How sure are we that sex is binary?

The question is whether biological sex, the chromosomes, the hormones, and the anatomy are really as binary as we have been told. And the answer, it turns out, is less obvious than most people, including many scientists, would assume.

The most common argument used to defend the binary is anisogamy: the fact that sexually reproducing species produce two distinct types of gametes, large ones (eggs) and small ones (sperm), with no intermediate form. This is true, anisogamy is one of the most consistent features of sexual reproduction across the animal kingdom, and nobody is opposed to that. But using gametes alone to define the totality of biological sex, mainly in humans, is a bit like defining a person by their blood type. It captures one biological dimension while ignoring dozens of others, and building an entire theory of human biology on a single feature, however consistent, is not how science is supposed to work.

Biological sex is not only defined by gametes. It is a composite of chromosomal makeup, gonadal structure, hormone production, how sensitive each tissue is to a given hormonal signal, and the way all of these interact across a lifetime of development. And at each of these levels, variation is not the exception; it is the rule.

More Than XX and XY

Take chromosomes first. The standard model is simple: XX means female, XY means male. Clean, memorable, and taught from early childhood as though it were as fixed as the periodic table. But chromosomes are not a blueprint that executes itself. They carry genes, and genes need to be read, interpreted, and activated, or silenced, by a cascade of molecular signals that unfolds across development and continues throughout life.

Some of the genes most critical for sexual development are not even located on the sex chromosomes. Others are present in both males and females but expressed differently depending on timing, hormonal context, and molecular signals that can be modified by environment and experience, a process known as epigenetics. The same stretch of DNA can produce different outcomes depending on whether certain genes are switched on or off, and those switches can be influenced by everything from prenatal hormone exposure to stress to nutrition. This is one of the most well-established findings in genetics.

Add to this the fact that the proteins that receive and translate hormonal signals, the molecular machinery that allows testosterone or estrogen to actually do something inside a cell, vary between individuals in ways that meaningfully alter how the body and brain respond to the same hormonal environment. Two people with identical chromosomes and similar hormone levels can develop quite differently depending on how efficiently their cells read those signals. The binary is already complicated before you have left the molecular level.

Then there are hormones. Testosterone is commonly described as the male hormone, and estrogen as female, a shorthand so pervasive it has become almost invisible. Yet both are present in all human bodies, produced not only by the gonads but also by the adrenal glands, by fat cells, and even by the brain itself, which synthesises its own steroid hormones locally to regulate cognition and mood sometimes independently of reproductive status. Outside of specific events like pregnancy or ovulation, the average levels of estrogen and progesterone do not differ as dramatically between men and women as is often assumed. And while testosterone is on average higher in men, levels fluctuate throughout life, and they respond to behaviour, stress, social context, and age in ways that are anything but fixed. Nurturing behaviour lowers testosterone in men; sexual thoughts can raise it in women. These reflect how dynamic and context-dependent hormonal biology actually is, and they sit uncomfortably with any model that treats hormone profiles as stable, binary, and biologically predetermined.

These are the arguments at the heart of Agustín Fuentes’ 2025 book Sex Is a Spectrum: The Biological Limits of the Binary. Fuentes, a biological anthropologist at Princeton University, is careful not to dismiss the reality of biological sex or the fact that the large majority of humans develop along trajectories we recognise as male or female.

What he challenges is the assumption that these two outcomes represent discrete natural categories with a sharp boundary between them, rather than the most common positions along a continuous and multidimensional biological space. The binary, in his account, is a useful description of the most frequent outcomes, not an accurate map of the full biological territory.

What Happens in the Brain

Perhaps nowhere is this complexity more striking than in neurodevelopment. For decades, neuroscience operated under the assumption that brains could be sorted into male and female types, with distinct structures and characteristic functions following from biological sex. Popular books were written about it, therapeutic and educational interventions were designed around it, and it became one of those ideas that moved from scientific hypothesis to cultural common sense with very little friction along the way.

The actual research, however, has been far more ambiguous than that trajectory would suggest.

A large-scale analysis published in the Proceedings of the National Academy of Sciences examined brain imaging data from over 1,400 individuals and found that very few people had a brain composed entirely of features at the male end or entirely at the female end of the measured distributions. The vast majority of brains were a mosaic: some features more commonly associated with males, others more commonly associated with females, combined in patterns that were largely unique to each individual. When the researchers looked at whether people clustered into two distinct brain types, they did not. The categories dissolved into a continuum when examined carefully enough.

This mosaic does not arise randomly. It is the product of the same multifactorial processes that shape the rest of sex biology: genetic background, hormonal exposure across different developmental windows, and the continuous influence of environment and experience across a lifetime. These factors do not simply add up in a predictable way; they interact, and the same hormonal signal can produce different outcomes depending on which tissues are involved, which genes are active, and at which moment in development it arrives.

Gender identity, within this framework, emerges not from a single organising switch but from the accumulated history of these interactions across brain circuits involved in self-perception, embodiment, and social cognition. Neuroimaging studies show that these circuits respond preferentially to stimuli aligned with a person’s experienced gender identity, regardless of sex assigned at birth, suggesting that what we call gender identity is not a social label imposed on a neutral biology but a deeply embedded feature of how the brain represents the self.

What Science Has Had to Unlearn Before

None of this means that biological sex is not real, or that the patterns we observe between males and females across populations are meaningless. They are real, and they matter. But the history of science is full of cases where a category that seemed self-evidently fixed turned out, on closer inspection, to be far more complicated than the prevailing model allowed. Consider one of the most foundational principles in all of biology, the central dogma, the idea that genetic information flows in one direction only: from DNA to RNA to protein. It has been taught in every biology classroom for decades as one of the pillars of modern science. Earlier this year, a study published in Science described bacterial proteins that use their own structure as a mould to synthesise DNA, bypassing the nucleic acid template entirely. A principle that generations of scientists were trained to treat as settled turned out to need revision.

It is worth considering that whenever claims about gender and biology are presented as self-evident truths, it is important to question their validity. Public debate on this rests, at least in part, on the premise that biological sex is a clear, fixed, and binary fact that science has settled. What recent research actually seems to point to is that sex is a multidimensional biological space, that the boundaries between categories are less sharp than the textbook version implies, and that the people who fall outside the most common presentations are not exceptions or unnatural to the rule but part of the same biological continuum that produces the rule in the first place. Pride Month, among other things, seems like a reasonable moment to ask whether the certainty being said in every corner and debate was ever as solid as it was claimed to be.