Trump says there are ‘two sexes.’ Experts and science say it’s not binary.

On the first day of his second administration, President Donald Trump signed an executive order declaring that the United States recognizes “two sexes, male and female” — but according to experts and a significant body of academic research, the definitions he used are oversimplified and inaccurate.

Trump’s order — which was followed Feb. 5 by another directive seeking to ban transgender athletes from women’s sports — signaled that attempting to legislate who is male and who is female is high on his agenda.

However, Maurine Neiman, a University of Iowa professor who has studied the biology of reproduction for 25 years, said: “While there are some areas of active debate, scientists are in wide agreement that biological sex in humans as well as the rest of life on earth is much more complicated than a simple binary.”

Eve Feinberg, professor of obstetrics and gynecology at Northwestern University, echoed the sentiment. "It's scientifically incorrect," she said of the order in an interview. "And I think it's a disservice to people who don't fall into one of those two sexes."

Michael Ulrich, a professor of health law at Boston University, said Trump’s executive order is in all likelihood targeting transgender people. “It’s trying to explain away people,” he said. “They want to try to present it as this extremely simple issue — as if it’s really just one or the other, you’re male or you’re female.”

The White House did not respond to a request for comment.

Sex is widely understood to refer to a label assigned at birth based on one's anatomy that may or may not match a person's gender. The Gender Equity Unit at Johns Hopkins University defines sex as "the biological and physiological reality of being male or female or intersex based on external genitalia, hormones, and chromosomes," and gender identity as reflecting "one's innermost concept of self as male, female, a blend of both or neither."

Trump’s order — and additions the administration has made to certain webpages — dismisses gender identity. It instructs the federal government to enforce relevant laws based on what the order describes as “immutable biological classification” — male or female, determined “at conception.”

But those categories are not so simple, some experts say.

Neiman of the University of Iowa points, as an example, to people who are considered intersex, meaning their sex characteristics do not fit a female/male sex binary.

“While these cases are a minority, sex is legitimately difficult to assign for close to 2% of the U.S. population,” she said. “We can find real exceptions in real people to whatever rule we might apply to define sex, whether it be sex chromosomes, the size of reproductive cells, hormone levels, internal organs, or genitals.”

“The executive order demands that we fit a spectrum into a nonexistent binary box,” Neiman added.

The order also seeks to define female as "the sex that produces the large reproductive cell," meaning the egg; and male as "the sex that produces the small reproductive cell," meaning sperm.

However, Neiman said, “there are people who by other criteria we would call males who can’t produce sperm at all. There are people we could call females using other criteria who don’t produce any eggs.”

Trump’s order also says sex is determined “at conception,” but Feinberg, the Northwestern professor, says that term is religious rather than scientific. If by “conception,” the order means fertilization, Feinberg notes that “internal reproductive organs don’t form at the time of fertilization. They form much later in the development of the fetus.”

Fertilized eggs do have sex chromosomes, which are associated with certain developmental pathways — XX chromosomes with the development of female reproductive organs and XY chromosomes with male reproductive organs. But “sex chromosomes do not always determine the reproductive pathway that the individual follows,” Feinberg said.

For example, there are conditions in which a person may be born with a Y chromosome but not develop male reproductive organs, Feinberg said. These include an insensitivity to androgen, the male sex hormones. Those with 5-alpha reductase deficiency, meanwhile, may “develop along that ‘female’ pathway” and then at puberty, “it can switch to become more of a male pathway,” Feinberg points out.

The American Society for Reproductive Medicine (ASRM), in a fact sheet released this month, points out that it is not uncommon for individuals to have atypical combinations of chromosomes — such as those with Klinefelter syndrome, a common condition in which a male assigned at birth has an extra copy of the X chromosome. Chromosomal variations such as XXY, XYY, and others occur in an estimated 1 in 1,500 to 1 in 2,000 live births, the ASRM said.

“Such natural variation, which is neither a disease nor a disorder requiring medical intervention, illustrates the complexity of biological sex,” the ASRM said in the fact sheet.

“It is crucial to understand that biological sex is determined by biology, not politics,” the group said.

Trump’s order is rippling through federal agencies and bureaucracy, posing a challenge to a wide range of government functions, including scientific research and passports.

In the weeks since it was issued, the Trump administration has sent notices terminating grants for transgender health services and research, the Washington Post reported. The nation’s premier health agencies also abruptly took offline several webpages that monitor HIV, health risks for youths, and assisted reproductive technologies to comply with Trump’s order, though a federal judge ordered last week that they restore them.

In the directive, Trump demands that government-issued IDs including passports, visas, and Global Entry cards reflect a person’s sex as he has defined it. Within 24 hours of the order, the State Department began holding the processing of some passports and other documents for people who had applied to update their sex designation, according to the American Civil Liberties Union.

So many parts of society are broken down into the sex binary that “it’s hard to think of all the different, potential implications” of the order, said Ulrich of Boston University. It “certainly allows for really harmful treatment in almost any area,” he said.