Biological sex is not as simple as male or female

By Tina Hesman Saey

Sex is messy.
It’s not just about chromosomes. Or reproductive cells. Or any other binary metric. Many genetic, environmental and developmental variations can produce what are thought of as masculine and feminine traits in the same person. And so sex, scientists say, should be viewed in all its complex glory.

“Sex is a multifaceted trait that has some components that are present at birth and some components that developed during puberty, and each of these components shows variation,” says Sam Sharpe, an evolutionary biologist at Kansas State University in Manhattan.
Yet a definition of biological sex put forth by U.S. President Donald Trump designates people as either male or female based solely on the size of the reproductive cells they make.
Millions of Americans don’t fit that narrow definition through no fault of their own — and many don’t even know it.

In an executive order signed January 20, the president asserts that there are two immutable human sexes and that females are defined as persons “belonging, at conception, to the sex that produces the large reproductive cell.” Males, according to the order, make the smaller cell.
On February 19, newly instated Health and Human Services secretary Robert F. Kennedy Jr. announced that the agency, which oversees most federally funded health research, will use these definitions in making policies.

In a slight variation from the executive order, HHS defines males as people “of the sex characterized by a reproductive system with the biological function of producing sperm.” And females as people with reproductive systems that make eggs.
“For me, the definition is really painful because it reduces a human being to their chance of reproducing,” says Anna Biason-Lauber, a pediatric endocrinologist at the University of Fribourg in Switzerland.

What’s more, Trump’s definition is not biologically accurate and leaves out people who carry certain genetic variants and don’t make any reproductive cells, or gametes. The order makes no exceptions for them. “What does that mean for people who don’t have gametes?” Sharpe asks. “That is, as of now, an unanswered question. But it’s an important question to answer, because you can’t have a definition of sex that doesn’t apply to everyone.”
Any definition of sex used to determine who can get an identification card or use a public restroom needs to account for variation, they and other researchers say.
Sex is complicated....