Robert J King Ph.D.

Hive Mind


Terf Wars: What Is Biological Sex?

Can it change?

Posted Mar 03, 2020

Is she a "good girl" or a "bad girl?" Is he a "real ally?" You aren’t a "proper" Irishwoman. No "true" Scotsman would say that.

Is that person a "real" woman?

Humans love categories, and humans love distinctions. That’s how we’ve evolved to think. Categories—nouns and adjectives—are quick ways to sort the world into appropriate emotional, and behavioral responses. “Stay away from that 'poisonous' tree." "Fight for your 'noble' country.”

“Poisonous” isn’t really a thing (as Paracelsus pointed out, the dose is the poison). Countries only exist if we believe in them. We let children hone this set of key categorizing skills by giving them sorting toys, dress-up toys, games where the pieces are in one square, or another square, never in-between.

None of this categorizing is a problem unless we start insisting that these categories are deeply reflective of external nature. It’s not, we like to think, that some categories are just useful, or helpful, or shorthand filing systems admitting of exceptions—but that they are essences built into the very fabric of reality.

Some people are very worried about some particular essential categories at the moment. “Are these people—transwomen, for example—real women?” “Are these people—transmen, for example—real men?”

“Yes,” says one group, these people possess a particular essence. “No!” shout those who disagree. “We won’t allow these people into our fiercely defended spaces.” Anyone who spends anytime online lately has witnessed vicious, personal, sometimes threatening back and forths, about which essences (“I feel like a woman” or “You haven’t suffered oppression” or “I have a womb, you don’t”) should win out in our shared social spaces.

I can hardly honor some of what is going on online with the word “debate.” It is too personal, too heated, and too painful. Issues of identity and safety are often like this. However, I would like to offer one perspective on the issue of who is, or is not, essentially a man, or essentially a woman.

No one.

Both sides are wrong. Oh, people are not wrong about how we should treat other people (we should be kinder) about how they feel (we need more honesty) or about whether oppression has occurred (it has and will continue, alas). I doubt that anyone is lying about any of that. But most everyone I have listened to is wrong about whether this or that person has the essence of male or female.

The reason that they are wrong: No one has the essence of male or female.

Let’s be crystal clear here. This is not because “science has shown that sex is a spectrum” (this is a hopelessly unhelpful picture no matter how well-intentioned) and it’s not because “science is just some other way of knowing” (an even more hopeless picture, albeit less well-intentioned). There do exist intersex people—quite a few of them, in fact—but they are not the issue here either. Their existence does not invalidate the general pattern that humans are sexually dimorphic.

No one has the essence of maleness or femaleness, for one simple reason: Since the 17th century, what science has been showing, in every single field, is that the folk notion of an “essence” is not reflected in reality. There are no essences in nature. For the last three hundred years or so, the advance of science has been in lockstep with the insight that is what really exists are processes, not essences.

Being is, as being does

We have a name for this in science, “functionalism.” But, don’t worry about the technicalities or the jargon. The essence (see what I did there?) of functionalism is this: Something is, is what it does. Or, to put it another way, the scientific revolution largely replaced nouns, with verbs.

This scientific revolution has been quite general. Alchemy—that ancient study of essential (magical) properties, became chemistry, the study of processes; such as the ionic bonding you studied in high school. Astrology—that ancient study of essential (magical) properties of stars became astronomy, the study of physical processes; such as the gravity you studied in high school. Biology dropped the idea of an élan vial—a magical life essence—in favor of the various metabolic process that underpins life. You studied these in high school, too. Those high school studies of yours often came with equations. There’s a reason for that: Formulae express processes.

Before your eyes glaze over… can you see a theme here?


Science (processes) has replaced magic (essences).

We don’t make plain in high school how big a revolution this was—but its implications are massive. And, the proof that this functionalist way of thinking about the universe is right? We applied this process-style thinking to the universe, and you can now read what I’m writing on machines that embody a bunch of these processes. Electricity. Logic gates. That’s all computers are—a physical way to embody processes. They turn verbs back into nouns again.

And it is just one way that we were right. Engines. Medicines. When we understand the verb—the doing—we understand the noun—the thing. What something is, is what it does. For instance, it doesn’t matter if you are reading this blog on an iPhone, listening to it on a podcast, have printed it out, or projected it on a screen. The physical realization—the noun—doesn’t matter. The transfer of information, the verb, does matter.

The universe is, essentially, processes. Humans are a bunch of processes, processes that evolved by natural selection to increase fitness. Some of those processes make us appear male or female.

