New Science is Challenging Views of Sex and Gender

Laws than ban gender-affirming care imply that gender is a defined binary. Recent science doesn’t agree.

By Katie Stachowicz

Sex is not what you think it is. No, not that sex — I’m talking about biological sex. Your chromosomes, your genitals, what’s on your birth certificate. Except, according to science, sex isn’t any of those things.

As the heated debate around sex and gender leads to bans of gender-affirming care for minors in 18 states at the time of this article, it’s a good time to consider the question; What actually is sex? Nearly every bill banning care for transgender individuals equivocates sex and gender, citing “genetics”, “anatomy”, or, even more vaguely, “biology”. The basis of their argument is that sex and gender are indisputable facts established by science. Yet, a deeper look at scientific research presents a more nuanced picture.

Decades of scientific research (and non-Western views of gender) conclude that sex and gender, while linked, are separable. But trying to define them is a bit trickier. In general, when referring to sex, scientists are referring to gamete, or “sex cell”, production. Females typically make ova, and males typically make sperm. Gender, on the other hand, is typically taken within psychology and sociology as an individual’s own sense of identity. This makes it hard to study in animals, although some studies have tried.

The term “gender binary” has been thrown around a lot recently, especially on conservative news sites. This dismissiveness towards non-Western views of gender is unfounded and incorrect. When it comes to expanding our definitions, we need to look even further than gender. Sex isn’t a binary either — at least, not in the way we typically think.

“Sex is messy,” says Yesenia Garcia, a neuroscience Ph.D. candidate at Emory University who has studied sex differences in the brain. “It’s not one single thing, but a collection of variables that interact and inform one another.”

When we assume that everyone in one category has certain characteristics, we run the risk of doing medical, psychological, and social harm to anyone who does not perfectly match up with that list. We need to open our minds, look at what the science is saying, and realize that sex, like gender, exists in shades of gray.

Sex and gender and chromosomes

This is a common argument: XX means you’re a girl, and XY means you’re a boy; end of story. Studies show a genetic component to sex, so there is some truth in this, but it’s much more complicated. Between 1-2% of people across the world are estimated to be intersex, meaning that they don’t fall neatly into the established binary. To put this in perspective, that’s a number comparable to the combined populations of California, Texas, and New York.

The term intersex can mean lots of things. For some, it means they were born with Androgen Insensitivity Syndrome, a condition where individuals with XY chromosomes don’t respond to the testosterone in their bodies. Testosterone, often referred to as the male sex hormone, is critical for developing male-typical features in humans. If an individual can’t respond to testosterone, even with XY chromosomes, they tend to be assigned female at birth and identify as female throughout their lives.

“Because we use the term sex to encompass things like secondary sex characteristics, the ‘it’s just genetics’ argument falls apart,” says Megan Massa, PhD, a neuroendocrinology researcher and assistant teaching professor at Emory University who uses they/them pronouns. They cite intersex conditions such as polycystic ovary syndrome, where XX individuals develop features often associated with males such as increased body hair density and facial hair.

Outside of the human world, there are many ways to determine sex outside of X and Y chromosomes. Turtle eggs become male or female based on the temperature the egg incubates in. Several species of fish, including clownfish, use social factors like the availability of females to change their sex in adulthood. People aren’t fish or turtles, obviously, but as far as nature is concerned, we’re just other animals. Our method of sex determination isn’t the only way.

Sex and social perception

The easiest element of sex to see, and so what people usually use to determine the sex of an infant, are genitals. However, genitals are not always great indicators of what’s going on in the body.

Garcia, who identifies as nonbinary and queer and prefers they/them pronouns, argues that assigning sex based on just one factor is unscientific. “People who claim that sex is based solely on chromosomes or birth designations have a very limited understanding of biology,” says Garcia. “Practicing scientists know that there is a big diversity of intersex conditions that lead to incongruence of chromosomes versus physical traits.”

