During his inauguration speech Monday, President Trump promised to make America binary again.
“It will henceforth be the official policy of the United States government that there are only two genders, male and female,” he said. Within hours, he had signed an executive order to that effect, asserting a new legal definition of sex that strips federal recognition of the gender identity of some 1.6 million trans and nonbinary Americans.
The order directly contradicts a number of existing laws and recent court rulings and is likely to face legal challenges. It also defies decades of research into how human bodies grow and develop.
The document represents “a dramatic failure to understand biology,” said Rachel Levin, a Pomona College neuroscientist who studies the development of sex.
In the order, sex is described as “an individual’s immutable biological classification as either male or female,” and, despite Trump’s own conflation in his address, it says sex does not serve as a synonym for gender identity.
Scientists who study gender define it as someone’s sense of self, reflected in their attitudes, feelings, and behaviors, which is different from sex, which is generally defined based on an individual’s anatomy, physiology, genetics, and hormones. Researchers told STAT that both are best understood as multi-dimensional concepts influenced by both biological and social factors.
During Trump’s first term in office, his administration floated the idea of defining sex as immutably either male or female under Title IX, the law barring sex discrimination in schools. The proposed rule, which was never taken up by agencies tasked with enforcing Title IX, defined sex based on the genitals a person is born with. The new administration’s definition backs up even further, to the moment of conception.
Neither is a sound place to draw a line, scientifically speaking. The process of sex differentiation is a sequence of events that in some people takes place over years and can go any number of different ways. But the moment in time where a sperm fuses with an egg to create a zygote is an especially nebulous one.
At that point, all humans — whether they inherited an XX, an XY, or an atypical combination of sex chromosomes (XXY or XYY, for instance) — are all the same, just one big cell. While it’s possible to know what that zygote’s genetic makeup is, it’s impossible to determine whether it will go on to become, as the new order defines it, someone who “produces the large reproductive cell,” meaning an egg, and therefore female, or “the small reproductive cell,” meaning sperm, and therefore male.
“It’s a definition that’s not testable, and it’s not policeable, so it’s kind of bullshit,” Levin said. “There’s no easy way to identify a male and a female; there are more common ways that humans develop, but it’s a big, messy, fascinating world of correlations — some that line up with that binary, some that don’t.”
About 24 hours after conception, that cell will divide, making it an embryo. About two weeks of cell division later, a set of neutral gonads will start to develop. These cells will have equal potential to become either testes or ovaries. It won’t be until between 8 and 10 weeks post-fertilization that genetic signals begin to reach those cells, telling them to go one way or another.
The most important signal comes from the SRY gene, which is found on the Y chromosome. When it flicks on, it sends a message to the gonad to become testes, which immediately begin secreting testosterone. In the absence of a functional SRY gene, an embryo develops ovaries, which don’t begin cranking out hormones, mostly estrogen, until puberty.
The relative ratio of these hormones is, generally speaking, what determines the kind of external genitalia a person develops. But there are exceptions, and there are many ways for bodies to develop that don’t fall into either a strictly “male” or “female” category.
The SRY gene can malfunction, leading to individuals who are genetically male, but physiologically female. Sometimes a person with XY chromosomes doesn’t develop androgen receptors, meaning their body doesn’t recognize the testosterone it produces, a condition known as androgen insensitivity syndrome.
An estimated 2% of the population has one of these intersex traits, according to interACT, an intersex advocacy organization. In a statement, interACT described the executive order as an attempt to erase the existence of transgender and intersex people grounded not in science, but in a discriminatory worldview. “The Trump administration purports to talk about ‘biological realities,’ but demonstrates that they lack any understanding of the relevant science,” the statement said.
“Saying that sex is 100% binary ignores a lot of scientific evidence to the contrary,” said Francisco Sánchez, a psychologist at Arizona State University who studies gender identity. “Even though the vast majority of people may neatly split into those categories, there are still people who don’t. We know that variation exists. But the bigger issue is should the government really be basing policies on outdated and overly simplistic understandings of sex that ignore decades of biomedical and psychological research?”
Beyond bucking basic tenets of biology, scientists are worried that enshrining the new, nonsensical definition into government policies will lead to the elimination of federal funding for research into the health outcomes of transgender, nonbinary, and intersex individuals, worsen already patchy public health data collected on these marginalized populations, and exacerbate existing health disparities.
“Why would you pay someone to study something that you’ve said doesn’t exist?” Levin said.
Over the last decade, the National Institutes of Health has funded an increasing number of research projects focused on the health of transgender and nonbinary people, according to data from its Sexual and Gender Minority Research Office. (As of publication time, the office’s web pages had been removed.) The shift followed a 2016 decision by the NIH to classify sexual and gender minorities as a group whose health is significantly worse than the general population. The Biden administration also prioritized transgender women for receiving interventions as part of its efforts to end the HIV epidemic in the U.S. by 2030; transgender people have higher than average risk of contracting HIV.
It remains unclear what the future of those projects will be under Trump round two. His executive order instructs government agencies to erase the concept of gender identity from all statements, policies, and other communications as well as “take all necessary steps, as permitted by law,” to end funding of projects that acknowledge the existence of gender identity.
Elvisha Dhamala, a neuroscientist at the Feinstein Institute for Medical Research who studies how sex and gender influence the brain, is concerned that the type of work she does will be de-prioritized for federal funding under the new administration’s policies, given that they discount the fundamental science behind it.
But she’s more worried about the policies’ potential to fuel prejudice against transgender, nonbinary, and intersex individuals. “It’s only going to make them more susceptible to harm and discrimination in basically all spaces that they exist in, which is every aspect of our world,” she said. “I find it quite scary as a whole.”
Neither the NIH nor the Centers for Disease Control and Prevention responded to STAT’s questions. On Tuesday, the Washington Post reported that federal health agencies had been directed by the Trump administration to pause all external communications until at least the end of January.
Mike England, a spokesperson for the National Science Foundation, told STAT in an email that the organization is reviewing all the new executive orders “and implementing them accordingly.”
Jonathan Wosen contributed reporting.