Are There More Than Two Human Sexes?

Description

SciShow's 'There Are More Than Two Human Sexes' uses disorders of sex development to claim that sex is more complicated than male and female.

Let's explore what they mean.

Sources

[1] Green, H. (2019). There Are More Than Two Human Sexes. SciShow, YouTube.

[2] National Health Service. (2019). Differences in sex development. NHS.

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[8] NIH. (2020). Androgen insensitivity syndrome. Genetics Home Reference, National Library of Medicine.

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[13] NIH. (2020). Persistent Mullerian Duct Syndrome. Genetics Home Reference, National Library of Medicine.

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Transcript

In 2019, the science education YouTube channel, SciShow, released a video called 'There Are More Than Two Human Sexes.' Host Hank Green says in the intro that, "In high school biology, we learn that humans are born with either XX or XY chromosomes, and that a person's internal and external sex organs match those chromosomes. It turns out, however, that sex isn't that straightforward."

In this video, Green describes several congenital medical conditions of the reproductive system to prove that there is a spectrum of sex. These rare conditions (known as Differences in Sex Development, DSDs for short, and classically known as intersex) produce atypical chromosomal, hormonal, and genital variations.

But, do these congenital medical conditions form new sexes, creating a sex spectrum as SciShow claims? Or are these conditions simply variations within males and within females, revealing the diversity of the sex binary?

To answer this question, we have to define what sex is.

SciShow defines sex as "chromosomes, hormones, gonads, and genitals." This definition of sex implies that any atypical combination within these four categories provides evidence for a sex spectrum. The problem, however, is that even in cases where these categories do not align, the genetic and hormonal mechanisms still produce a male or female. Why?

Because individuals with DSDs still develop reproductive systems organized around either sperm or ova.

And here we discover a crucial component to the definition of sex that Green never mentions: gametes.

A male is the sex that develops a reproductive system organized around small gametes, and a female is the sex that develops a reproductive system organized around large gametes. As two evolutionary biologists note, sex-related traits vary, but the ultimate distinction between sexes is found in gametes: "In many species, a whole suite of secondary sexual traits exists, but the fundamental definition of sex is rooted in this difference in gametes, and the question of the origin of the two sexes is then equal to the question of why do gametes come in two different sizes."

Any definition of sex that does not include the reason why we have a sex in the first place (namely, to support the production of sperm or ova) is missing the mark. By ignoring the definition of sex, Green is able to claim that variations in chromosomes, hormone production, and genital morphology form new sexes.

The issue with Green's assertion of a sex spectrum is that DSDs do not form new sexes but, rather, show the variation within males and within females. To create a new sex, a new reproductive function would have to be evident, which would require the existence of a third gamete type. However, individuals with DSDs still develop a system organized around sperm or ova, not something new.

Let's review several of the congenital medical conditions that Green discusses.

For example, individuals with XXY and XYY chromosomes are males (as they develop a system organized around sperm) and individuals with X or XXX chromosomes are females (as they develop a system organized around ova).

Understanding more complex DSDs reveals the underlying mechanisms which produce the sex binary.

Conditions like Complete Androgen Insensitivity Syndrome (CAIS) show us the independence of the SRY gene and androgen receptors. Despite SRY activation, the XY fetus with CAIS develops a female phenotype because of an insensitivity to androgens. Swyer syndrome shows us that when the SRY gene is not activated in an XY fetus, female reproductive structures develop, leading to a female phenotype. XX male syndrome shows us what happens when the SRY gene translocates to an X chromosome, causing male development in an XX fetus. And individuals with genetic mosaicism, who have a mix of XY and XX cells, develop as either males or females, not both and not in between.

Now imagine having a difference in sex development, such as Congenital Adrenal Hyperplasia (which can cause atypical genital development in girls), and you find that your condition is being used to argue that you're not female. Or imagine having Persistent Mullerian Duct Syndrome, PMDS (which causes parts of the Mullerian structure, such as a partial uterus, to remain in boys), and someone claims that you're not male.

Girls with Congenital Adrenal Hyperplasia are female, as they developed towards the production of ova, and boys with PMDS are male, as they developed towards the production of sperm.

As you can see, each DSD develops towards one or the other reproductive system, and the individual is either male or female, not something in between, nor a third sex.

If Green defined biological sex as the two evolved reproductive anatomies organized around two gametes, then he could not make the argument that there are more than two sexes.

So, if you see the SciShow video being presented, understand that:

1) They do not use the fundamental definition of sex.

2) They present decades-old research about DSDs, not new scientific discoveries.

3) They 'other' people with complex medical conditions as neither male nor female.

Hank Green is correct that sex development is more complex than what most of us learned in high school, but this does not mean there are more than two human sexes. There are only two gametes, and therefore only two sexes. The complexities of sex differentiation and development show us the amazing variation of anatomy and physiology within males and within females. We do not have to deny science to accept this diversity.

I'm Zach, for the Paradox Institute.

© 2020 Zachary A. Elliott, All Rights Reserved.