Let’s rewind all the way to the embryo. The current running theory of sex development in humans depends on the SRY gene, which is on the Y chromosome. This gene gets the ball rolling on developing the testes and male-typical sex organs. Without a functional version of this gene, even with XY chromosomes, an individual won’t develop male sex organs. Similarly, DAX1 on the X chromosome encourages development of female-typical sex organs. Without a functional version, female-typical sex organs likely won’t develop. After sex organs develop, they can start to make their own hormones. However, there is sometimes a disconnect between hormone production and how a body responds to them. Androgen Insensitivity Syndrome is one of those. Another, called aromatase deficiency, is most diagnosable in XX individuals. Estrogens, a type of hormone important for female-typical development, are made from testosterone using a protein called aromatase. If an XX individual doesn’t have enough working aromatase, they’ll have a lot more testosterone than average. Often, this testosterone boost occurs at puberty, at which point the increase can lead to more male-typical puberty development than female-typical puberty development.

In cases where infants have atypical or “ambiguous” genitalia, doctors often perform surgeries to put them firmly in one sex category or the other. These surgeries are controversial and rarely medically necessary. Historically, doctors advised them to avoid “gender confusion” without explaining that the surgeries can cause permanent scarring, incorrect gender assignment, chronic incontinence, and lifelong pain. While the medical community’s view is becoming more nuanced, the issue of forced surgery remains a painful problem for many in the intersex community. Understanding that sex is a complex, multifaceted issue will prevent these extraneous surgeries from causing lifelong health problems.

“Current legislation proves that it is trying to create a binary as opposed to enforcing what already exists,” Massa says. “It says trans kids can’t change their sex, get these surgeries, but intersex kids must change and get these surgeries. If there was a binary that it was just enforcing, there wouldn’t be this distinction between two different groups of people.”

Sex and brain development

The brain doesn’t develop until the second trimester of pregnancy, long after sex organs. While many studies suggest that hormones produced by male-typical or female-typical sex organs can influence brain organization, other scientists propose that some cases, where the brain and genitals develop separately, may be more common than we might think.

“When your sex is assigned at birth, are you looking at gamete production? No,” argues Massa. “And whatever you’re using that you think ‘aligns’ [with gamete production] – it doesn’t have to.”

A 2018 study from the University of Tel Aviv published in Frontiers in Human Neuroscience found no differences in brain structure between self-reported male and female students. While there were a few small brain areas that seemed to show slight differences overall, when looking at individual brains, the differences were indistinguishable. “What is typical of both males and females is a brain comprised of a ‘mosaic’ of features,” the authors wrote, “some in the form more common in males and some in the form more common in females.”

The concept of brain areas being thought to be “male” or “female”, such as the sexually dimorphic nucleus of the preoptic area (or the sdnPOA), falls apart under closer scrutiny. For a few years, it seemed that this area was significantly larger in men than in women. However, once a researcher thought to include non-heterosexual people in their study, they found that a gay man’s sdnPOA was more similar to a straight woman’s than to a straight man’s.

If you’re confused about what sex is, congratulations – you’re thinking like a scientist! It’s clear that a lot of different things contribute to (or come from) sex: genes, hormone exposure, body development, and behavior, to name a few. But these things don’t always “match up”: just because one is masculine doesn’t mean the rest have to be. This means that one broad definition of sex, scientifically, just doesn’t work.

When we think in a binary that doesn’t exist, we risk misclassifying and mistreating people. When using sex for a specific purpose, whether that’s in scientific research or in law-making, we should define it by how we’re using it. An endocrinologist – a scientist or doctor who studies hormones – cares about your estrogen or testosterone levels, not what’s on your driver’s license. An advocate for gender equity in sports ultimately wants to know if biological factors are giving certain people advantages over others in an athletic game, not what’s in their pants.

Using rigid, predetermined, and out-of-context data points to define a person’s sex is not only dehumanizing — it goes against the evidence. Science has been telling us for years that sex is more complicated than XX and XY. It’s time that we listen.