Antiscientific sentiment bombards our politics, or so says the Intellectual Dark Web (IDW) . Chief among these antiscientific sentiments, the IDW cites the rising visibility of transgender civil rights demands. To the IDW, trans people and their advocates are destroying the pillars of our society with such free-speech–suppressing, postmodern concepts as: “trans women are women,” “gender-neutral pronouns,” or “there are more than two genders.” Asserting “basic biology” will not be ignored, the IDW proclaims. “Facts don’t care about your feelings.”

The irony in all this is that these “protectors of enlightenment” are guilty of the very behavior this phrase derides. Though often dismissed as just a fringe internet movement, they espouse unscientific claims that have infected our politics and culture. Especially alarming is that these “intellectual” assertions are used by nonscientists to claim a scientific basis for the dehumanization of trans people. The real world consequences are stacking up: the trans military ban , bathroom bills , and removal of workplace and medical discrimination protections, a 41-51 percent suicide attempt rate and targeted fatal violence . It’s not just internet trolling anymore.

Contrary to popular belief, scientific research helps us better understand the unique and real transgender experience. Specifically, through three subjects: (1) genetics, (2) neurobiology and (3) endocrinology. So, hold onto your parts, whatever they may be. It’s time for “the talk.”

BIOLOGICAL SEX: HOW YOU GET IT

Nearly everyone in middle school biology learned that if you’ve got XX chromosomes, you’re a female; if you’ve got XY, you’re a male. This tired simplification is great for teaching the importance of chromosomes but betrays the true nature of biological sex. The popular belief that your sex arises only from your chromosomal makeup is wrong. The truth is, your biological sex isn’t carved in stone, but a living system with the potential for change.

Why? Because biological sex is far more complicated than XX or XY (or XXY, or just X). XX individuals could present with male gonads. XY individuals can have ovaries. How? Through a set of complex genetic signals that, in the course of a human’s development, begins with a small group of cells called the bipotential primordium and a gene called SRY.

A newly fertilized embryo initially develops without any indication of its sex. At around five weeks, a group of cells clump together to form the bipotential primordium. These cells are neither male nor female but have the potential to turn into testes, ovaries or neither. After the primordium forms, SRY—a gene on the Y chromosome discovered in 1990 , thanks to the participation of intersex XX males and XY females—might be activated.*

Though it is still not fully understood, we know SRY plays a role in pushing the primordium toward male gonads . But SRY is not a simple on/off switch, it’s a precisely timed start signal, the first chord of the “male gonad” symphony. A group of cells (instrument sections) must all express SRY (notes of the chord), at the right time (conductor?). Without that first chord, the embryo will play a different symphony: female gonads, or something in between.

And there’s more! While brief and coordinated SRY-activation initiates the process of male-sex differentiation, genes like DMRT1 and FOXL2 maintain certain sexual characteristics during adulthood. If these genes stop functioning, gonads can change and exhibit characteristics of the opposite sex. Without these players constantly active, certain components of your biological sex can change.

There’s still more! SRY, DMRT1, and FOXL2 aren’t directly involved with other aspects of biological sex. Secondary sex characteristics—penis, vagina, appearance, behavior—arise later, from hormones, environment, experience, and genes interacting. To explore this, we move from the body to the brain, where biology becomes behavior.

THE BRAIN: WHERE STUFF GETS “MADE UP”

When the biology gets too complicated, some point to differences between brains of males and females as proof of the sexual binary. But a half century of empirical research has repeatedly challenged the idea that brain biology is simply XY = male brain or XX = female brain. In other words, there is no such thing as “the male brain” or “the female brain.” This is not to say that there are no observable differences. Certain brain characteristics can be sexually dimorphic: observable average differences across males and females. But like biological sex, pointing to “brain sex” as the explanation for these differences is wrong and hinders scientific research .

