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Morpheus offers the pills in the MatrixThis is the first part of a series about the complex biological realities of sex. Though the posts build on one another, each can be understood alone.

Content note: this post contains images and language that may not be safe for work.

1. Introduction

I first learned about the social construction of sex from a lovely trans woman named Kiki.

She said, “You may have heard before that gender is socially constructed, while sex is biological. But I’m here to tell you that what you’ve heard isn’t true. Sex is socially constructed too. So are you ready for the truth? Are you going to take the red pill or the blue pill?”

Three years later, I was diagnosed by my gynecologist with polycystic ovarian syndrome (PCOS), which means that my body produces hormones intermediate between “typical men” and “typical women.” What I learned from Kiki gave me context in which to understand what this meant about my body and who I am. But it’s still very hard for me to talk about. My hormones affect me in ways that are hard to see, so even most of my lovers don’t know. I can count the number of people in my personal life who know this about me on my two hands.

I picked the red pill. If you read on, you can take the red pill too.

The problem with calling sex “biological” is that biology is complicated. Hardly anything in biology fits into two neat categories like “male” and “female.” To give you an idea of how complicated sexual development really is, let’s go to the very beginning. How do sexual characteristics develop in a human embryo?

2. The Biology

A. Development of the Internal Genitalia

In the sixth week of an embryo’s development, a piece of primordial tissue called the pronephros splits off into a baby kidney and a baby internal reproductive system. This system consists of three parts. There’s the Müllerian ducts, which can develop into fallopian tubes, a uterus, and a vagina. There’s the Wolffian ducts, which can develop into a seminal vesicle, vas deferens, and epididymis. Then there’s the gonads, which can develop into ovaries or testes. At this stage the gonads are called indifferent, which I find kind of hilarious, because I imagine the gonads just chilling inside the embryo going, “Yeah, whatever, I just don’t care about sex differences.”


So what determines what happens to all of these parts? It’s complicated. Very complicated. But I’ll try to cover the highlights.

The embryo doesn’t start to differentiate into male/female traits until 7 weeks in. What determines whether the gonads become testes or ovaries is the presence of a gene called SRY, which is typically found on the Y chromosome (though as with everything in biology, there are exceptions: SRY sometimes wanders off to another chromosome, which means you can have a person with XX chromosomes and testes).

SRY influencing development

Not everyone has XX or XY chromosomes. Some people have XXY or XYY or XXX or just X. But if the embryo has a Y chromosome, the SRY gene will nudge the indifferent gonads toward becoming testes. This means that even if you have testes, you might not be XY in your chromosomes.

The SRY gene causes some cells in the gonads to begin commitment to sperm development, and to pump out two hormones: the famous testosterone, and the less-known anti-Müllerian hormone, which usually (but not always) causes the Müllerian ducts to break down. (If they don’t break down, the fetus will be born with testes, a uterus, and Fallopian tubes.)

The Wolffian ducts usually develop instead, into the organs that create the non-sperm components of semen and deliver them to the testes. (If they do not develop, this results in an individual with testes who does not ejaculate and is infertile, because the sperm don’t have a nice semen package in which to leave the body.)

In the absence of SRY, some genes on the X chromosome, such as DAX-1 and Wnt-4, kick in. The cells in the gonads commit to egg development. Hormones secreted by the ovaries usually cause the Wolffian ducts to degrade, though sometimes there are remnants. If you have bumps on the sides of your vagina, they may be remnants of the Wolffian ducts you had as an embryo.

The Müllerian ducts usually develop into a uterus, Fallopian tubes, and a vagina, though how much of the vagina comes from the Müllerian ducts is controversial. Don’t you love that there’s a big scientific controversy about vaginas? Scientists aren’t sure whether the entire vagina comes from the Müllerian ducts, or just the upper vagina.

Now, in some cases, the gonads get mixed signals and become intermediate between ovaries and testes, and may be able to produce both eggs and sperm. If that happens, the hormones produced by each part interfere with the other, and the typical result is infertility or only one type of gonad fully functioning. The Wolffian and Müllerian ducts may both develop, one to a side, or just one or the other might develop, or neither.

B. Development of External Genitalia

Meanwhile, on the outside of the embryo, between its tiny growing legs, is a structure that looks like this:

early external genitals

The development of the external genitalia happens later, in weeks 9 through 12 of development. A complex interaction of hormones determines how the external genitalia develop, which means that there are many possible outcomes of genital development. I’ll try to cover as many possibilities as I can. I will refer to the image above as well as the image below, called the Prader scale, which shows some of the different ways the external genitalia can develop.

Diversity of genitalia

Picture links to source (warning: describes variation in sex development with pathologizing language.)

Part 1 is the sensitive head of what anatomy textbooks call the “genital tubercle” but I prefer to call the “phalloclitoris,” because as we will see, the penis and the clitoris are so similar that in this story (and maybe in general) it doesn’t really make sense to call them different things. The development of the head of the phalloclitoris is mostly the same in everyone. Testosterone causes it to get larger, but it has just as many nerve endings no matter how big or small it ends up.

