The Trump administration says there are two sexes. The science says they're wrong.

The Trump administration intends to legally define sex as strictly male or female. But the science says that sex can be quite varied -- and not so easily boxed into a narrow category.

This past weekend, The New York Times revealed that the U.S. Department of Health and Human Services has plans to define one's sex based exclusively upon the biological genitalia they're born with -- ostensibly to make the definition of sex more consistent under civil rights laws that ban discrimination.

But herein lies a problem: A person's biological sex doesn't always fit unilaterally into male or female. And perhaps more importantly, one's gender -- how one learns and chooses to socially identify across the male and female spectrum -- can be much more varied.

"The idea that there are two categories that everyone can fit into is just a little bit nutty," Anne Fausto-Sterling, professor emerita of Biology and Gender Studies at Brown University, said in an interview.

"The development of sexual systems is just much more complex," she added.

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Yet, such a narrow government classification -- if ultimately adopted as a federal rule after being subject to a mandatory 60-day public comment as soon as this fall -- would inhibit at least 1.4 million American transgender adults from being legally recognized as anything other than the sex they were designated at birth.

"The biology doesn’t say there are two sexes,” Andrea Ford, a medical anthropologist at the University of Chicago, said in an interview.

Biological sexes "do not always correspond in predictable ways or fall neatly into two categories," said Ford.

Beyond genitalia, there are gonads (testes and ovaries), hormones, and chromosomes that can manifest in a multitude of ways.

For example, babies with male chromosomes (XY) can can be born with testes but ambiguous genitalia, which can raise questions of gender assignment. Some women naturally produce lots of testosterone. People born with two XX chromosomes -- who are typically female -- may have a specific gene for male genitalia. And some people live for decades unaware that they share attributes of both sexes.

"It’s convenient for people to organize things into a recognizable category, but there’s definitely an infinity of ways that sex expresses itself," said Ford.

What's more, gender and sex are different, but often strongly linked, said Arthur Arnold, a research professor at UCLA's Department of Integrative Biology and Physiology, in an interview.

Gender, however, isn't restricted by the limitations of our bodies and physiology.

"What we're fundamentally talking about is gender here," said Arnold. "It's really a battle of social nomenclature. Are you going to constrain people by saying they are one sex?"

For instance, people may be uncomfortable with the gender they've been assigned, known as gender dysphoria. Accordingly, "some people may cross-dress, some may want to socially transition," and others may decide to medically transition with hormone therapies or gender affirmation surgery notes the American Psychiatric Association.

There might be something in our bodies or genes that predisposes us towards becoming a certain gender. But as of now, there's not a conclusive sexual or biological explanation for what propels us to choose a gender or place along the gender spectrum.

"It’s a paradox that’s unresolved," said Arnold.

The wide recognition of sexual and gender diversity isn't nearly new.

This makes the government's recent suggestion that there are scientifically just two sexes -- which are determined at or before birth -- perplexing at best and harmful to those who don't fit neatly into those categories, at worst.

Research into gender in general isn't a new area of study.

"This goes back to the 1950s, and it's just gotten more and more clear it's right with time," said Fausto-Sterling, noting early and seminal work by sexual identity researcher John Money.

Centuries before the 1950s, Native Americans recognized -- and accepted -- the reality of other genders. In many tribal cultures, both males and females chose genders that weren't exclusively male or female.

"These highly-respected individuals experienced the gender spectrum fluidly and they were never condemned for who they were," according to the University of California at Santa Barbara.

Yet, in the 21st Century, Trump administration officials seek to put sex, and accordingly gender too, into a box of their choosing.

"It’s an ideological stance that they’re trying to reflect onto science -- but the science doesn’t say that,” said Ford.