Let’s just take the most famous example of sexual dimorphism in the brain: the sexually dimorphic nucleus of the preoptic area (sdnPOA). This tiny brain area with a disproportionately sized name is slightly larger in males than in females. But it’s unclear if that size difference indicates distinctly wired sdnPOAs in males versus females, or if—as with the bipotential primordium—the same wiring is functionally weighted toward opposite ends of a spectrum. Throw in the observation that the sdnPOA in gay men is closer to that of straight females than straight males, and the idea of “the male brain” falls apart .

Trying to link sex, sex chromosomes and sexual dimorphism is also useless for understanding other brain properties. The hormone vasopressin is dimorphic but is linked to both behavioral differences and similarities across sex . Simply put, the idea of a sexual binary isn’t scientifically useful, and nowhere is this more obvious than in the brain. It also happens that transgender people have the brains to prove it .

It’s easy to see sexual dimorphisms and conclude that the brain is binary; easy, but wrong. Thanks to the participation of trans people in research, we have expanded our understanding of how brain structure, sex and gender interact. For some properties like brain volume and connectivity, trans people possessed values in between those typical of cisgender males and females, both before and after transitioning. Another study found that for certain brain regions, trans individuals appeared similar to cis-individuals with the same gender identity. In that same study , researchers found specific areas of the brain where trans people seemed closer to those with the same assigned sex at birth. Other researchers discovered that trans people have unique structural differences from cis-individuals.

THE BODY AND THE BRAIN AND THE HORMONES BETWIXT

As if the brain and body weren’t complicated enough, another biological factor influences the expression of biological sex in an individual: hormones. Anyone who has gone through puberty has felt the power of hormones firsthand. But like all things biology, hormones cannot be limited to the pubescent idea of “estrogen = female and testosterone = male.”

For one thing, all humans possess levels of estrogen, progesterone and testosterone with sex differences not as prominent as is popularly thought . During infancy and prepubescence, these hormones sit in a bipotential range, with no marked sex differences. Through puberty, certain sex hormones like estrogen, progesterone and testosterone become weighted toward one end of a spectrum. But in developed adults, estrogen and progesterone levels are on average similar between males and nonpregnant females. And while testosterone exhibits the largest difference between adult males and females, heritability studies have found that genetics (X vs. Y) only explains about 56 percent of an individual’s testosterone , suggesting many other influences on hormones. Furthermore, measurements of sex hormones levels in any one individual wildly vary across the range of “average” values regardless of how close or spread apart you take the measurements. The binary sex model not only insufficiently predicts the presence of hormones but is useless in describing factors that influence them.

Environmental, social and behavioral factors also influence hormones in both males and females, complicating the idea that hormones determine sex. Progesterone changes in response to typically male-coded social situations that involve dominance and competition . Estrogen, typically linked to feminine-coded behavior, also plays a role in masculine-coded dominance/power social scenarios . Though testosterone levels are different between males and females on average, many external factors can change these levels, such as whether or not a person is raising a child. Differing testosterone levels in both men and women can predict certain parenting behaviors . Even the content of a sexual fantasy can change testosterone levels. The fact is, behavior and environment—like cultural gender norms and expectations—influence sex-related hormones, and the biology of the body and brain itself.

SCIENCE AND SOCIETY: BETTER TOGETHER

While this is a small overview, the science is clear and conclusive: sex is not binary, transgender people are real. It is time that we acknowledge this. Defining a person’s sex identity using decontextualized “facts” is unscientific and dehumanizing. The trans experience provides essential insights into the science of sex and scientifically demonstrates that uncommon and atypical phenomena are vital for a successful living system. Even the scientific endeavor itself is quantifiably better when it is more inclusive and diverse . So, no matter what a pundit, politician or internet troll may say, trans people are an indispensable part of our living reality.

Transgender humans represent the complexity and diversity that are fundamental features of life, evolution and nature itself. That is a fact.

*Editor’s Note (6/18/19): This sentence was edited after posting. It originally referred to participants as transgender.