Binary Definitions of Sex Sound Simple. Biology Isn’t -

-Which is why legislatures should be careful about how they use it

A popular claim in current debates is that human sex is “binary” and can be defined purely by gamete size: Males produce small gametes (sperm). and Females produce large gametes (ova). In evolutionary biology, that distinction (anisogamy) explains why two reproductive strategies exist. At the level of species-level reproductive roles, it’s a clean and useful model.

But legislation does not operate at the level of abstract evolutionary theory. It operates at the level of individual citizens navigating society and herein lies the problem.

Gamete production is not directly observable in everyday life. Most people are not actively producing gametes at any given time. Some never do. Some no longer do. Some cannot. Some have differences of sexual development that complicate chromosomal or gonadal categories. A menopausal woman produces no ova. A teenage girl may not yet. An infertile woman may never have. None of that makes them socially or legally unintelligible as women so a definition that works in evolutionary biology does not automatically scale to public policy.

Policing a law requires operational criteria relying on observable or documentable traits. But observable traits in between humans overlap heavily. In fact in comparison with other species of great ape we show relatively little dimorphism; Height, shoulder width, voice pitch, muscularity, fat distribution, jawline shape, hairline and hair distribution all overlap to some degree due to underlying developmental and genetic processes which, like gamete production are not casually observable. These include; Hormone variation which also overlap between males and females and chromosomal exceptions (XX, XY, XXY, mosaic patterns) that can result in gonads that diverge from the karyotype. Biology is not being denied here it is being taken as seriously as it would need to be to consider making social legislation based on it.

Sex is a real biological category at the population level, if your focus is on population genetics, but at the margins, where law actually bites, human bodies do not present as neat binary switches so when legislation tries to enforce a strict binary definition based on reproductive roles, it faces an enforcement problem: someone must decide who counts and that is where the issue stops being theoretical.

If access to spaces is restricted on the basis of “biological sex,” then enforcement relies on visual assessment or documentation checks. Visual assessment disproportionately targets women who do not conform to narrow femininity norms; tall women, athletic women, butch lesbians, women with PCOS, women with naturally higher androgen levels, women with short hair, women with deep voices etc.

In other words, simplistic binary definitions do not just affect trans or intersex people. They create vulnerability for all women who sit at the edges of feminine presentation. Ironically, a policy intended to protect women may subject many women to suspicion.

This is not an argument that sex is meaningless. It is an argument that biological abstraction is not the same thing as administrable law. Good legislation should answer three questions: Can this rule be applied without invasive verification, what is the false positive rate and who bears the social cost of misclassification? If a rule cannot be enforced without humiliating "innocent" women, and we already have examples that it can't, it is not a well-designed rule.

I am not saying that conversations about who should have access to single sex spaces are invalid. Social and cultural concerns are also a thing, as are issues of fairness in physical sports, which I think are for the administrators to discuss with participants. But, evolutionary biology explains why two gamete types exist, it does not tell us how to police a bathroom door. Reducing human complexity to a "sex is binary" may feel clarifying, but when slogans become statutes, edge cases become people, and people deserve better than being reduced to their gamete size.