More than 99.9 percent of animal species that have developed since the emergence of male and female sexes reproduce sexually. And 95 percent of those animal species—including humans—have male and female sexes in separate individuals, whereby organisms are either male or female for their entire lives.

It may be surprising, but the two sexes follow a universal biological definition that applies to all species with male and female systems: the male sex is the phenotype (or structure) that produces the smaller gametes (i.e., sperm), while the female sex is the phenotype (or structure) that produces the larger gametes (eggs). The sperm are numerous and fast, contributing half the genetic material of the parent, but no resources for the survival of the fertilized egg (zygote). The eggs are relatively few, and very slow, contributing half the genetic material of the parent and all the resources for the zygote’s survival. Combine these two different gamete types together and a genetically unique individual is formed.

The technical term for this system is known as anisogamy (from the Greek aniso, meaning unequal; and gámos, meaning marriage), which involves the fusion of two gametes with different size and form. It is so efficient for reproduction and producing genetic diversity that it has evolved independently in nearly all lineages of multi-cellular organisms.

Biologists consider the evolution of the two sexes mathematically inevitable, because this system maximizes the efficiency of sexual reproduction—providing resources to the offspring through investment in large, nutrient-heavy eggs, while also maximizing gamete fusion through the production of many sperm that can quickly find an egg. This efficiency explains why the same system has evolved independently so many times.

Since the origins of the sexes, male and female have developed specialized reproductive anatomies for producing and releasing sperm and eggs. Sometimes, species combine these anatomies into one individual, where an organism is both male and female at the same time, and therefore, can produce both gamete types (a type of male-female system known as hermaphroditism). Mammals, however, separate these anatomies, and so individuals are either male or female through their entire lives; and therefore, can produce only one gamete type, not both (a type of male-female system known as gonochorism—from the Greek gone, generation; and chorizein, to separate).

Like humans, most species with male and female sexes have gonads (testes or ovaries). Testes are the factories for sperm, and ovaries are the factories for eggs. They also have genitalia to transport or receive these gametes so they can meet and fuse with each other. The gonads and the genitalia form the reproductive system.

This is how we identify sex in individual organisms: we observe the set of structures that produce and release either gamete type. These comprise what is known as the sexual phenotype. While the reproductive anatomies can be widely diverse in appearance across species, the sexes are always defined in the same way: by reference to their two different roles in reproduction, as differentiated by gamete type.

Yet these undeniable evolutionary facts have not escaped the deconstruction efforts of theorists and activists who seek to subvert and redefine the meaning of male and female for social and political purposes.

One popular belief—which denies the idea that male and female are objective, biological realities—is known as the sex spectrum. It includes the claim that (1) sex exists on a continuum, (2) male and female categories are socially constructed, and (3) one’s internal sense of self is the most accurate parameter for sex.

Many proponents of the sex spectrum are driven by the idea that the two sexes, as constructed categories, were created by society to oppress and control sexual minorities. Some of these proponents know that male and female are real, ancient realities, yet argue against them out of a sense of compassion for those who insist on denying them. Others believe that male and female do not capture the full reality of human experience, arguing that individuals exist on a continuum of traits between male and female. In all cases, they urge that male and female must be superseded by a subjective, feeling-based reality, whereby one’s internal sense of self is prioritized over the reality of our sexed bodies.

The sex spectrum has become so popular that even mainstream science journals such as Nature have published pieces claiming that “the idea of two sexes is simplistic.” And a 2022 Scientific American article claimed that the male and female sexes are mere “myths” originating in the nineteenth century.

It is not that all such theorists believe physical bodies themselves are socially constructed (though some do). Rather, sex-spectrum pseudo-scientists believe that the line dividing male bodies from female bodies is culturally and temporally defined. In other words, the criteria dividing male from female is thought to be determined by place and time, not biology.

In advancing the argument that sex exists on a continuum between male and female, these theorists tend to apply a mix of rhetorical strategies, each of which is worth examining in turn.

This first strategy is the foundation of the sex-spectrum claim: By detaching sex from its evolved function (reproduction), and constructing a definition based around the variation in people’s bodies (such as chromosomes, gonads, genitalia, hormones, facial hair, breasts, etc.), sex-spectrum proponents can then insist that human variation challenges the idea that there are just two sexes.

According to the sex spectrum, male and female are actually clusters of specific traits, and those who do not exhibit these clusters are neither male nor female. Proponents typically indicate these clusters as averages, or peaks, in a bimodal distribution (a range of data with two modes, or two clusters of data). Males represent one peak, and females represent another peak, while people in between the peaks of male and female are deemed to be intersex.

