Last week, the New York Times reported on a Department of Health and Human Services memo proposing to define gender legally “on a biological basis that is clear, grounded in science, objective and administrable.” If enacted, this policy would constitute the Trump administration’s strongest attack yet on the civil and human rights of trans Americans. Such a definition of gender would create a legal fiction in which trans people do not exist. The trans community accurately sees this memo as an existential threat and has organized its response under the hashtag #WontBeErased.

In making this argument, the Trump administration acts as though non-binary conceptions of gender and sex are radical and new and can thus be dismissed out of hand. This isn’t surprising given that Roger Severino, the director of the HHS Office for Civil Rights, labeled Obama administration protections for trans people “a radical new gender ideology” and “a new definition of what it means to be a man or a woman [imposed] on the entire nation.” These contentions rest upon the notion that throughout history, people have understood biological sex as being simple and obvious.

This claim is simply false.

The concept of biological sex has changed frequently over time. This reality is even reflected in the two definitional criteria proposed by HHS for determining gender: genitalia first, and then chromosomes if necessary. These criteria themselves come from two different times: basing sex on genitalia dates to the 18th and 19th century, while a reliance upon chromosomes became popular after World War II. Demonstrating how quickly our understandings of sex change, both criteria are out of date with contemporary scientific research.

In short, the Trump administration is relying upon a historical falsehood in saying that the definition of sex is old and well established, and it moreover relies upon specific historical interpretations of sex that no longer reflect scientific consensus.

Historically speaking, biological sex has never had a stable definition.

Within European and American history, the common ways of defining biological maleness and femaleness have changed dramatically over the past 2,500 years. The ancient Greeks and Romans understood humanity to have not two sexes but one. Rather than a conception of males and females as biological opposites, as is common today, they regarded females as imperfect variations on the male form. Moreover, according to the old medical model of four essential humors, if a woman were to somehow acquire a greater degree of heat, then she would become biologically more male.

Although there is much debate among scholars as to when this “one-sex model” stopped holding sway, it was not until the 1700s and 1800s that the popular conception of sex began to really look like the one supported by the Trump administration, with men and women thought of as fundamentally different biological types.

Within this framework, sex was considered an absolute characteristic — one was either female or male, with no possibilities in between. The standard metric of distinction was genitalia, but then, as now, doctors ran into trouble whenever they tried to use penises and vaginas as absolute differentiators. Besides the fact that the organs could be altered through surgery or accidental occurrences, there were also frequent reports of people raised as women whose “clitorises” were later determined to be small penises, or vice versa. In such cases the “true” sex of the person was often decided through exploratory surgery to determine whether the person possessed testicles or ovaries.

In part because of cases that defied the binary, as well as new research into biological chemistry, by the end of the 19th century many scientists no longer believed that sex could be described accurately in such black-and-white terms.

The discovery of sexual hormones also played a crucial role in this development. The leading scientists of the era came to regard biological sex as referring primarily to a person’s balance of sexual chemicals. This definition of sex exploded the old sexual binary, replacing it with a broad spectrum of sexes. Scientists such as Magnus Hirschfeld used this conception of sex to argue for the fundamental humanity of all sexual minorities — in this view, everyone possessed a biological sex that existed in an intermediary state between male and female.

This new science also offered the possibility of transforming one’s sex through surgical intervention. In the 1910s, the endocrinologist Eugen Steinach became the first scientist to offer convincing proof of the existence of gonadal hormones through an experiment in which he transplanted ovaries into male guinea pigs and testicles into female guinea pigs. The result was that the animals developed the secondary sexual characteristics associated with the organ that was present in their body, rather than the organ they had been born with. As Steinach later wrote in his intellectual autobiography, his work “had converted males into females” and thus shown that “a one hundred percent man is as non-existent as a one hundred percent woman.”

After the Second World War, scientists drifted back toward a binary conception of sex. The Nazis had repressed many of the scientists who had advocated a broader idea of sex (including Hirschfeld and Steinach) because these ideas undermined the gender roles that they considered essential for creating the perfect Aryan race.

Additionally, new research into molecular biology after WWII began to popularize the idea of XX and XY chromosomal karyotypes. In contrast to the spectrum implied by sexual hormones, molecular biology seemed to correspond with the idea of being either male or female.

However, ideas of non-binary biological sex did not disappear entirely. For decades after the end of the war, Steinach’s acolyte Harry Benjamin continued to deploy a nonbinary conception of sex, even publishing the first book-length medical study of sexual reassignment in 1966. Other prominent sexologists of the postwar period — notably John Money, Christian Hamburger and perhaps Alfred Kinsey — also endorsed a non-binary idea of sex in one form or another. Moreover, studies of the media enthusiasm for Christine Jorgensen, a trans woman who became a minor celebrity in the ‘50s and ‘60s, suggest that the American public in those years may have been more willing to consider ideas of non-binary sex than we might suspect.

In recent years, the scientific support for a binary conception of sex has largely eroded, due in part to the increasing realization that chromosomes tell only part of the story. Hundreds of thousands of people possess a chromosomal karyotype that does not correspond to XX or XY. Moreover, just as the blueprint of a house doesn’t tell you what happened after it was built, biologists and medical doctors increasingly focus on the significance of environmental and nongenetic factors when examining sexual biology. Today, most biologists agree that biological sex is an incredibly complex subject, one that cannot be reduced to a question of genitalia or chromosomes but that ought to be thought of in terms of a wide variety of factors.

The Trump administration’s proposed definition of sex, however, ignores all this accumulated scientific knowledge. It instead seeks to return to the antiquated 19th-century idea that a person must be either male or female, determined if necessary by a hidden biological arbiter. By contrast, the definition of sex offered by the trans community is more historically — and scientifically — rooted. It hews much more closely to the understandings put forth by science for most of the past century.

Ultimately, that means it is not the trans community, but rather the policies of the Trump administration, that would “impose a new definition of what it means to be a man or a woman on the entire nation.” And it would be doing so for the purpose of labeling trans Americans as abnormalities fit only to be defined out of existence.

Fortunately, in this struggle trans Americans have both science and history on their side.