ, which of course they do, along a myriad of atrributes.
Lying in wait is the essentialism trap. Saying a “female brain” can pop up in a male body assumes there’s a fixed “female brain” ideal—while tossing out the idea of a male/female split in a sexual dimorphic mammalian species. That’s a mess. Either traits stick with the body’s sex, or they’re untethered, which doesn’t match what we know scientifically. The mistake is thinking brain variation trumps biological sex, when it’s just variation within it.
The problem is also mixing up categories and traits. Science defines “male” and “female” by reproductive function, not by every detail matching a stereotype (which are also culturally and socially constructed). The fallacy acts as if sex depends on a checklist—hand size, brain wiring, etc.—instead of on its biological base. A man with delicate hands isn’t less male; a woman with exceptional spatial skills isn’t less female. Variation of attributes across categories—even when they overlap—doesn’t undo the categories.
People overreach with averages. Joel’s research suggests that brains are a mix of traits—not cookie-cutter “male” or “female.” Some stretch her findings to say male and female brains don’t exist at all. But, on average men’s and women’s brains (as well as a myriad of other trait) do differ, but the differences are large, and that ties back to the anthropological truth of sexual dimorphism. Variation doesn’t mean the categories vanish; it means people within them aren’t clones. The gender binary is real. And gender is immutable.
freedom and reason 