This constructed definition of male and female suggests that anyone who falls outside these clusters of traits is no longer a “true” male or female, but rather more male or less male, more female or less female, both or neither.

For example, according to this popular understanding of the sex spectrum, males with smaller penises are implicitly to be considered less male than males with longer penises, whereas females with larger breasts and wider hips would be more female than females with a larger clitoris and smaller breasts. This logic is even applied to chromosomal disorders, whereby males with an extra X chromosome are considered “less male,” and males with an extra Y are considered “more male.”

Once you understand this rhetorical technique—whereby the sex spectrum categorizes people according to a stereotyped and arbitrary ideal of what bodies “should” look like—the problems with it become obvious.

Male and female are not a mix of traits, but rather two distinct phenotypes that produce two different gamete types. Defining them as a mix of traits that can all vary independently ignores the evolutionary function of the sexes and the science of sex differentiation and development.

The function of the sexes is to produce two differently sized gametes to combine so that new individuals can be produced. The gonads, genitals, and hormones work together to support this function. These elements cannot be mixed and matched, because these traits develop in a sequential, hierarchical order. Each trait builds on the other: The chromosomes have genes that determine the development of the gonads (the gamete factories), which in turn secrete hormones that help build the internal and external genitalia (the transportation systems for the gametes).

The reason why there are two average values in this distribution when sex differences are plotted in this way is because there are only two groups being compared: the two sexes. One distribution shows variation in anatomy, physiology, and behavior within males, and the other distribution shows variation in anatomy, physiology, and behavior within females.

For instance, height differences between the sexes can be considered bimodal, and they overlap significantly. Yet this does not mean that sex itself is bimodal, or that short males are suddenly “less male,” or that tall females are suddenly “less female.” There is one distribution of height for males, and one distribution for females, and where the distributions overlap is where the sexes have traits in common.

Arguing that male and female cannot be defined because they share traits in common would be like arguing that apples and oranges cannot be defined because they too share traits in common—such as size and shape. And trait overlap between two categories certainly does not always mean that the categories are completely constructed by society.

While variation in sex-related traits is common, this variation exists within the evolved male-female system. Across all species that reproduce through anisogamy, male and female are defined according to the phenotype they develop with respect to either sperm or eggs. As sex-difference researcher Marco Del Guidice writes: “The biological definition of sex is not just one option among many equally valid alternatives; the very existence of differentiated males and females in a species depends on the existence of two gamete types.”

It’s notable that sex-spectrum proponents fail to define the axes of their “spectrum” of sex. Measurable units are never given, nor is it explained how exactly any individual can be accurately plotted on the distribution. All data sets must be defined with units, or else they cannot be plotted. But because the sex spectrum fails to provide a model (or even any kind of consistently applied logic) for how individuals can be predictably placed on such a spectrum, it fails basic scientific standards.

Having detached sex from reproduction, theorists and activists will then cite the existence of medical conditions that negatively impact the development of male and female reproductive systems, known as disorders of sex development (DSDs), to bolster their case. Because these disorders (often described as intersex conditions) do not always fit their narrow trait-cluster-based definition of the sexes, they claim such conditions as exceptions to the categories of male and female.

For example, proponents of the sex spectrum will often point to chromosomal conditions such as XO, XXX, XXY, and XXXY as additional sexes. But when we return to the biological definition of sex, all such conditions collapse into two simple outcomes: males and females.

Those with XO and XXX chromosomes develop the phenotype that produces large gametes (i.e., eggs), and those with XXY and XXXY develop the phenotype that produces small gametes (i.e., sperm). This is due to the genes within the chromosomes, which control the development path. Genes such as SRY, the sex-determining region on the Y chromosome, develop fetuses into males, whereas genes such as WNT4 and RSPO1 develop fetuses into females. Atypical chromosome combinations are not examples of additional sexes, nor are they exceptions to male and female categories; they are developmental disorders within the two categories.

Other disorders of sex development result in a sex opposite of what is expected from the chromosomes, such as an XX male or an XY female. This occurs due to gene translocations or mutations. For example, when the SRY gene translocates onto an X chromosome in an XX zygote, the fetus will develop testes and a penis. This results in a male despite the absence of the Y chromosome.

Other disorders result in under-masculinization of genitalia in males (smaller penis and testes) or over-masculinization in females (larger clitoris and fusion of the labia). But again: each one of these developmental disorders, when understood within the biological definition of sex, is not an exception to male and female; rather, they are genetic disorders within males and within females.

