Sex and gender are binaries? Sorry, that's a scientific falsehood

Illustration of DNA and chromosomes

Ask many Americans these days and they will insist upon the scientific validity of a binary definition of sex. This line of thinking holds that there are only two sexes available for humans to inhabit: male or female.

As a doctorate-carrying scientist, however, I attest that this is false.

Biological sex can be defined in many ways. And when it is accurately defined, it's never binary. How so, you might ask? Let me explain.

First, let's start with what we can see — external genitals — the penis, testes, vulva and vagina. They must be binary, right? That's how we assign sex at birth! I’m so sorry, but, nah. Think about the frequency of people with naturally red hair. Roughly the same frequency of people are born with intersex characteristics, which means their reproductive organs/genitalia are somewhere between what is typical of what we usually describe as male (testes, penis) or female (ovaries, uterus). People with intersex characteristics are not as rare as you may think or as some recent politically charged articles may lead you to believe. Science reminds us that sex, here defined as genital sex, is not binary.

If you have XX chromosomes you’re a girl, and XY you’re a boy, right? Not necessarily. People can be born not just with XX and XY, but also XYY, XXX, XXY or XO.

Next, let's consider the chromosome. XX and you’re a girl, XY and you’re a boy, right? Not necessarily. To put it briefly, chromosomes can have really beautiful and varied arrangements. People can be born not just with XX and XY, but also XYY, XXX, XXY or XO. Chromosomal sex is not binary.

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Let's move on to hormonal sex. What I can tell you about this, friends, is that the levels of estrogen and testosterone in bodies is a distribution — just like you have ranges for things like thyroid functioning levels. And guess what? Cis men (men assigned male at birth) and cis women (women assigned female at birth) have both testosterone and estrogen. The same is true for trans men (men assigned female at birth), trans women (women assigned male at birth) and nonbinary folks (people who do not identify singularly as male or female). Estrogen is made from testosterone, and it's always happening in all of our bodies. Some bodies do it at a slower rate than others. Hormonal sex is a distribution, not a binary.

Now, let's consider the most reductive definition of sex. The gametes. What are gametes? Reproductive cells. Eggs and sperm. Sounds binary, right? As a human, you either produce eggs or sperm, yeah? Nah. On average, most cis women and trans guys are born with all of the eggs they may eventually ovulate with. But some are born without them. Some have their ovaries removed. So, they have no gametes. What about them? Cis men and trans women don't even start producing sperm until the onset of puberty. So, before puberty, they have no gametes. None. Some cis men are sterile. What about them? As you can see, some people, for these reasons, don't produce or have gametes at all. Therefore, there are three states: no gametes, eggs or sperm. It's a triplet, a trifecta. Gametic sex is not binary.

Fine. All those ways we categorize sex are not binary, you may be agreeing or conceding. But gender surely is, yes? Men are from Mars and women are from Venus, right? Nah. Here we now refer to sex of the mind, or what I’d like to call cerebral sex. This is one way to think about gender — how we, in our heads, think about ourselves and how we fit into our society and culture. While a doctor may need to know your genital or hormonal sex, society at large should always respect, define or treat people based on their cerebral sex. Cerebral sex, or gender, is the biological sex that may be furthest from the binary. Here we refer you to the lived experiences of your community members — they may be trans men, trans women, nonbinary folks and two-spirit people, amongst many other genders that are just a Google away. Don't "believe" that these genders "exist"? They exist because people inhabit them, and because our brains and lived experiences have dictated them. Your gender exists because you inhabit yours. Cerebral sex/gender is not binary.

If you have XX chromosomes you’re a girl, and XY you’re a boy, right? Not necessarily. People can be born not just with XX and XY, but also XYY, XXX, XXY or XO.

Now, you may still be disagreeing with me. You might be thinking that the binary definitions of biological sex are the true definition and that the variations I’ve described are just "exceptions to the rule." I challenge you, though — how good of a definition of biological sex can it be if it does not capture the lived biologies and experiences of millions of humans? I argue that my definitions of biological sex, each one I’ve provided, are more biologically accurate than a binary view of sex, no matter the definition you choose — they more fully encapsulate the truth of nature and humanity.

So, dear reader, next time someone asks you if sex is binary, ask them, "How are you defining sex?" If they can't answer, explain to them the different ways we can define biological sex. Explain that, no matter how you define it, biological sex is nonbinary.

Ash Zemenick received their doctorate from UC Davis in 2017 and is the lead director and creator of Project Biodiversify and manager of UC Berkeley's Sagehen Creek Field Station. Project Biodiversify (www.projectbiodiversify.org) runs workshops on inclusive teaching and develops teaching content to diversify biology education.