How many sexes?

Is TE the only relay/ultra site that covers both Bayesian inference and complex human biology? Or is it merely drugs and sex?

It’s hard to write about biological sex. Our language about sex is overloaded, and our bias toward wanting the world to be as simple as it sometimes appears interferes with seeing things as they are.

With some sensitivity ‘disorders of sexual development’ (DSD) is the preferred medical umbrella phrase for women who do not have two X chromosomes or have markedly different development. It is often presented as ‘differences of sexual development,’ an obscuring phase: every woman alive can recall different rates of development between herself and her friends during puberty and think, of course there are differences in sexual development. But that is not what the phrase represents. ‘Intersex’ is also used, preferred by many intersex people. An inclusive phrase: it ranges from gender dysphoria all the way to women with a Y chromosome and internal testes.

We think of two sexes, but there are eleven chromosomal sexes, even counting XY as only one. (It’s not.) There are X (Turner syndrome), XX, XY, XXX, XXY (Klinefelter), XYY, rarely XXXX, XXXY, XXYY, and extremely rarely XXXXY and XXXXX. We mistake the ‘XX-or-XY’ majority for the totality.

Testosterone is our chief sex-determining hormone. Testosterone is highly ‘conserved,’ meaning it’s so useful that it’s been around in the same form for hundreds of millions of years. It’s produced by bacteria, plants, fish, insects, trees. Testosterone and some of its derivatives (called androgens) are necessary for male sexual development, but estrogen is also derived (via aromatase) from testosterone. Lipophiles: they’re made from cholesterol, which is a fat.

In humans testosterone follows a disjoint (non-overlapping) bimodal distribution. Height is a bimodal distribution, but it is conjoint (overlapping). Most men are taller than most women, but plenty of women are taller than plenty of men. Testosterone is not like that. The highest XX women have testosterone levels four-or five-fold lower than the lowest XY males.

If height was like testosterone, the shortest healthy adult male would be taller—by a lot—than the tallest healthy adult (XX) female. During puberty boys’ testosterone levels are 20 to 30 times higher than girls.

There are some DSDs that elevate XX women’s testosterone. Polycystic ovarian syndrome (PCOS) is an example, and it is not a surprise that, according to what we know so far, the subset of PCOS women who also have elevated (for women) levels of androstenedione are represented in elite sport at roughly three times their presence in the general population. (Note that PCOS women still have testosterone levels within the XX band.) This is not so for XX girls/women with congenital adrenal hyperplasia (CAH), a DSD also characterized by high testosterone levels. (Boys can also be born with CAH but unlike with girls it has no measurable behavioral effects.) CAH can have health consequences for girls. CAH girls show boy-leaning play preferences, even with rigid gender-ideological parents, and are much more likely than their non-CAH sisters to choose ‘thing-centric’ professions, like carpentry or engineering. (They on average make more money than their sisters, too.) The evidence is not extensive but even non-CAH XX girls exposed to higher levels of in-utero testosterone show a greater preference for ‘boy’ play. (Play, like height, is bimodal but overlapping; CAH and high in-utero testosterone girls play like girls, simply mixing in a slightly greater dash of boyish activities. Roughhousing is an example.)

There is a subset of DSD girls and women who have Y chromosomes, and whose bodies correspondingly produce sperm rather than eggs. There is a range of somatic expressions: complete androgen insensitivity syndrome (CAIS) and PAIS (partial), to 5-alpha reductase deficiency (5-ARD). CAIS women lack ovaries and a uterus but otherwise are physically and behaviorally women, indistinguishable from their XX sisters. They have internal testes, but the ‘default’ body for humans is female, and CAIS women, who produce testosterone and other androgens but do not have receptors for them—their bodies ignore them—grow up and live as girls and women.

5-ARD is different. These women also have internal testes producing male levels of testosterone. Unlike CAIS or PAIS, they have functional androgen receptors (which are coded on the X), but a deficiency in 5-alpha reductase, an enzyme needed to convert testosterone to dihydrotestosterone (DHT), which is a more potent androgen variation that during puberty causes male secondary sexual characteristics to develop. 5-ARD levels vary: some 5-ARD childhood girls have enough of the enzyme that at puberty they express/become boys, e.g., what seemed to be a clitoris develops as a penis, testicles descend outside the core, &c. Some don’t, and live their lives as women.

Identity and biological status don’t always track. For instance CAIS women don’t commonly discover their chromosomal status until their friends start menstruating at puberty and they don’t. (Parents and other adults can … not behave well when they learn their little tomboy has a Y chromosome. Lucky is the child with emotionally generous parents.) The increase in trans people (gender dysphoria) has been correlated with hormone-disruptors (microplastics, other synthetic chemicals) in the air and water, but research is in early stages, and is both time-consuming and combinatorially difficult.

XY individuals have roughly 12% higher hemoglobin levels. Hemoglobin carries oxygen; oxygen metabolism is generally regarded as the limiting factor in any kind of high-output endurance athletics. As a runner, you may be able to sprint at the same pace that Eliud Kipchoge or Brigid Kosgei run marathons but you quickly hit oxygen debt, while neither he nor she are even close.

Does higher testosterone (with functional androgen receptors) result in superior athletic performance? No and also yes. Not generally for men within the normal male range, where there’s a ‘sufficiency’ model, viz., unneeded testosterone is biologically ignored. For women, unequivocally yes, when it ranges outside the healthy-XX-female band.

Evidence: the standing women’s world records and top-ten lists in track and field. Women who are nudged (or forced, as was the case with Russian, Eastern European, and Chinese athletes with state-sponsored doping) in the direction of male levels of bone density, muscle composition, and oxygen-carrying capacity are stronger and faster than women of similar or even greater talent who are not synthetically jacked.

The analogy with Michael Phelps is frequently raised: Caster Semenya, the most well-known 5-ARD XY woman, as the female equivalent of Phelps. This is a false analogy. Phelps was (is) at the edge of the male distribution in physical capacity, which is entirely unsurprising in an elite athlete. He is still within the male range. There are many NBA players with longer arms and bigger feet. There are no XX women, truly not one in the world, with testosterone levels as high as Semenya. Phelps is an extreme male, but Semenya is not an extreme XX female.

This does not mean Semenya and others with 5-ARD XY are not women. It does mean that simple dichotomous thinking, that there are exactly two sexes, is in error.

For sport, it means that XX women, no matter their talent, competitiveness, and the intelligence of their training, cannot win against moderately talented XY women. Inclusiveness is good, but not without examination. In the case of sport, inclusion of 5-ARD XY women disadvantages all XX women.

In careful academic writing a distinction is made between sex, a biological condition, and gender, an interpersonal and social status. Sport is appropriately segregated by biological sex, not by gender. For fairness, women’s sports should include XX women only.

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