A correspondent for the conservative website National Review recently declared that Laverne Cox, the transgender actress gracing the cover of Time magazine, “is not a woman.” Kevin D. Williamson went on to argue that transgender identities are a “delusion … that transcends the biological facts in question,” and referred to Cox throughout with male pronouns, despite her expressed preference for female ones. Williamson wrote that the “subjective impressions” of gender should not supersede the “biological fact” of sex, which, according to him, is simple science.

Now, my biological education peaked at age 3, when the boy next door and I played doctor and discovered that the differences in our hair length weren’t our only physical differences. Based on a 3-year-old’s understanding of science, Williamson’s confident claims about “biological fact” seem logical.

But some people are real doctors. So when Williamson’s article provoked outrage from the transgender community and its allies, I called some experts. When it comes to sex, is it true that biology is simple?

That answer, resoundingly, was no. The biology of sex is complex, and there’s a lot we don’t understand. A new book, Sex Itself: The Search for Male and Female in the Human Genome, even makes the case that we should abandon the term “sex chromosomes” entirely, because it encourages an “empirically wrong” binary understanding of sex.

But Williamson’s article isn’t just one guy being stupid about science: It represents a whole world of unquestioned assumptions. The conservative, supportive response to Williamson generally seemed to be, as one Twitter user declared: “A penis is a penis and a vagina, a vagina. … Kindergarten level biology. … Not hard.”

Even those of us who distinguish between sex and gender, who understand that gender is a cultural construct, too often reduce sex to a binary. But when we dig into the science of sex, it turns out that the fundamental categorizations we take for granted—male and female—are not fundamental at all. The assumption that there are only two sexes is wrong. Sex, like gender, is a construct—and it varies.

“As human beings, we want to put things into categories,” said JoAnne Keatley, the director of the Center of Excellence for Transgender Health. “But nature is not that rigid. It doesn’t conform to our need to put things in boxes.”

Biologists use the term sex to refer to gamete production—whether an organism produces sperm or eggs. But some individuals fall on intermediary points of that spectrum or change which type of gamete they produce over the courses of their lives. Some plant and animal species, such as papayas, even have an established third sex.

In people, too, genetics alone is often not enough to determine someone’s biological sex. Once you understand the basic rules of sex traits, you can start to understand the exceptions to those rules.

Some of those exceptions include androgen insensitivity syndrome, in which a person with a “male” Y chromosome can have female sex characteristics such as breasts and a vagina. Babies with a condition called 5-alpha-reductase deficiency often appear female at birth, with female external genitalia, and then develop male genitalia during puberty. Turner syndrome (in people who have a single X chromosome), Klinefelter’s Syndrome (in people who have XXY chromosomes), and many other genetic variants also result in intersex individuals, all of which make biological sex difficult to reduce to an absolute binary.

“A kindergarten understanding of biology is great for kindergarteners, but we’re adults,” says Melissa A. Wilson Sayres, a geneticist who studies sex chromosome evolution. “We teach children rules, and as adults we learn the exceptions.”

We tend to dismiss people who don’t fit into our rigid binary understanding of sex as “defective.” But in some parts of the world, communities where intersex conditions are more common have an understanding of sex that embraces those variations. For example, the Sambia people of Papua New Guinea, who have high rates of hermaphroditic conditions, have three different words to describe three distinct sexes.

About one in 100 people, or roughly 70 million people worldwide, have a recognized intersex condition. But most transgender people probably do not. If, as Williamson tweeted in response to the outcry his article provoked, “biology is biology,” where do most transgender people fall biologically?

Grown-up science has some things to say about that, too. A number of studies have suggested that there may be a biological explanation for transgender identities. The androgen receptor known as NR3C4 plays a critical role in the development of primary and secondary male sex characteristics, and one study found that male-to-female transgender people have longer repetitions of the gene for that receptor, which reduced its effectiveness at binding testosterone. That study concluded: “There is a likely genetic component to transsexualism.”

According to another study, a genetic variant of the enzyme CYP17, which influences the sex hormones progesterone and pregnenolone, can be linked to people with female-to-male transgender identities. That study concluded that CYP17 is a “candidate gene” of female-to-male transgender identities. Male-to-female transgender identities have also been linked to prenatal androgen exposure.

The science goes on and on. The most important sex organ, as they say, is the brain, and studies have found measurable differences in transgender brains. One study of the stria terminalis, a region of the brain that influences sex responses and sexual behavior, found that male-to-female transgender people had a female-sized stria terminalis. A follow-up study found that in male-to-female people, even ones who had never gone through a course of hormone therapy, the number of neurons in the stria terminalis was equivalent to the number in women. The neuron counts in female-to-male individuals were consistent with those in men.

In other words, biology suggests that sex is not a subjective feeling or a “delusion,” but rather a matter of genetics, hormone exposure, brain composition, and any number of unknown variables, and that sex can’t be neatly divided into two categories. Williamson’s article wasn’t merely cruel and politically incorrect—it was scientifically incorrect.

Of course, science still has a long way to go to fully understand the biological and medical details of sex, gender, and transgender identities. But if Williamson is sincerely interested in the “stubborn concreteness” of sex as opposed to the “infinite malleability” of gender, as he claims, the science we do have on the biology of sex suggests that he owes Laverne Cox, and the transgender and intersex communities at large, an apology. He also owes his readers an apology for not doing basic reporting before he wrote an article attacking a vulnerable community based on his kindergarten-level understanding of sex.