Part 2 is a membrane that gives rise to the urethra and the anus in everyone, and to a vagina in some. The first thing that happens to structure 2, in everyone, is that the bottom part pinches off into an anus. What remains is called the urogenital sinus. In some individuals, the story ends there. They have one opening, from which they pee, but also has erotic nerve endings and produces lubricant (though it is often shallower than a vagina; see stage 3 in the Prader scale.) In some individuals, it pinches off into a urethra and a vagina. In the remaining group, it closes up like a zipper into just a urethra. If these individuals have a penis, the urethra usually lengthens up to the tip (but it might not migrate all the way up; see stage 4 in the Prader scale).

Part 3, in everyone, forms the body of the phalloclitoris. Now, here’s where things can get hard to explain, because sex education in this country is woefully inadequate. It is obvious to most everyone what the body of the penis looks like: it’s the shaft, everything that isn’t the head. But not everyone is aware that the clitoris has a body too, not just a head. In most individuals who have a clitoris, only the head is externally visible. But the body of the clitoris is just as long as the body of the penis. It looks like this:

clitoris anatomy

Those four balloon-like things around the vagina are the body of the clitoris. A penis is just like this, just external and sewn up along the bottom edge. Except, of course, not always: some people are born with an external phalloclitoris that opens up along the bottom, like the clitoris in the image above. This all comes from structure 3 in the picture. Structure 3 can also develop labia minora. Anyone who has been sexy-intimate with labia minora, their own or someone else’s, won’t be surprised by this: both the body of the phalloclitoris and the labia minora feel very good when stimulated.

Part 4 can swell into labia majora, or fuse together along the bottom edge into a scrotum. Or something in between can happen: labia majora that form “pouches” like a scrotum, or a scrotum that doesn’t completely seal up along the middle. See the Prader scale image for some of the different ways Part 4 can develop.

That’s it for the external genitalia. The last part of sexual development happens around week 26: the descent of the gonads. You may have heard about the descent of the testes. If a fetus with testes has a scrotum, most of the time, the testes will descend into it before birth. If the fetus has testes but no scrotum, or the signal to descend never reaches the testes, they will remain in the abdomen undescended, possibly for the rest of the person’s life, possibly not. What you may not know is that the ovaries (usually) descend too. When the ovaries descend, they attach to the ends of the Fallopian tubes.

3. The Implications

Those are the biological facts of sexual development. It should be clear to you now that the outcomes of sexual development don’t fall into two obvious categories of male and female. One implication that jumps out at me is that while we don’t know how a sense of gender identity develops in the brain, because there are so many possible outcomes of sexual development in the genitalia, it wouldn’t surprise me at all if we find that there are many possible outcomes of sexual development in the brain. The likelihood of someone growing up to have a penis and a strong sense of female identity is at least as high as someone growing up to have a beard and a vagina, or testes and a uterus.

Another implication is that “biological sex,” in reality, is a spectrum, or maybe even more complicated than a spectrum. However, societies divide this spectrum into socially constructed categories: sexes.

So where do we draw the dividing line? This may seem arbitrary to you, and it absolutely is. Not all societies have divided up this spectrum the same way. For example, in India, some people with genitals in between the typical male and typical female are classified as a third sex, hijra. Where does Western society draw the line? Until the 2000s, the standard was basically this: is the location of the urethra in right place, and the size of the phalloclitoris big enough, that the baby can eventually stand to pee, and be able to insert the phalloclitoris into a vagina?

Even if you are not a regular reader of this blog, the ideology of sex and gender behind this dividing line should be clear. For decades, the medical marker of maleness was a penis that fit the standards of masculinity: standing to pee, and having heterosexual intercourse. These standards had serious consequences. Any baby with a phalloclitoris that didn’t meet medical standards was subjected to unnecessary surgery to reduce the phalloclitoris to an “acceptable” size for a clitoris, raised as female, and kept in the dark for the rest of their life about the genitalia they were born with. These days, the standard used for assigning sex to intersex babies is chromosomal sex. XX, you’re female, XY, you’re male.

But there’s more. While some babies are born with genitalia ambiguous enough for parents and doctors to get into a kerfuffle, there are many intersex conditions that have nothing to do with external genitalia and may go undetected. For example, there are those individuals with XX chromosomes and a wandering SRY gene attached to their genomes somewhere. Those people may manifest, in their gonads, internal genitalia, and external genitalia, as typical males. But until they get karyotyped and have a look at their chromosomes, they may never know they are intersex. There are also conditions that cause male-assigned people to produce high amounts of estrogen and related sex hormones or female-assigned people to produce testosterone and related sex hormones. The effects of these sex hormones are sometimes highly noticeable, but sometimes they are harder to detect.

This means that even if you don’t think you are intersex, you could be. I know because it happened to me.