A third strategy often employed to support the sex-spectrum thesis is an appeal to the diversity of sex-determination mechanisms in other species as a means to argue that male and female are unreliable categories.

Sex-determination mechanisms are the regulatory systems, both genetic and environmental, that determine the development path an organism will take for its sex. Proponents of the sex spectrum are correct that sex-determination mechanisms are widely varied across species. But they are incorrect to conclude that this somehow invalidates the categories of male and female in humans.

Ironically, in fact, it is the opposite: the diversity of sex-determination mechanisms across species solidifies the biological categories of male and female in humans. That is because while there is a wide diversity of sex determination mechanisms across anisogamous species, every one of these mechanisms result in the production of males and females. As geneticist Leo Beukeboom and evolutionary ecologist Nicolas Perrin write in their book, The Evolution of Sex Determination:

Among the many surprises and oddities that biological investigations on sexes are revealing, one is particularly intriguing: even in higher eukaryotes with well-differentiated male and female roles, the initial steps of sex-determination pathways are bewilderingly diverse and extraordinarily labile. Evolutionary biologists seek to understand this conundrum: what might drive the surprising dynamics of such a fundamental process that, at the end, always leads to the same and simple outcome: the production of males and females?

In other words, while there is a wide diversity of mechanisms that develop an organism’s sex across species, the male-female outcome is the same. For example, unlike humans, who have a chromosomal sex determination system of X-Y, birds have a chromosomal sex determination system of Z-W. Eggs with ZZ develop as males, whereas eggs with ZW develop as females.

Other sex determination systems do not use chromosomes at all. For example, in crocodiles, sex is determined by environmental temperature, a process known as temperature sex determination (TSD). Crocodile eggs incubated at around 34 degrees Celsius and above develop as males, whereas eggs at temperatures around 30 degrees Celsius develop as females. Based on this range of temperatures, we would not say that sex in crocodiles exists on a spectrum. Rather, it is the range of temperatures at which sex will be determined that exists on a spectrum. The resultant sex is either male or female.

If sex-determination mechanisms were the same as biological sex, then species that determine sex by a range of temperatures would indeed have infinite sexes. Developmental biologist Emma Hilton has explained the absurdity of this kind of false claim:

By understanding that a range of determination temperatures can all lead to the same sex in crocodiles, one can then make parallels with different chromosome configurations leading to the same sex in humans. If we flip the argument so that different determination conditions define different sexes—an XY person is a different sex to an XXY person—we must carry that through to crocodiles. It would be dishonest to argue the premise for only humans. And effectively, because temperature is divisible into very small categories indeed, we end up with infinite crocodile sexes. They are named: 20.000001 [Degrees Celsius], 20.000002, 20.000003, etc.

After conflating the diversity of sex characteristics in humans with the supposed continuum of sexes, sex-spectrum advocates commonly cite the diversity of sex characteristics in other species to destabilize the public understanding of male and female. Common examples are hyenas, clownfish, seahorses, slugs, flowering plants, fungi, and other intriguing sexually reproducing species.

Let’s explore them.

First, the hyena. Hyenas have only two sexes, male and female, but they also have unique sex characteristics that at first glance seem to contradict the understanding of sex in humans. Specifically, the female hyena does not have an external vaginal opening but rather an external structure that looks like a penis. Because of this, proponents of the sex spectrum will argue that the sex characteristics of the female hyena are evidence that the divisions between male and female are arbitrary.

In fact, the penis-like structure on the female hyena is not a penis. It is a large clitoris with a central canal through which the female urinates, copulates, and gives birth. More importantly, when we return to the biological definition of sex, we can see she is a female because she has the phenotype that produces eggs. Thus, the female hyena is not an exception to the male-female dyad, but rather just another example of the profound diversity within the two sexes.

Clownfish have only two sexes, male and female, just like humans. And yet unlike humans, they are sequential hermaphrodites: they change sex based on environmental context (from male to female). In this process, not only do their surface-level sex characteristics change, but their entire reproductive role changes. The testes become ovaries, and they go from producing sperm to producing ova.

Sex-spectrum proponents will often use clownfish to argue that male and female are socially constructed categories. And yet at any moment, we know which clownfish are male and which are female based on whether they have the phenotype that produces sperm or the phenotype that produces eggs. And unlike clownfish, humans cannot change sex. Regardless, the sexes in both humans and clownfish are anything but socially constructed.

