A leaked memo from the Trump administration doubles down on a claim that sex and gender identity can be boiled down to “immutable biological traits.” In other words, it asserts that a person’s genitalia dictates whether they are sexually male or female and should identify as a man or a woman. It is biologically clear, however, that those binaries are not clear-cut, says reproductive endocrinology expert Dr. John Theisen. That’s what makes this memo so disturbing.

The memo, leaked to the New York Times, uses an incorrect interpretation of biology to put an unchangeable timestamp on someone’s sex that begins at birth. Theisen, an assistant professor of reproductive endocrinology, infertility, and genetics at Augusta University, says that the science referenced in this memo is a misinterpretation of biology altogether.

“I don’t think it is reasonable or even possible to strictly define sex based on parameters found at a person’s birth,” he tells Inverse. “While it is reasonable to assign sex at birth based on the physical appearance of unambiguous genitalia, an attempt to define a rigid immutable sex on this basis is not consistent with what we know to be true about the biology of sex and sexual differentiation.”

The memo invokes biology in two statements used to make its claims, neither of which pass Theisen’s sniff test.

This leaked memo has prompted similar outcry to the administration's earlier anouncment that they would ban transgender troops from serving in the military Flickr

“Sex means a person’s status as male or female based on immutable biological traits identifiable by or before birth.”

This statement suggests there are unchangeable physical traits that can place an individual into one of two buckets: male or female. The New York Times report indicates that those “physical traits” refer at least in part to genitalia, which Theisen says is a useless exercise.

"There are numerous situations where a person’s biologic sex is not clear at birth.

“There are numerous situations where a person’s biologic sex is not clear at birth,” he says. He points to a condition called congenital adrenal hyperplasia, in which a genetically female individual produces high amounts of male hormones, like testosterone. In that case, the infant is be born with “ambiguous genitalia” with characteristics of both male and female sex organs.

This rare condition, reports the Intersex Society of North America, occurs in one in 13,000 births. But its existence illustrates that it is possible for a person to have hormones and genitalia are typical of both genetically male and female individuals.

More importantly, it shows that there’s no clear-cut biological trait that can identify people as male or female with complete certainty. No evidence mandates that genitalia should define biological sex more accurately than hormones do. There is evidence that a two-category male-female system isn’t even adequate anyway, says Theisen.

“The sex listed on a person’s birth certificate, as originally issued, shall constitute definitive proof of a person’s sex unless rebutted by reliable genetic evidence.”

Theisen interprets this section of the memo as a way to respond to cases in which more than one “immutable trait” is at play, suggesting ambiguity. It defaults to genetics to provide “reliable genetic evidence” that someone adheres to one of two biological sexes.

“Some might say that the reported use of the phrase, ‘any dispute about one’s sex would have to be clarified using genetic testing’, would address some of the above issues, but that’s assuming that gender, biologic sex, and sex chromosomal compliment (whether a person has XX or XY chromosomes) are always linked. They aren’t,” he says.

"Even at a genetic level the use of a binary male and female is not sufficient.

It is true that males tend to have an X and a Y chromosome, whereas females tend to have two X chromosomes. But as Theisen points out, two chromosomes are not enough to completely define sex: in people with androgen insensitivity syndrome, for example, a person can have an X and a Y chromosome (and be genetically male) but be unable to respond to the “male” hormone testosterone.

“This essentially leads to individuals who develop completely as women, not masculinized in any way, but who, at a genetic level, are males,” Theisen says. This syndrome is is just one example: “The list of sex chromosome compliments that stray from the standard XX or XY, is too numerous to cover here, but suffice it to say, that even at a genetic level the use of a binary male and female is not sufficient.”

The human body, in other words, is far too complicated to define in dualities. Using biological evidence to create an artificial gender binary is likely more of a political tool than one grounded firmly in scientific evidence, despite what this memo seems to claim.