Human Biology Is Not Binary

It is often said that each of us is either male or female biologically. It is said that even if a person feels that their gender identity does not match their biology—that, for example, despite having a penis and scrotum, they feel like a woman, or despite having a vagina and breasts, they feel like a man—their biology is still clear and consistent.

This conflict between what they feel and what they are biologically seems to be a strictly human thing because certain other animals are simply male or female.

At first glance, this line of thought may seem to make sense. Although more and more people now identify as transgender or nonbinary, the gender they identify with is different from the sex they were assigned at birth, which is generally the sex that corresponds to their genitals.

So one might say that a person’s visible body (at birth) is unambiguously of one sex, even if their felt gender does not match this sex. One might argue that their gender is not part of their biology and thus that there is a conflict between their psychology and their biology.

But biologists know this view is incorrect. Biological sex characteristics do not fit a binary classification, whether we consider genitals or brains, and whether we look at people or nonhuman animals.

Our genitals can be intermediate between a penis and a clitoris. We can also mismatch between our chromosomes (typically XX for females and XY for males), our gonads (ovaries or testes), and our genitals. These mismatches occur for a wide variety of reasons. Understanding what causes them helps to begin with the typical development of anatomical sex characteristics in humans and other mammals, which occurs prenatally.

Development of Human Anatomical Sex Characteristics

Typically, if an embryo has a Y chromosome, then a gene on that chromosome called SRY (for Sex-determining Region of the Y chromosome) causes genes on other chromosomes to synthesize a series of proteins that turn an undefined pair of internal structures into testes; in the absence of a functioning SRY gene, these same structures become ovaries instead.

If testes develop, they begin producing androgens—male-promoting steroid hormones like testosterone, secreted into the bloodstream—which in turn cause a single set of undefined external structures to become a penis and scrotum; in the absence of androgens, these same structures become a clitoris and labia, respectively.

These androgens also cause one set of internal structures to develop into ducts that will later deliver sperm from the testes to the penis; these structures disappear without androgens. Meanwhile, the testes secrete a different hormone that causes the second set of internal structures to disappear; in the absence of this second hormone, these internal structures instead develop into the fallopian tubes that will deliver eggs from the ovaries to the uterus.

But this is just the most typical series of events. Many variations occur. For example, some people have different chromosome combinations, such as XXY or XO (just one sex chromosome, an X).

Others, regardless of their chromosomes, have intermediate amounts of androgens. We all secrete androgens from our gonads and adrenal glands, regardless of sex. An intermediate level of androgens or a high androgen level but reduced sensitivity to androgens leads to embryonic development of genitals that are intermediate between a clitoris and a penis, and intermediate between labia and a scrotum.

For example, in congenital adrenal hyperplasia (CAH), the adrenal glands produce extra androgens. In an XX embryo, these extra androgens cause what would otherwise become a typical clitoris to grow into what appears to be either an elongated clitoris or a short penis.

In androgen insensitivity, the protein receptors that respond to androgens and mediate its effects either do not respond well or do not respond at all. In this case, in an XY embryo, the structure that would otherwise develop into a typical penis becomes a very short penis or, if there is complete insensitivity to androgens, a typical clitoris. XY people with complete androgen insensitivity appear typically female, or perhaps one should say ultra-feminine, as they have no body hair, for example.

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In still other cases, due to weak production of another androgen, dihydrotestosterone, an XY individual is born with typical female genitals, but at puberty, when testosterone production rises dramatically, their genitals are transformed into a penis.

In the Dominican Republic, where this is common in some communities, these individuals are called “guevodoces,” which is typically translated as “penis at twelve,” though it literally means “eggs at twelve,” as their testicles also descend from their abdomen into their scrotum at this time.

Treatment of Intersex People

These and many other related conditions are collectively described by the umbrella term intersex or differences of sex development. Since the 1950s in the U.S., intersex infants have often undergone surgeries–sometimes repeatedly, as there are often surgical complications–to modify their genitals to make them look typically male or (more often) typically female. (As one surgeon famously said, it is easier to “make a hole” than to “build a pole.”)

The goal of these surgeries is simply to create genital anatomy that fits within the binary sex categories we are accustomed to. These surgeries typically have the parents' consent, but not of the infants, and the child may later identify with a sex that conflicts with their surgically modified genitals.

You might think intersex cases are unusual medical abnormalities that should not alter our understanding of typical, healthy people as simply male or female. But we should consider this statement carefully. How unusual are these conditions, and in what sense are they abnormal? Should we simply ignore these cases when we think about human characteristics as a whole?

First, how unusual are these conditions? Estimates vary greatly, probably because many anatomical variations are continuous, not discrete, and different biologists and physicians count these conditions differently. In addition, reporting such conditions varies dramatically in different parts of the world.

Anne Fausto-Sterling and her students at Brown University estimated, based on published studies, that 1.7 percent of the population has one or another intersex condition. Most of this 1.7 percent is due to CAH.

But this estimate does not include hypospadias, in which a baby is born with what appears to be a typical penis, but with urine exiting from somewhere along the underside of the penis, rather than the tip. The urine exit location likely depends on the number of prenatal androgens or the sensitivity of the androgen receptors.

Estimates of the frequency of hypospadias differ dramatically across countries, presumably because it is underreported in some cultures, where parents might view such a baby as insufficiently masculine. In much of Asia and South America, reported frequencies are low, but in North America and Europe, they are more than ten times higher–4.6 percent of babies in Denmark. Perhaps these babies should be added to Fausto-Sterling’s 1.7 percent estimate, yielding up to 6 percent of the population.

The visibility of this population, however, is reduced dramatically by surgery. In most cases of hypospadias in the U.S., the urethra is surgically rerouted so that children can urinate while standing without making a mess, as typical boys can do.

The concern is less about inconvenience and more about humiliation. As two clinicians once put it, the child “does not want to be discovered in what he may view as a feminine position for urinating.”

Second, in what sense is being intersex “abnormal”? A technical definition of “abnormal” would be outside the central portion of a normal distribution. As we’ve already seen, these conditions are surprisingly common in this strictly statistical sense.

But another sense of “abnormal” is a medical problem. Is being intersex a medical problem?

Here, it gets complicated. For example, some people with CAH require medical treatment due to a lack of adrenal hormones that regulate salt balance. (The precursors to these hormones are turned into androgens instead.)

But in other intersex cases, there are no symptoms unless you consider atypical-appearing genitals a symptom. Having a genital intermediate between a clitoris and a penis requires no treatment to preserve health. Urinating from the underside of the penis is an inconvenience but does not threaten health. It may be a social problem, but it is not a medical problem.

So as a whole, it is at least questionable whether intersex conditions are abnormal, either statistically or medically.

For these and other reasons, human biology is not binary.

A version of this post appears on Medium.