A Viral Twitter Thread Gets Sex Wrong

How scientific language is used as a smokescreen to obscure an underlying ideological agenda.

Image Credit: Modified from Tyler Nix on Unsplash

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“Falsehood will fly, as it were, on the wings of the wind, and carry its tales to every corner of the earth; whilst truth lags behind; her steps, though sure, are slow and solemn…” – Thomas Francklin, 1787

In the era of social media, it seems as though the internet is overrun with posts, articles, and threads claiming to debunk outdated notions of sex and gender. Last week, a particularly viral Twitter thread caught the attention of millions, which purported to debunk “transphobic” notions about the biology of sex. As of this writing, the thread has garnered over 7000 retweets, reaching the screens of nearly two million users.

However, as with many viral threads of this nature, the content is not new or original. The thread credits someone named Margaret Mosberger-Kijonka for the content, though a 2017 article in The Observer credits a biology teacher at Arlington High School in Lagrangeville, New York, named Grace Pokela as the source. Despite its mysterious origins, viral threads can be powerful vectors for spreading misinformation about biology, and so its claims should be addressed.

To the layperson, threads like this appear to deftly do away with archaic and oppressive beliefs that have somehow managed to persist well beyond their expiration date. In reality, however, they only demonstrate that twisting the strands of fact into knots is far easier than untangling them.

Misunderstandings about the fundamental nature of biological sex are widespread among all parties involved in the debate. It often happens that someone will point out another person’s misunderstanding and attempt to disprove it with yet another misconception. This is precisely what is occurring in this thread, where a self-proclaimed “science teacher” is presenting an excessively simplified depiction of the connection between sex chromosomes and sex, only to proceed on to reveal their own ignorance on the subject.

Below, I will address each claim made in the thread. My goal is to provide deeper insight into the issue at hand and to help readers recognize and address these and similar arguments when they arise in the future.

The thread starts by presenting a “transphobic post” that was made in response to someone asking for a definition of the word “woman.” The post supposedly claimed that, “In a sexual species, females have two X chromosomes and males have an X and a Y, I’m not a bigot it’s just science.”

In response, “a science teacher” rises to the occasion to set the record straight and put bigotry in its place.

The science teacher responds with the following:

First of all, in a sexual species, you can have females be XX and males be X (insects), you can have females be ZW and males be ZZ (birds), you can have females be females because they developed in a warm environment and males be males because they developed in a cool environment (reptiles), you can have females be females because they lost a penis sword fighting contest (some flatworms), you can have males be males because they were born female, but changed sexes because the only male in their group died (parrotfish and clownfish), you can have males look and act like females because they are trying to get close enough to actual females to mate with them (cuttlefish, bluegills, others), or you can be one of thousands of sexes (slime mold, some mushrooms.)

I’ll clarify this confusion piece by piece. But before we do, I need to lay down some basics.

First, it is important to understand that not all sexually reproducing species have “sexes.” That’s because when we speak about an individual’s sex we are referring to whether they have the function of producing small gametes (sperm) or large gametes (ova). Males by definition have the function of producing small gametes, and females large gametes.

Species that reproduce by combining gametes of different sizes are called anisogamous. But some species are isogamous, meaning they reproduce by combining gametes of the same size. Because of this, while isogamous species still reproduce sexually, they do not have “sexes.” Instead, they sometimes have what are called “mating types,” which denote the sexual (in)compatibility of equal-sized gametes.

In light of these facts, the “transphobic post” is indeed wrong in its claim that “In a sexual species, females have two X chromosomes and males have an X and a Y.” And it’s wrong for several reasons. First, it’s wrong because not all sexual species contain males and females. Second, not all species with males and females determine sex using chromosomes. And third, not all species that determine sex using chromosomes do so via X and Y chromosomes.

As the science teacher correctly points out, birds use Z and W sex chromosomes to determine sex, and some reptiles use temperature to determine sex. There are indeed a variety of what are called “sex determination mechanisms” in nature, but these describe the process by which certain genes trigger and regulate sex development that result in males and females. But while different species may mechanistically determine sex in different ways, an organisms’s sex is always defined the same way—by whether they have the function of producing sperm or ova.

While birds use different chromosomes than mammals to determine an individual’s sex, and some reptiles use temperature, we know who the males and females are by the type of gametes they do or would produce. We know that hermaphroditic flatworms, which are both male and female, take on the female role when they lose a penis sword fight because we can observe who is delivering sperm and who is having their eggs fertilized. We know that clownfish change their sex because we observe them changing from sperm producers to egg producers. And we know that males mimic females in some species because of the type of gametes they produce.

In short, while the science teacher is correct to point out that sex can be determined in a variety of ways, and that nature also presents a diversity of mating strategies, it is nevertheless still true that females in humans—i.e. “women”—generally have XX chromosomes. But regardless of sex chromosome composition, “women” refers to members of the female sex.

