When biology isn’t black and white: The science of sex and gender

Published on Psychology Today

NOTE: Psychology Today edited this post to make it less "political". What is below is the post as originally written and submitted.

Since taking office in January of 2025, the Trump administration has taken at least a dozen major federal actions against the transgender community. For example, the federal government now defines sex as only male or female, there is no federal recognition of gender identity, including on passports, and there are new restrictions on gender-affirming care for federal employees. Republican led states have followed suit, and more than 600 anti-LGBTQ bills were introduced in state legislatures in the past few years, with at least 27 states enacting major restrictions or outright bands on gender affirming care for minors.

Why is there so much hostility toward the transgender community? Some say it’s to protect children, particularly from transition surgeries. But these surgeries are incredibly rare, and transition surgeries on genitals are not even allowed for minors in the US. Another issue is letting transgender women into sports, which is thought to give transgender women an unfair advantage. But transgender participation in sports is also incredibly rare. For example, the NCAA has roughly 510,000 college athletes, and NCAA President Charlie Baker claims that he knows fewer than 10 of those athletes who are transgender—that’s less than 0.002%.

What’s more likely the cause of all this buzz is that transgender rights is an ideological issue that conservatives see as a way to push back against progressive values and policy. But regardless of how you feel about policy, when you ask the scientific community whether gender is binary, the answer is “no, not necessarily,” and any argument for gender as binary falls apart quickly when you look at biology.

“Gender is just part of our biology.” Most people like to think of gender as part of our biology. Each cell in our body has 46 chromosomes. A father’s sperm and a mother’s egg each have only half—23 each. At conception, the chromosomes of the sperm and the egg match up into 22 identical pairs, with the 23^rd pair being the sex chromosome. In most cases, a person with an XX chromosome will become female and a person with an XY chromosome will become male.

Sex is what is written into the chromosomes or what is dictated by our biology—another word for this is genotype. Gender is what actually gets expressed—how we look, how we act, and how we feel. This is called phenotype, which is the interaction between the genes (genotype) and the environment. Sex doesn’t necessarily map on perfectly to gender, and the environment plays a role in determining the gender of each person. Most of the time, a person’s sex and gender are the same, but this doesn’t necessarily have to be the case. And sometimes not even sex is determined by sex chromosomes.

In fact, the sex of many species of animals is determined entirely by environmental circumstances and not by their chromosomes at all. For example, several species of coral reef fish and clown fish (yup, we’re talking about Nemo) don’t have sex chromosomes at all and can actually switch genders if their schools require it. Likewise, alligators, crocodiles, turtles and some lizards don’t have sex chromosomes either—their sex is mostly determined by the temperature of their nest during incubation. 

“Boys have a penis and girls have a vagina.” This argument falls apart even more when we think about the relationship between our sex chromosomes and our external genitalia. Going back to human sex chromosomes, all fetuses start out undifferentiated. At some point during prenatal development, genes on the Y chromosome will cause the fetus to grow testes; these testes begin producing androgen, which is then what causes the development of the rest of the male anatomy.

However, there are cases when the fetus is not receptive to the androgen that is being produced. In these cases, the fetus will have testes, but the rest of the male genitals will not fully develop. When the baby is born, it will appear like a girl with female external genitalia, but no actual uterus or fallopian tubes. In most cases, the baby will be raised as a girl and will not know that she is has an XY chromosome until puberty when she never gets her period. This condition is called Androgen Insensitivity Syndrome. The opposite can also occur, when a fetus with an XX chromosome produces too much androgen and develops ambiguous or partial male genitalia. This condition is called Congenital Adrenal Hyperplasia.

This suggests that even sex characteristics are not all-or-none: What determines whether a fetus develops male or female sex organs is about the amount of androgen produced by the fetus, when it’s produced, and how sensitive the fetus is to that androgen. In other words, there is a clear biological reason that there is more variation in both sex and gender than can be explained by a simple dichotomy of male/female, boy/girl, man/woman, or even penis/vagina.

When not to trust your gut

If scientists support the idea that gender is not necessarily binary, why don’t people believe it? One possibility is that it goes against many people’s basic intuition, or their gut. But unfortunately intuition isn’t always right, and we shouldn’t always believe what our gut tells us. There are a ton of biases that are common in humans and very hard to shake, like the idea that coincidences are meaningful, that first impressions are most important, or that if you flip a coin 5 times and heads keeps coming up, tails is due to come up next (it’s still 50/50). We also prefer explanations that are easy to understand. Explanations that favor gender as binary are simple, appeal to our own intuitions, and for many of us, to our own gender identities. We prefer things that are black and white, and we resist the gray area in between. But in reality, most things aren’t just one thing or another and instead fall on a spectrum. Indeed, life is much more colorful than just black or white; it’s beautiful and complicated, just like gender.  

Photo: Foreign, Commonwealth & Development Office/Flickr