When I was 18, I was diagnosed with PCOS, polycystic ovarian syndrome. This happens when the ovaries produce unusually high levels of androgens (male sex hormones). PCOS is not classified by the medical community as an intersex condition. However, what the medical community designates as “intersex” or not is motivated by politics, not biological facts. The goal of the way variation in sexual development is defined is to label as few people “intersex” as possible, so they don’t have to live with the “shame” of the diagnosis. The only conditions that are called intersex are ones that can’t be explained away to a child’s parents as a “slight genital abnormality.” Thus, doctors are able to claim that only 1 in 1500 babies is born intersex.

A much more pragmatic definition of intersex, as proposed by Dr. Cary Costello at the University of Wisconsin-Milwaukee, is when a body does not fully differentiate into male or female. By that definition, people with PCOS are intersex, because the condition we were born with makes our androgen levels higher than most women’s and lower than most men’s. Our androgen levels also reduce the levels of female sex hormones in our bodies so that they are intermediate between the typical levels for men and women. Our bodies are not fully hormonally differentiated between male and female. It is thought that up to 5% of female-assigned people may have PCOS. That would mean that at least 1 in 40 people are intersex. The medical community, and society at large, is not ready to accept that figure. If 1 in 40 people don’t fit into our boxes of “biological sex,” then there’s no way to deny that our boxes don’t do a very good job of classifying people. Many people would find that frightening.

I don’t find it frightening. I find it delightful. I am so happy that there is so much sexual diversity in the world, and that biology is too complex and beautiful to jam into two little boxes. When I was diagnosed with PCOS, I wasn’t horrified or scared. I was relieved. Finally, I had an explanation for why my body never followed anything resembling a regular menstrual cycle. I knew why my sex drive would suddenly, drastically change: my hormones were shifting from a female sex hormone-dominated bouquet to a male sex hormone-dominated one, or vice versa.

When I was diagnosed with PCOS, my gynecologist offered me the option of hormone therapy to make my hormonal profile less androgenic and more typically female. Since I was an adult, I could choose whether to take that option or not. I tried it out for a few months, and I hated it. It changed me in a thousand subtle ways that added up to a profound alienation from my own body. I didn’t feel like myself anymore. So I stopped the hormone therapy and went back to my intermediate, intersex state.

Children who are diagnosed with intersex conditions usually don’t get that choice. Their genitals may be operated on, resulting in permanent loss of sexual function. They may be given hormones for years to feminize or masculinize them, causing some of them to go through a partial puberty at age four. The choice of which sex to assign them to, as I explained above, is utterly arbitrary. Many more intersex children end up identifying as transgender than in the general population, knowing that they were born with the very genitalia that they desperately wish hadn’t been taken from them with a surgeon’s knife.

The entry for Androgen Insensitivity Syndrome, an intersex syndrome that results in intermediate genitalia, on Medscape has this to say about how to treat children with this condition: “The ultimate medical goal of treatment is to restore external genitalia as close to a nonambiguous appearance as possible while retaining full sensation, the ability for sexual satisfaction (to include penetrative intercourse), and, ideally, fertility.”

Maybe some people with Androgen Insensitivity Syndrome want to be nonambiguous. Maybe they want to have penetrative intercourse. But when they’re babies, you can’t possibly know. I remember how miserable I was on the hormone therapy that made me “typically female.” I can’t imagine what it would have been like if I’d been forced to be on them all my life. No one should ever have to go through that. Nonconsensual, unnecessary surgery is morally wrong, and I extend my deepest sympathies to all intersex people who have been violated that way.

You hear all kinds of stories about “biological sex.” At the Olympics, they determine the sex of athletes by measuring their testosterone, because supposedly testosterone is what gives male athletes an advantage over female athletes. You also see scientific studies about how testosterone makes men more aggressive than women, more sexual, better-adapted to be hunters back when they were cavemen. If these stories are true, then I have the advantages of a male athlete. I am aggressive, sexual. I am a caveman hunter. If the way men and women behave is an inevitable consequence of biology, then where do I, and other intersex people, fit in?

We don’t. Because the stories aren’t about us. They aren’t about biology, which is messy and complicated. They’re fables. They’re folk tales we tell each other so we’ll keep believing in the great patriarchal fantasy that there are two sexes that are completely different from each other, and that one is better than the other. Because biology.

Well, I’m a person too. So are other intersex people. So are non-intersex people who don’t fit into the patriarchal narrative of how we’re supposed to live. And this is our story.

Next up in the series: what is sex anyway, and why does it exist?


Embryology lecture on sexual differentiation: note that this resource is a) highly technical and b) uses pathologizing language to describe variations in sex development.

Interface Project FAQ: more about intersex people and their concerns

Intersex Roadshow: a blog by Dr. Costello, an intersex trans man, about intersex issues, variation in sexual development, and gender liberation

Intersex Society of North America FAQ: Extensive information about the history of intersex rights