Genetics can be complex, but that doesn’t mean biological sex is a mystery

In short, everything that has sex, has a sex.

In December 2019, Twitter exploded when J.K. Rowling, the author of the immensely popular Harry Potter book series, sent a tweet that boiled down to this:  biological sex is real.

The tweet was in support of Maya Forstater, a woman who lost her job due to tweets she wrote that prioritized a person’s biological sex over gender identity in a number of circumstances.

Some called Rowling’s tweet a dog-whistle for anti-transgender views — an accusation frequently lobbed any time a public figure, especially a woman, suggests that distinctions between transwomen and natal women might be necessary.

Others sought to throw a wrench in her central claim by listing exceptions to the usual script of XX sex chromosomes = female, and XY sex chromosomes = male.

The point they were making was this: biological sex is irrelevant when faced with the complexity of the human genome.

And it is true that biology is complex. The medical literature is full of cases that, while rare, add up to numerous exceptions to the XX/XY dichotomy.

There is the case of a female with XY sex chromosomes who gave birth to a daughter who also had XY sex chromosomes. There is also the case of a female with both XX and XY sex chromosomes dispersed across different cells, and whose unique genetics was traced back to two opposite-sex zygotes (fertilized eggs) fusing into a single embryo. There are males with XX sex chromosomes — a condition known as De la Chapelle syndrome — who can attribute their male development to genes usually found on the Y chromosome ending up on the X.

These cases alone are enough to illustrate that all-or-nothing rules cannot define the relationship between sex chromosomes and observed sex.

However, as far as the legal and political issues that motivated Rowling’s tweet go, directing the conversation toward the complexities of the human genome is a red herring.  That’s because humans don’t interact with one another at the microscopic domain of chromosomes and genes. Humans recognize sex by observing anatomy in the form of primary and secondary sex characteristics.

Primary characteristics are external and internal organs directly related to sexual reproduction. In human females, they are the vulva, vagina, cervix, uterus and ovaries, which produce eggs. In human males, they are the penis and testes, which produce sperm.

While body parts can change in some amazing ways across species, the consistency of females making eggs, and males sperm, is what makes “male” and “female” steady categories across all sexually reproducing life forms — from trees, to bees, to people.

In short, everything that has sex, has a sex.

As the name suggests, primary sex characteristics are the primary means of defining sex. But for obvious reasons, they’re rarely how people determine the sex of people around them.

For this, we have secondary sex characteristics.

Secondary sex characteristics are strongly associated with sex, with some examples being body hair, height, voice pitch, and fat distribution. While they don’t define sex, these traits cluster in such a way that means most adults appear unambiguously male or female —  babies, monkeys, dogs, and software can all reliably discern sex based on secondary characteristics alone.

Despite widespread anatomical and physiological differences between the sexes, science and industry have historically conducted research and designed products with just the male in mind, a decision that has led to disastrous outcomes for women, as author Caroline Criado Perez chronicles in her book Invisible Women: Data Bias is a World Designed for Men.

Secondary characteristics appear during puberty, shaped in large part by spikes in the sex hormones estrogen and testosterone in females and males, respectively.

By taking cross-sex hormones and getting cosmetic surgeries, trans-identified people can feminize or masculinize some features. However, even with these interventions, it can be difficult — and, in the case of some traits, impossible — to overwrite anatomy. Gendered clothes, make-up and hairstyles can serve as traditional cultural signifiers of sex, the same way they do for non-trans people. But at the same time they strengthen stereotypes that feminists have been striving to dismantle for centuries, especially if a person only adopts the behavior as a means of social signifying rather than expressing personal style.

No matter the individual trans person’s path, there are no guaranteed results. Some end up “passing” as the opposite sex. Others, despite their best efforts, do not. Furthermore, some transgender people — whether due to health reasons, financial constraints, or personal preference — do not seek to change their appearance at all.

The result is a diverse community — something LGBTQ groups frequently refer to as the “trans umbrella” — united by identity rather than any particular anatomy.

Self-identification laws seek to prioritize that identity over any other attribute when determining legal sex. With that in mind, savvy rhetoric that categorizes biological sex — even the concept of male and female — as outdated, old fashioned, or even “essentialist” is a useful tool to that end: if biology has caveats, why not just let people identify however they like?

The problem with self-identification is that it doesn’t stop biological sex from being relevant — especially for females. The same biological traits that define and distinguish them as female — their primary and secondary characteristics — are the ones that can make them vulnerable.

That vulnerability doesn’t stem from any intrinsic defect of the female body. Male and female bodies are different — not better or worse than one another. It stems from the specific way that males can hurt females, and a patriarchal society that centers males — as Criado Perez showcases in her book — and minimizes the harm that they, as a class, perpetrate against females.

Radical feminist activists have sought to raise awareness about how self-identification laws and policies set up a system in which trans-identified males can harm females. The multiple cases of self-identified transwomen sexually assaulting — and even impregnating — female inmates in prisons show that the concerns of radical feminists aren’t just theoretical. They are already happening.

But instead of engaging with these feminists in order to come up with solutions that protect the safety and dignity of both trans-identified people and women, the reaction of trans activist groups has been to try and shut discussion down.

In March, the British feminist historian Selina Todd was banned from speaking at an event commemorating the 50th anniversary of a women’s liberation conference — a talk she was invited to speak at months earlier — because of her association with Woman’s Place UK, a group organizing against self-identification laws.

Prior to a February panel organized by  Women’s Liberation Front (WoLF)  critiquing gender identity at the Seattle Public Library (SPL), trans activists groups petitioned the libraryto cancel the event. The SPL stood firm, but the event required heavy police presence, and protesters surrounded the event to intimidate attendees before and after the event.

But screaming will not make concerns go away. In fact, these acts of intimidation and violence are only affirming them. And rhetoric that downplays biology is acting in the same vein.

Trans rights activists are right that biology has its caveats. A female simultaneously giving birth and having a Y chromosome wouldn’t be a thing (albeit a very, very,very rare thing) if it did not. But self-identification comes with its own drawbacks, almost all affecting females. Rhetoric that tries to downplay this fact is bad science and bad faith.

M.C. Kluge is a science writer and feminist.

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