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Our Biology Is Not Binary

Many of us have biological traits that are not simply female or male.

A few days ago, the U.S. Supreme Court ruled that according to the 1964 Civil Rights Act, people cannot be fired for being gay or transgender. This momentous decision should finally protect LGBTQIA people from employment discrimination across the U.S.

Transgender, gender non-conforming, and intersex people, however, are still treated poorly. They face practical hurdles to such things as using a bathroom in schools and public buildings and having identification cards that do not misgender them.

The common belief that we are all biologically either female or male is one reason for these obstacles to ordinary life. Many would say that our sex is unambiguously female or male from birth, even if our gender identity ends up being something else.

But as biologists have long known, this common and intuitive idea is misleading. The persistence of the female-male binary categorization, despite biological facts to the contrary, shows how ingrained the sex dichotomy is in Western cultures.

How do we decide if someone is female or male? Many would say that males have a penis and a scrotum with testicles and females have a clitoris and a vagina, and that this biological division is clear at birth and even earlier. But this is not necessarily the case.

For a variety of biological reasons, many of us have external genitals that are intermediate between a clitoris and a penis and intermediate between labia and a scrotum. Others have typically male chromosomes but typically female genitals. Still others have intermediate gonads, called ovotestes. Collectively, all these situations can be described by the umbrella term, intersex.

In teaching human physiology to thousands of undergraduates over 22 years, I have been struck by my students’ reactions to learning about intersex biology. I break my class into small groups to discuss challenging questions. My favorite question is about the sex of someone who has XY (typically male) chromosomes, had prenatal secretion of androgens (including testosterone) typical of males, but has never had functional receptors that respond to androgens. This is a real situation called complete androgen insensitivity syndrome.

In the first trimester of pregnancy, androgens cause an undefined structure to become a penis; in the absence of androgens, it instead becomes a clitoris. Androgens have numerous other anatomical effects in prenatal development and puberty. My students have already learned the basics of prenatal sex determination at this point, so they usually figure out that because this person has no response to androgens, they will have a typical clitoris and labia (and not a penis or scrotum), as well as a typically female adult body shape, including breasts, no body hair, and a higher-pitched voice. (They will also have undescended and thus externally invisible testes.)

I ask the students to vote individually (using electronic “clickers,” which keep their votes confidential) whether this person is female or male; in recent years, I have also offered intersex as an option. Most students choose male, with intersex in second, and female a distant third. We then have a class-wide discussion, in which students are typically more intellectually and emotionally engaged than for any other question in my course. After we are all clear about this person’s anatomy and physiology, I have sometimes polled the students again. On the second vote, some students switch their vote from male to intersex, but still only about 5% vote for female, despite the fact that this person would look and sound just like a typical female, even naked. In some ways, such as lack of body hair, this person might be considered ultra-feminine. So why do so few vote for “female”?

Many seem to feel that no matter what someone looks like, sounds like, or acts like, if they have a Y chromosome, they are male. Many react to the combination of a Y chromosome and a female body as troubling or impossible.

There are numerous other combinations of biological sex characteristics that also challenge notions of a simple sex dichotomy. These include people whose genitals are intermediate between a clitoris and penis. This can occur for several reasons, including because they have XY chromosomes but receptors that are partially insensitive to androgens, or because they have XX chromosomes but were exposed to extra androgens from their adrenal glands prenatally. Collectively, depending on how you count, intersex people make up somewhere between less than 0.1% and more than 1% of the population. In Western societies, we have largely hidden the existence of intersex people, apparently because they don’t conform sufficiently to our ingrained sex categories.

For generations, U.S. physicians have surgically altered infants’ genitals (obviously without their informed consent) to make them fit better into one or the other of our binary sex categories. These surgeries often have to be repeated to address complications and typically cause long-term pain, scarring, physiological problems, and reduced sexual pleasure when they are adults, as well as symptoms of post-traumatic stress disorder in some cases. Some physicians and others have argued, however, that there is nothing worse for a child than being a social outcast because they don’t fit neatly into one of our two sex categories. Such infant genital surgeries continue today in the U.S., despite the opposition of intersex activists and organizations.

Our biology is not binary. It is time we recognize that many of us have combinations of biological traits that do not fit neatly into either a female or a male box. Acknowledging this should help us climb out of these boxes and embrace our biological diversity related to sex and gender.

Read more about this topic here.


Kessler, S.J. (1998) Lessons from the Intersexed. New Brunswick, NJ: Rutgers University Press.

Karkazis, K. (2008) Fixing Sex: Intersex, Medical Authority, and Lived Experience. Durham, NC: Duke University Press.

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