Essences—being magical—can admit of no exceptions—but processes can go awry. That doesn’t make the distribution of processes random in our population (far from it) but it does mean that every time someone tries to pin down an essence of man or woman, there are going to be exceptions. That’s not a problem—unless you really really want to believe in essences. And some of us really do. And it’s causing a lot of grief.

But, humans are exceptions to the rules, right?

When it comes to applying the process/essence insight to humans, the push-back towards magical (essences) style beliefs is the hardest, because we feel like magical beings. Psychological science doesn’t always help us out here. For example, we stick people into categories in the DSM-5, that bible of mental disorders. It’s very useful for portioning out treatments, but sometimes we forget that all those pathological categories are just markers for underlying processes. Sometimes it's helpful to think of someone as “a schizophrenic,” but often we need to remind ourselves that all we really have is a person with schizophrenic symptoms. The old philosophical puzzle of consciousness is the most obvious place where otherwise sane, rational, scientifically literate, people still babble on about magical essences, but I don’t want to talk about consciousness today. I want to talk about sex.

Is there a magical essence, making you a real man or a real woman? (Already I can hear “Yes!” and “No!”)

Let’s talk about some of the processes involved.

To a biologist, being male means having small, fast-moving sex cells (gametes), while being female means having large energy-rich ones. The technical term for this is “anisogamy” (having sex cells—gametes—of differing sizes) and it results from some very ancient selection processes. Once a piece of genetic material starts going down the “small and fast” route, it is under selection to carry on down that path, becoming smaller and faster over time. Ditto, selection on the “large energy-rich” route.

The small gametes are sperm, the large ones are eggs. The strategic options that they build around themselves to reproduce are you and me (and all the wondrous sexually reproducing organisms you can see). Does that mean that (say) if the processes that normally make women produce eggs fail then they are no longer women? Of course not. But what starts this (normal) set of processes off in the first place?

In humans (and some others) it is the SRY gene, which sets off a cascade of other functions on the Y chromosome. In 99 percent+ of cases, it does. Except, when it doesn’t (for example, very rarely the SRY gene is found on the X chromosome). Are the chromosomes sex essences then? Remember back to your school biology: XY for men, XX for women? In 99 percent+ of cases, yes, they match up. An XX person looks and acts female, an XY person looks and acts male. Except those times when they don’t (for instance, with some intersex conditions, like Turner’s syndrome).

Chromosomes aren’t a sex essence either. What the chromosomes do is to set off a cascade of hormones that prepare the body and brain for whichever sex it is going to be. Well, in 99 percent+ of cases they do, except when they don’t (such as the rare condition, 5-α reductase deficiency, where girls turn masculine at puberty).  

Is there then a sexual essence to be found in the hormones that result from those chromosomal arrays (simplifying enormously, testosterone-like stuff for the guys, estrogen-like stuff for the gals)? Yes, in 99 percent+ of cases, except when it isn’t (people with Turner’s syndrome might have high levels of androgens, but be insensitive to them, for example).

All of us who went through puberty got to see some of the bodily and behavioral effects of these hormones (all the fun of schooldays). Is there an essence of sex to be found here? Yes, in well, probably less than 99 percent of cases this time. Things like gender non-conformity are not that rare, and neither is (atypically) fancying the same sex. And there are all sorts of interesting processes such as cases of congenital adrenal hyperplasia (CAH)—where despite the intense social pressure to act as female—the CAH people insist on following their own inner (male-typical) interests. These people are not that rare either. And such people are a tough challenge to simplistic “social roles” models of human sex-typical behavior as well. Because, now we are talking about a person interacting with various bio-social influences, and taking on things such as social norms, gender roles, diet, accidents, foetal hormone levels, and some other stuff that is much more responsive to human will, than the SRY gene.

These processes—in brain and body—and some of them bio-social now—are much less sexually dimorphic than that 99 percent+ figure. However, and despite what some over-enthusiastic advocates might have you believe, scientists can still routinely tell that there is a difference between men and women at the population level of magnification.

And, we can routinely tell the difference between male and female brains, too. This all matters, because assuming that human brains are sexless silly-putty has medical implications. Personality differences are measurable at large scales of magnification too, and these increase as cultures get more egalitarian, which is, once again, tough to explain for anyone who thinks that sex differences are just software downloaded from culture (and don’t even get me started on how you would even begin explaining where this almighty powerful culture came from in the first place, on the blank slate model).

