As a biologist, Dr. Ari Berkowitz is concerned that more people aren’t aware of a fundamental fact about biological sex. We might expect that a biologist would insist on the objective nature of “male” and “female” as innate, fixed, and unchanging biological realities. Evolutionary biologist Colin Wright, for example, argues for sex realism, a position which, he says, derailed his plans to become a biology professor.
But no. According to Berkowitz, “Biological sex is as nuanced as gender.” He’s written a whole book about it. The thesis is in the title: The Binary Delusion: How Biology Defies the Myth of Two Sexes.
As he explains in a recent article at The Conversation, “Biological sex is neither binary nor a spectrum.” Instead, it’s “multidimensional.” This is because “there is overlap between females and males in each sex-related biological trait.” And by that he means both primary and secondary sex-related traits. Let’s examine his case.
Primary Sex Characteristics and Intersex Anomalies
The physical structures related to human reproduction, such as chromosomes and the internal and external organs, are called primary sex traits. They are discernible in utero and present at birth. Those features that develop around puberty — like facial hair and a deeper voice in men or breast development in women — are called secondary sex traits.
Most people understand that secondary sex characteristics can vary, but Berkowitz leaps beyond this uncontroversial observation and claims that primary sex characteristics vary as well, and in more or less the same way. His argument is based entirely on the existence of intersex conditions.
The term “intersex” describes individuals whose anatomy does not fall neatly into the category of male or female. A classic example of true intersex, writes Dr. Leonard Sax,
is an XX/XY chimera: an individual who is an amalgam of female and male. Such individuals often have ambiguous genitalia — not quite male, not quite female — and they may even have an ovary on one side and a testicle on the other side. An XX/XY chimera can arise as a result of simultaneous fertilization of a single ovum by both an X-carrying sperm and a Y-carrying sperm. Documented human XX/XY chimeras are exceedingly rare, with only a handful of cases reported in the literature worldwide.
How rare is the condition of intersex? Sax writes that it occurs in 0.02 percent of the human population.
Berkowitz cites a Department of Health and Human Services document, “Advancing Health Equity for Intersex Individuals,” which puts the number at 1.7 percent, but he goes on to argue that it may be as high as 6 percent. First let’s address the 1.7 percent figure, and then we’ll look at how Berkowitz bumps it up to 6 percent.
Sax traced the 1.7 percent number to a single source, a paper published in 2000 by Anne Fausto-Sterling. She arrived at this data point by expanding the diagnostic criteria for intersex to include deviations in hormone levels, a condition that had never before been categorized on its own as intersex. This resulted in a drastic inflation of the number of intersex individuals.
To be sure, there are situations in which a genetic or developmental anomaly can result in both abnormal hormones and a true intersex condition, such as the case of South African athlete Caster Semenya, whose story is told in the 2011 documentary Too Fast to Be a Woman? But people with hormonal imbalances are not ipso facto amalgams of male and female. As Sax points out, a person with a hormonal abnormality nonetheless still possesses either XY chromosomes and the standard male body parts and sex traits, or XX chromosomes and the standard female parts and traits. In other words, each is either a man, or a woman. They are not part one and part the other. When you remove the conditions Fausto-Sterling added to the diagnostic criteria, the percentage of intersex individuals drops by a factor of 100 to less than 0.02 percent.
Nonetheless, the 1.7 percent figure, which is sometimes rounded up to “nearly 2 percent,” took flight and has become an accepted data point, as evidenced by the 2025 HHS document Berkowitz cites.
But he goes on to claim that the prevalence of intersex may be as much as 6 percent:
Collectively, intersex variations occur in 0.2% to 6%, depending on which variations you choose to count.
To establish this higher number, he adds in another condition as a qualifying factor for intersex. He reports (without citation) that 4.6 percent of babies born in Denmark are born with hypospadias, and then suggests, “Perhaps these babies should be added to Fausto-Sterling’s 1.7 percent estimate, yielding up to 6 percent of the population.”
Hypospadias is an incomplete formation of the male urethra. Is it a variation? If variation means nothing more than something different from the norm, then sure. But is it intersex? Well, no. Berkowitz tacitly acknowledges this, not once, in fact, but twice — first when he suggests that the anomaly may be “underreported in some cultures [because] parents might view such a baby as insufficiently masculine,” and then again when he speculates that the “visibility of this population … is reduced dramatically by surgery [which is performed] so that children can urinate while standing without making a mess, as typical boys can do.”
