Why There Are Only Two Sexes


Men are men, and women are women. There is no third option.

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Are there only two sexes?

A generation ago, this might have seemed like a silly question. But given the rise of gender theory, transgenderism, intersexuality, and all of their related phenomena, the question now appears to be both complex and pressing. . .

What differentiates human males from human females? Is it the number of sex chromosomes? Is it the possession of the appropriate sex organs? Is it the amount of testosterone or estrogen? The difficulty is that none of these standards always works: some individuals are born with extra chromosomes, such as XXYY or XYY. Some individuals are born with both pairs of sex organs. Some females have higher testosterone levels than many men.

Any single instance of an outlier counts as a serious objection to the binary sex distinction. Individual human beings are by nature either male or female just as by nature a number is either odd or even (by nature I mean the essence of a thing or what necessarily follows upon a thing’s essence). If one encountered even one number that is neither odd nor even, then this strange number would be enough to show that numbers are not “by nature either odd or even.” If one encountered even one triangle that was four-sided, this would be enough to show that triangles are not “by nature three-sided.” Similarly, if one encountered even one individual who did not fit the binary sex norm, then this single counterexample would be enough to disprove the traditional sex distinction.

Luckily, there are no non-odd-nor-even numbers. Nor are there no four-sided triangles, because four-sided triangles are a contradiction in terms. But is an intersex human being a contradiction in terms?

Biologically, intersex individuals seem to exist, as do people with other non-binary sex conditions. Hermaphrodites have male and female sexual organs. Some individuals with male organs have XX chromosomes. Some genetically male individuals have incomplete female sexual organs. What non-arbitrary but universal standard can there possibly be for determining sex?

What Determines Sex?

Our sex—male or female—is determined by our basic capacity to engage in sexually reproductive acts. Consider the following thought experiment by Christopher Martin:

Suppose we met a race of creatures—fairly clearly non-rational animals—that was very different from us: on Mars, say. And the question arises: are these creatures sexed? and if so, can we distinguish male and female? We need to think now how we would go about finding out these answers. We would not do it by investigating their psyches, nor even merely by just looking at (or cutting up) individuals. We would try to find out how they reproduced and what was the role of the different organs of the different individuals involved in reproduction. Thus, sex is a biological and teleological notion. Anything else which is called sexual is so called ultimately because it has some relation to this process, to these organs.

If we observe that the members of a species reproduce asexually, then we rightly conclude that neither male nor female exist in that species. But if we observe that two are required for reproduction to occur, we rightly conclude that the species reproduces sexually by the union of the two. We name these two types differently—as male and female—based upon the roles they play in reproduction. Such is why Aquinas held to a binary account of sex: “The distinction of the sexes is ordained in animals to the generation which occurs through coitus.” If human beings had no ordering to reproduction, or no sexual reproduction occurred, not only would one have no concept of gender, there would be no biological sex in human beings.

There, thus, can only be two biological sexes for human beings. In syllogistic form, what I am arguing is this:

  1. Biological sex is defined in relation to the roles played in sexual reproduction.
  2. Sexual reproduction involves only two, namely, male and female.
  3. Thus, biological sex is only two, namely, male or female.

Defects occur in nature, but defects imply a norm from which they deflect. A castrated man is still a male; a female with a mastectomy is still a female. The fact that one is born with ambiguous genitalia does not do away with one’s true sex. That it is hard to identify someone as male or female does not mean one is neither. Identical twins are hard to distinguish, but they are still distinct persons. Epistemological problems need not entail ontological ones.

Consider the case of plants that reproduce sexually. When we discover a plant missing parts of its sexual organs, we do not thereby conclude that we have discovered a third sex. Rather biologists rightly concur that what you have found is a defective plant. Likewise, in human beings, when one has an extra chromosome, or defective genitalia, you have just that: a sexual defect at the physical level. Such people often are wonderful, loving, and morally upright persons, but physically something has gone wrong.

Hermaphrodites are individuals with both pairs of sex organs. While in very rare cases some human beings have both pairs of genitalia, in no case whatsoever has it ever been observed that both pairs are fully functioning. True human hermaphrodites with both male and female sexual organs that fully function don’t exist. Such is why no cases of self-fertilization have ever been recorded in human beings.

Even if we did discover an individual human being with both pairs of fully functioning sex organs, such a case would not disprove the binary distinction. What you would have is someone who is both male and female; one who is able to act either as male or female depending upon the other sex with which that individual desired to reproduce. Hermaphroditism, rather than disproving the traditional binary distinction, actually reinforces it. We would not even know hermaphrodites existed, let alone be able to speak of them, unless we knew of the male-female binary.

Differentiating Potencies

How we fundamentally distinguish male and female then is based upon the two biological roles in reproduction. A human individual that has the basic capacity to reproduce with the female is biologically and truly a male. A human individual that has the basic capacity to reproduce with a male is biologically and truly a female. Male and female are defined in reference to each other, which is why they are correlative terms.

One must distinguish, however, between two types of “capacity.” Males are still males even when they are not actively reproducing with a female, or if they are unable to reproduce due to sterility, castration, or a genetic or physical defect. The sense of “capacity” or “potency” in question here is a fundamental one. A mechanic that doesn’t have the proper tools is still “capable” of fixing your car, but not in the same way in which a mechanic with the proper tools is “capable” to fix your car in the here and now. A male is the type of organism that is capable to impregnate the female. In other words, he could impregnate her, given that he has the appropriate functioning organs. A female, however, cannot impregnate another female.

For the sake of argument, let us grant that it might be physically possible for a female to have a complete sex change operation that she had a fully functioning male genitalia, male hormones, and male chromosomes that were fully integrated into her body. In this case she would cease to be female, but become a male. Even if complete sex change operations were possible (which they will most likely never be), such operations would be no argument against the traditional binary distinction. In such a case, the woman would not become a third sex; she would cease to be female and become a male.

Given that human beings reproduce sexually, they are biologically either only male or female. Men are men, and women are women. True hermaphrodites with fully functioning sexual organs do not exist in human beings. There is no tertium quid.

John Skalko is an adjunct professor at St. John’s Seminary and a PhD Candidate in Philosophy at the University of St. Thomas, Houston.

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