Seahorses have only two sexes, male and female, in separate individuals. Though they are not sequential hermaphrodites like clownfish, they have a unique form of sex: the males carry the developing babies, a trait usually found in mammalian females. During reproduction, the female seahorse deposits thousands of eggs in the male’s brood pouch, which the male then fertilizes. For the next few weeks, the male carries the babies before they are released from his pouch. He even provides them with prolactin, the same hormone that produces milk in pregnant mammals.

Sex-spectrum proponents may use male seahorses as an example to show the supposed constructed nature of how we define male and female: Look, males in some species can carry babies! And yet the male seahorse is defined as male precisely because it shares a unifying trait with all other males across species: the phenotype that produces sperm. Therefore, they are male in the same way that male humans are male. They are not exceptions to the two sexes. They merely exhibit in-sex variations.

Slugs have only two sexes, male and female, but most slug species are simultaneous hermaphrodites. This means that they have both male and female sexes in the same individual, and therefore, they can produce both gametes and fertilize themselves.

Sex-spectrum advocates, and even Margaret Atwood, have used the strange world of “slug sex” as a supposed refutation of the male-female dyad. Yet these hermaphrodites only reinforce the biological foundation of male and female. Unusual as they may be, they have only two sexes: their phenotype produces both sperm (male) and eggs (female). Thus, they simply provide an interesting example of how the dual system of sex can be composed in other species.

Flowering plants also get trotted out as supposed exceptions to the two sexes. But again, to repeat a theme: all plants that reproduce through the fusion of two gametes of differing size (anisogamy) simply represent the variation within the dual system of sex. Some of them have male and female flowers on the same plant, and others have flowers that are all male or all female. Female flowers produce the eggs, and male flowers produce the sperm.

Proponents of the sex spectrum often claim that humans can be hermaphrodites, but this is not the case. Humans, like all other mammals, are gonochoric (gone, generation; chorizein, to separate). This means that individuals are either male or female through their entire life cycle.

In mammals, the genetic and hormonal mechanisms that regulate sex development are mutually antagonistic: as one reproductive system develops, it inhibits the other from developing.

In humans, if the gonads differentiate into testes during fetal development, they release two hormones: anti-Müllerian hormone and testosterone. The former disintegrates the vestigial female structures, and the latter builds the male structures. Found most often in species with genetic sex determination, this mutually antagonistic system explains why there are no human hermaphrodites.

While individuals can have vestigial remnants of an opposite-sex reproductive system caused by genetic mutations—such as a male developing with a partial uterus, or a female with a mix of internal ovarian and testicular tissue—both male and female reproductive roles cannot fully develop and function in the same individual. As the principles of evolutionary and developmental biology would predict, there has been no documented case of a human having both fully developed reproductive systems in the same individual.

If there were such cases, or some are shown in the future, this would mean that it is possible for a human to fulfill both reproductive roles at the same time, and thus, such an individual would be male and female at the same time—a simultaneous hermaphrodite. But even in that case, it would be an example of how the two sexes can be composed in an individual, and not evidence against the existence of male and female.

Finally, when all else fails, sex-spectrum advocates will point to species that do not have male and female sexes—such as organisms that reproduce through gametes of the same size, a form of sexual reproduction known as isogamy (iso, same; gámos, marriage). Isogamy is most common among simpler organisms such as algae or fungi. It occurs when all gametes are morphologically similar, particularly in regard to size, and when the contribution of genetic material and resources to the offspring is shared equally between the two parents. (This differs from anisogamy because, as noted above, in anisogamy, the two gametes—sperm and egg—are morphologically different in size, and the contributions of resources to the offspring are highly unequal.)

Proponents of the sex spectrum claim that isogamous organisms can have tens of thousands of sexes. But this conflates mating types with sexes. Mating types are molecular mechanisms that regulate compatibility between gametes. Isogamous organisms such as fungi can have thousands of pairs of these complementary gamete genotypes (i.e., thousands of unique pairs of locks and keys), and therefore, they can have thousands of mating types—but not sexes.

Thus, citing fungi that do not reproduce through anisogamy as evidence against male and female is an obvious red herring. The only response required to such an argument is this: humans are not fungi.

Adapted, with permission, from Binary: Debunking the Sex Spectrum Myth, by Zachary A. Elliott. Published by Paradox Press. Copyright © 2023 Zachary A. Elliott.

Zachary A. Elliott is the founder of the Paradox Institute, which presents research and commentary on sex and gender in animated videos, articles, and books.