Is it back to the 1950s, then?

However, none of that measurable sexual dimorphism is any sort of justification for telling people what they can or cannot achieve socially or, god forbid, individually. It’s just the result of what we’ve measured. Descriptions, not prescriptions.

None of the processes mentioned are magical. No Harry Potter style “Sexiamus” spell gets cast at any stage of human development. However, it's also foolish to claim that two mountain peaks (of sexual dimorphic traits) with a range of interesting smaller hills in between—is some sort of spectrum. In a spectrum, each color is the same width.

There are not countless sexes, scattered across nature. Unless you are fungus, of course. Some fungus species do have thousands of sexes but, unless you are a fungus reading this, then you don’t belong to one of those. You are part of a sexually dimorphic species, like most complex animals. Indeed, the many, many, ways that species achieve the “two-sex” model is, like the many, many, ways eyes have evolved, good evidence that it’s a really useful way of reproducing genes.

All this still leaves us with plenty of problems in working out fair and equitable ways of living together, and of being kind to the vulnerable, while not encouraging the delusional. But we only have a chance of solving such problems if we stop thinking magically.



E.g. Scientific American, Visualizing Sex as a Spectrum. Accessed on 24/10/2018 and available at

Erika Kothe (1996). Tetrapolar fungal mating types: Sexes by the thousands. FEMS Microbiology Review 18 (1): 65-87.

Anisogamy and parental investment. A key researcher here is Hannah Kokko, who has produced a large and impressive body of sophisticated modelling of anisogamy and its effects at various levels—from population to single gene--and linking these levels via modelling of relevant behaviors such as parental care. A key insight is that sperm could evolve to have energy provision—like eggs do—but that costs would always outweigh benefits because it cannot be predicted which sperm will succeed. Thus there are two routes to generating two sexes. When gametes find each easily, then competition can generate a differential selection path for male and female gametes; but when there is limited gamete availability, this can also lead to anisogamy. For those who wish to pursue the details, central papers would include:

Lehtonen, J., & Kokko, H. (2011). Two roads to two sexes: unifying gamete competition and gamete limitation in a single model of anisogamy evolution. Behavioral ecology and sociobiology, 65(3), 445-459.

Mukherjee, S. (2017). The gene: An intimate history. Simon and Schuster.

 Alice Dreger, in particular, has been a vocal and well-informed proponent of not forcing people, especially not forcing children, into categories without their ability to make informed choices. An increasing number of people wish to be identified as “third sexers” for example, and reject the idea of early so-called corrective surgery. See Dreger, A. D. (1998). Hermaphrodites and the medical invention of sex. Harvard University Press; Dreger, A. D. (1999). Intersex in the Age of Ethics; and Dreger, A. D. (1998). “Ambiguous Sex”—or Ambivalent Medicine?: Ethical Issues in the Treatment of Intersexuality. Hastings Center Report, 28(3), 24-35.

Imperato-McGinley, J., Peterson, R. E., Gautier, T., & Sturla, E. (1979). Androgens and the evolution of male-gender identity among male pseudohermaphrodites with 5α-reductase deficiency. New England Journal of Medicine, 300(22), 1233-1237.

Diamond, M. (2006). Biased-interaction theory of psychosexual development: “How does one know if one is male or female?”. Sex Roles, 55(9-10), 589-600. The quote is from p. 594.

Diamond, M., & Sigmundson, H. K. (1997). Sex reassignment at birth: Long-term review and clinical implications. Archives of pediatrics & adolescent medicine, 151(3), 298-304.

Diamond, M. (1965). A critical evaluation of the ontogeny of human sexual behavior. The Quarterly Review of Biology, 40(2), 147-175.

If you just look for one thing at a time (technically called univariate analysis) then you get different results than with multivariate analysis. One interesting way that this has been shown is by comparing the brains of cis and trans-gendered people. When this is done it is found that the brains of trans-gendered people do seem to converge on a single-sex continuum as shown by differences in gray matter (GM). As one researcher put it “[S]ex has a major effect on GM irrespective of the self-perception of being a woman or a man”. Baldinger-Melich, P., Urquijo Castro, M., Seiger, R., Ruef, A., Dwyer, D., Kranz, G., Klöbl, M., et al., (2019). Sex Matters: A Multivariate Pattern Analysis of Sex- and Gender-Related Neuroanatomical Differences in Cis- and Transgender Individuals Using Structural Magnetic Resonance Imaging. Cerebral Cortex. The quote comes from p. 10.