A new report that the Trump administration hopes to legally define a person’s sex at birth — an unchangeable condition determined by genitalia — has prompted an outcry among the transgender community.

But beyond the political implications — and fears that any such change could ultimately mean the end of civil rights protections for transgender people — the proposal raises fundamental questions about something else: biology.

The proposal is “highly inaccurate and just an insult to science. Basic science,” said Rachel Levin, a Pomona College neuroscientist who studies the development of sex.

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According to a memo obtained by the New York Times, the Department of Health and Human Services is spearheading the effort to come up with a legal definition of sex “on a biological basis that is clear, grounded in science, objective and administrable.” On Monday, a top official at HHS called the report “misleading” but declined to comment on the “alleged, leaked documents.”

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Defining a person’s sex at birth — and making that status immutable — would be at odds with biologists’ research findings.

To start with, about 1 in 1,500 to 1 in 2,000 newborns have atypical genitals, according to the Intersex Society of North America. “But a lot more people than that are born with subtler forms of sex anatomy variations,” the group says, “some of which won’t show up until later in life.”

More broadly, researchers who study sex, gender identity, and gender expression say, it doesn’t make sense from a scientific perspective to narrowly define sex as a fixed condition based on a person’s genitalia or genetic makeup.

Human embryos start out with a set of gonads that have the potential to become testes or ovaries. At about eight weeks gestation, a single gene on the Y chromosome, called SRY, activates and sends a message to the gonads to become testes. In the absence of that functional SRY gene, an embryo develops ovaries, which don’t crank out hormones until puberty.

But the presence — or absence — of those hormones and their effect on the budding brain and genitals mean that they can develop independently of one another.

There are a number of steps that can lead to genitalia and bodies that don’t fall into either of two sharply delineated categories, said Levin. The Y chromosome might lack a functional SRY gene, for example. In that case the individual is genetically male but physiologically female.

“There will never be a single answer or a simple answer to gender identity,” Levin added.

Some babies born with male genitalia have an extra X chromosome, a condition that is known as Klinefelter syndrome. And in androgen insensitivity syndrome, a person has XY chromosomes but does not develop androgen receptors that bind to androgens. A baby with AIS might be born with a short vagina without a cervix, but undescended or partially descended testes.

“If we’re going to start just using genetics and sex chromosomes, we’re already going to be missing [those people],” said Melissa Wilson Sayres, who studies sex-based biology and the evolution of sex chromosomes at Arizona State University. “They’re not going to have a place to go.”

Such concerns prompted at least one science-based organization to issue a statement on Monday describing the proposal as discrimination masquerading as science.

“This proposed change pushes pseudoscience,” the Union of Concerned Scientists, a Cambridge, Mass.-based nonprofit, said. “It’s baseless and wrong for HHS to distort science to try to rationalize this change. It’s a charade, not a real argument. “

The failure to recognize someone’s gender would likely have consequences for health care and research.

Dr. Sari Reisner, an epidemiologist and professor at Harvard Medical School and the Harvard T.H. Chan School of Public Health, expressed concern that discounting a person’s gender identity or gender expression could exacerbate existing health disparities among transgender and non-binary people.

“By not accounting for someone’s gender, outside of biological binary, you’re missing a whole component of their health,” he said.

“If you just go by sex, you’re not going to accurately capture health disparities,” he added. “That has implications for interventions and for health care delivery.”

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