Lastly, the science teacher’s claim that slime molds and mushrooms have “thousands of sexes” is simply not true because these species are isogamous, and these “thousands of sexes” are simply mating types, not sexes.

Following the science teacher’s survey of sex determining mechanisms and mating strategies found in nature, she anticipates that the original post was referring to humans specifically. She then begins listing a variety of differences/disorders of sexual development (DSDs) to refute the claim that females are XX and males are XY.

Oh, did you mean humans? Oh ok then. You can be male because you were born female, but you have 5-alphareductase deficiency and so you grew a penis at age 12. You can be female because you have an X and a Y chromosome but you are insensitive to androgens, and so you have a female body. You can be female because you have an X and a Y chromosome but your Y is missing the SRY gene, and so you have a female body. You can be male because you have two X chromosomes, but one of your X's HAS an SRY gene, and so you have a male body. You can be male because you have two X chromosomes- but also a Y. You can be female because you have only one X chromosome at all. And you can be male because you have two X chromosomes, but your heart and brain are male. And vice - effing - versa. Don't use science to justify your bigotry. The world is way too weird for that shit.

To be clear, it is true that not all human females are XX and males XY. Remember, sex is a phenotype (an observable characteristics of an individual) and not a genotype (the genetic constitution of an individual organism). But the science teacher uses a series of (somewhat) legitimate exceptions to the universal claim that “females have two X chromosomes and males have an X and a Y” to then insert a totally false and ideological claim. I’ll go thruogh each of the cases.

Claim #1: “You can be male because you were born female, but you have 5-alphareductase deficiency and so you grew a penis at age 12.”

This is false. This condition, known as 5-alpha reductase deficiency or “5-ARD,” is a condition that only affects males. The Olympic “intersex” runner Caster Semenya is an example of someone with this condition. Without going into granular details, this condition impacts genital development in utero, so that infants are born with genitals that usually appear female or ambiguous, or more rarely predominantly male though often unusually small or malformed. They also have internal testes that produce testosterone during puberty that can cause their female-like or ambiguous genitalia to masculinize and form a functional penis.

People with this condition are not “born female,” though they may appear that way at birth given their female-like genitals. In countries without rigorous prenatal or neonatal genetic screening, they may be incorrectly recorded as female at birth.

Claim #2: “You can be female because you have an X and a Y chromosome but you are insensitive to androgens, and so you have a female body.”

This claim is also false. Complete androgen insensitivity syndrome (CAIS) is a condition that only affects males. While these individuals have genitals that appear entirely female and appear female on the outside due to their body’s unresponsiveness to testosterone, they still have internal testes which makes them biologically male.

I won’t deny that this condition tends to strain most people’s intuitions about males and females. But from a strictly biological perspective, they are male. However, for all social and legal intents and purposes, these people are generally accepted as female (but this does not make them literally female!).

Claim #3: “You can be female because you have an X and a Y chromosome but your Y is missing the SRY gene, and so you have a female body.”

This is largely true! I say “largely” because Swyer syndrome (as this condition is called) represents a difficult edge case given that their ovaries do not develop despite having all other female reproductive structures.

Claim #4: “You can be male because you have two X chromosomes, but one of your X's HAS an SRY gene, and so you have a male body.”

This is true. It’s a condition knows as XX male syndrome or de la Chapelle syndrome.

Claim #5: “You can be male because you have two X chromosomes- but also a Y.”

This is true—a condition knows as Klinefelter syndrome where males have an additional X chromosome, making them XXY.

Claim #6: “You can be female because you have only one X chromosome at all.”

This is also true, and is known as Turner syndrome.

Claim #7: “And you can be male because you have two X chromosomes, but your heart and brain are male.”

This statement is unequivocally false, and it’s absurd that it was included at the end of this list. An individuals sex is not determined by their “heart and brain.” Additionally, hearts and brains do not have a sex. While hearts and brains may differ in some ways due to biological sex, they do not define a person’s sex in the same way that primary sex organs (gonads) do, which have the function of producing sperm (testes) and ova (ovaries).

Ultimately, the aim of this piece was to confront the proliferation of ideological pseudoscience surrounding the biology of sex. As I demonstrated throughout, those who peddle such views often rely on superficially scientific language to lend an air of credibility to their arguments. However, once you look beneath the shiny veneer of scientific language, it becomes clear that their arguments are often riddled with inaccuracies and fallacies, serving only as a smokescreen to obscure an underlying ideological agenda.

By insisting in the end that any disagreement with their views is inherently bigoted, proponents of this pseudoscience reveal the true nature of their aims: to stifle debate and impose their own ideological worldview on others. In a time when the need for rigorous scientific inquiry has never been greater, we must reject such distortions of the truth and commit to seeking out and promoting evidence-based knowledge.

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