Males born with an abnormally formed urethra do not have female sex characteristics by any stretch of the imagination. They are not part male and part female. They only count as intersex based on a significantly revised definition of “intersex.” Berkowitz is explicit about this, when he describes “intersex” as an umbrella term to denote “differences of sex development,” which could be interpreted to mean just about anything.
Y Chromosome: The Differentiating Difference
Berkowitz begins by stating that all human embryos initially have the same reproductive tissues. I might have stumbled reading this had I not previously read Steve Laufmann and Howard Glicksman’s Your Designed Body (2024). They explain the processes of sexual differentiation:
Human embryos are programmed to become fertile females by default. To become a male, the embryo must invoke a precise sequence of specific molecules that, in effect, “switches on” maleness….
In the first several weeks, the human embryo remains sexually undifferentiated and develops tissues with the capacity to develop into either the male or female internal genitalia. The male tissue is called the Wolffian ducts and the female tissue, the Mullerian ducts.
At a certain point, a protein called Testis Determining Factor (TDF) induces the “switching on” of male development. And what induces the production of TDF?
As you might expect, the genetic specification for TDF is on the Y chromosome, so only males can manufacture this special molecule. As the embryo begins making TDF, the undifferentiated gonads take the cue to become testes.
Berkowitz doesn’t mention the Y-chromosome factor in sexual differentiation. Instead, he writes about sexual differentiation as if it’s just a matter of chance hormone levels:
All human embryos initially have identical reproductive tissues that later develop into genitals. If an embryo has lots of the hormones called androgens, like testosterone, it will form a penis and scrotum; if an embryo has less of these hormones or has cells that don’t respond to androgens, it will form a clitoris and labia instead.
Certainly the hormones play a role, but Berkowitz fails to mention the pivotal role of the Y chromosome, which codes for the formation of the testes that produce the abundance of male hormones in the first place.
Of course, the process of differentiation is more complicated than a simple switch. It involves enzymes and hormones and hormone receptors, but the fact remains that male sexual development only happens in embryos that have a Y chromosome. Nowhere does Berkowitz acknowledge this hard fact of biology.
Secondary Sex Characteristics and Sloppy Science
His article at The Conversation also discusses secondary sex characteristics. He includes charts on differences in breast size, body hair, voice pitch, waist-to-hip ratio, muscular strength, and size of brain hippocampus. But the fact that secondary traits don’t settle out in discrete, non-overlapping areas on a graph is unsurprising. Some women exhibit more muscular strength than some men, while some men may have a smaller waist-to-hip ratio than some women. Again, this is uncontroversial, but the way Berkowitz lumps primary and secondary characteristics together to conclude that “the multidimensionality of sex-related biological traits is a mosaic” is extremely sloppy science.
No Escaping the Binary
Returning briefly to the matter of identical reproductive tissues in the early embryo, Berkowitz employs an odd phrase to describe some of them:
primary sex traits — sex chromosomes, gonads, internal reproductive ducts that transport eggs or sperm, and external genitals — are not always consistently female-typical or male-typical.
Laufmann and Glicksman’s explanation makes sense of the clunky “internal reproductive ducts that transport eggs or sperm.” As they explained, these ducts are called by different names in the female and male embryos. But even in Berkowitz’s apparent attempt to obscure the sex distinction, the binary remains: eggs or sperm. Further down, in another strange wordplay, he refers to eggs and sperm as “large sex cells” and “small sex cells,” as if the only difference between them is their size:
Biologists usually define the sex of any animal based on whether it produces small sex cells (sperm) or large sex cells (eggs), which are produced by testes or ovaries, respectively. But physicians assign a person’s sex at birth based on genitals, which may or may not match their gonadal sex (ovaries or testes).
Here Berkowitz links to a paper on studies of animals as part of his argument against sex distinctions in humans. But setting that aside, apart from cases of true intersex conditions, in what situations are there mismatches in a newborn child between ovaries and female genitals? Or mismatches between testes and male genitals? I’m going out on a limb and guessing the answer to that is, there are none. The situation of recognizing the sex of a human is really not that complicated, unless one has intentionally set out to obscure the obvious reality of sexual dimorphism.
As much as Berkowitz tries to deconstruct the binary, he just can’t get away from it: sperm or eggs, testes or ovaries, male-typical or female-typical, lots of the hormones called androgens or less of the hormones called androgens. No matter how you slice and dice your terminology, it’s just not possible to fully escape the binary.
Berkowitz is a professor of Biological Sciences and the Director of the Cellular and Behavioral Neurobiology Graduate Program at the University of Oklahoma. His attempt to deconstruct the sex binary tells us little about the binary, but it reveals a lot about the state of the biological sciences in academia.