February 25, 2025
5 min read
Politics, Not Biology, Is Driving Legal Efforts to Classify Sex
Sex is not one single, simple, uniform biological reality, so biology cannot be invoked as a basis for “immutable” legal classifications
Amid the legal theater in the U.S. over sex and gender identity—triggered by a Trump administration decree that “‘sex’ shall refer to an individual’s immutable biological classification as either male or female”—a moment of clarity has come from a judge.
In an ongoing military service case, federal district judge Ana Reyes stopped Jason Lynch, the Department of Justice attorney who has been defending the executive order, to ask, “You understand, as a matter of biology, it’s just incorrect that there are only two sexes, right?” To which he responded, “I don’t understand that to be incorrect.” The judge then enlightened Lynch about the existence of intersex human beings, about whom he had confessed to have no knowledge. That’s despite the fact that Lynch has been representing the U.S. government on an “immutable biological classification” in a federal courtroom.
Another clear example comes in the February announcement by the Department of Health and Human Services that expands on the recent executive order’s “sex-based definitions.” This announcement informs federal agencies that, for the purposes of carrying out their duties, a “male” is anyone who has a reproductive system that produces sperm, and a “female” is anyone with one that produces eggs.
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The courtroom exchange and the HHS edict highlight why and how too much of the current discussion and legal arguments about sex are about ideology and not biology.
The “why” here is that most folks, apparently including ones at the HHS, are unfamiliar with how sex categories are understood by biologists and what the actual range of biological variation is in humans. This lack of familiarity enables idealogues to deploy distortions to achieve their agendas of exclusion. The decrees are barbs in culture wars, over gender issues and the rights of transgender people, as the New York Times has noted. The “how” is in the intentional assertions about sex categories and the human experience that are biologically inaccurate.
Clear definitions of categories matter in the law. The use of two sex categories to talk about a species is standard in biology. In many animal species, including people, however, there are individuals who are neither male nor female or who are sometimes both. In other species, there are two sexes, but they aren’t male and female (usually these are intersex and male). And a few species have only one sex (usually female). The biological reality is that “male” and “female” are not universal immutable biological classifications but rather descriptions of typical patterns in reproductive biology. These categories, male and female, are used by biologists who fully understand that they rarely represent all the relevant biological variation in any given species or identical sets of variation across different species.
Sex is not one single, simple, uniform biological reality. Thus, biology cannot be invoked as a basis for such in legal terms. That’s the bottom line.
Of course, men and women are not the same, and reproductive biology does structure important aspects of human bodies and lives. But none of the key biological systems associated with sex in humans (chromosomes, gonads, genetics, hormones, and so on) come exclusively in two “immutable” categories. Yes, most humans have either XX or XY chromosomes, but as Judge Reyes noted, some don’t. People with either testes or ovaries are most common, but some people have both, and a few have ovotestes. Usually those with testes can produce sperm, and those with ovaries produce ova—but not always. The chromosomes one has do not always predict one’s gonads or one’s genitals or even all the elements of one’s reproductive tract. It is true that most people have the “typical” combo of chromosomes, gonad and genitals, yet there are tens of millions of people alive right now who don’t. These people are not errors, aberrations or problems; they are a part of the range of variation in our species. They are all real people. In fact, many who have these variations don’t even know it. You might be one of them.
In making laws, then, we need to recognize what the actual range of variation in sex-related biology is and how it maps across everyone.
Consider height: Male individuals are, on average, about 8 percent taller than female ones. But most people are not average. If you drew a random set of 50 individuals (25 male and 25 female) and lined them up by height, the result would not be all the people who were male on one side and all those who were female on the other. Rather there would (usually) be a majority of male individuals in the taller half and a majority of female individuals in the shorter half. Averages aside, not every male person is larger than every female person. It’s not just height either. In a recent National Health and Nutrition Examination Survey dataset, in the fifth to 95th weight percentiles, nonpregnant females aged 20 and older weighed between 110 and 263 pounds, and males in the same age range weighed between 136 and 287 pounds. As such, much of the variation—that space between 136 and 263 pounds—is an overlapping mix of female and male adults. This overlap and frequent lack of clear sex distinction extends beyond height and weight to livers, brains, hearts, stomachs, intestines, bones, etcetera.
Yes, there are some typical bodily differences between female and male adults of the same height and weight, such as upper-body muscle density and some specific hormone levels (but not kinds of hormones). How those differences manifest, how they developed, and what they mean for any given experience or ability, however, are not anywhere near an “immutable biological classification.” The distribution of human biological variation, as it relates to one’s capacity to serve in the military, sports abilities, childcare capacities, interest in math, love of cooking and everything else, is not simply, cleanly or “immutably” divided between the categories of female and male.
This administration’s definitions of “sex” are intentionally designed to mislead the public about human biology. Any biologist knows this, and Judge Reyes pointed it out as an issue of legal concern. From a legal perspective, I guess those in the administration have the right to make any statement they wish as a reflection of their political ideology. But they do not have the right to invoke biology as a “natural” proof of their position—because it is not. There are few easy or universal biological “rules” dividing sex categories and human lives, but there are important variation and patterns. Biologists and other scientists need to help judges and attorneys understand what biology does and does not tell us about human sex and reject arguments that intentionally mispresent these data.
Falsehoods should not be enshrined in our laws. People’s lives depend on it.
This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.
