This week, the Idaho Senate overwhelmingly passed a bill that would segregate bathroom access in state-owned buildings by “biological sex.” If signed by the governor, any trans person found guilty of a felony for using the bathroom and could be imprisoned.
In a statement opposing the bill, the president of the Idaho Fraternal Order of Police warned that “officers responding to a complaint would be placed in the difficult position of determining an individual’s biological sex in order to enforce the statute.” In other words: Please don’t make us perform genital inspections (although we will if you really want us to).
Never mind that in 2024, Idaho legally defined biological sex as an immutable, binary characteristic that “can be observed or clinically verified at or before birth.” Surgeries that reconstruct genitalia mean that, for some people, these two assessments of biological sex will lead to different classifications. The discrepancy between definitions of “biological sex” (on one hand, the state’s legal definition of sex assigned at birth and on the other hand, the cops’ definition of external genitalia) underscores its emptiness.
This shape-shifting nature of is part of its utility as propaganda. It allows anti-trans activists and policymakers to constantly shift the goalposts for social inclusion and political rights. The International Olympic Committee (IOC) exemplified this dynamic this week with a new mandate for all female athletes to receive genetic testing, effectively barring some intersex and trans athletes from competing. The president of the IOC provided the following rationale: “[I]t would not be fair for biological males to compete in the female category.” What is a biological male? The IOC doesn’t know, either.
This rhetoric is effective because it incorporates the multitude of sexed characteristics under a single epithet and, in doing so, obscures the underlying biological complexities. For the anti-trans movement, these are inconvenient truths that must be glossed over.
In reality, sex exists beyond the binary, a constellation of descriptors including sex chromosomes, reproductive anatomy, external genitalia, secondary sex characteristics, gamete production, and sex hormones. On their own, none of these factors are binary, and a multitude of arrangements can exist.
Importantly, the point here is not to essentialize or medicalize. Biology is just a small part of our complicated and beautiful lives as humans. Rather, this is an invitation to acknowledge the multitudes of forms and physiologies that exist. There is no single way to be, and no amount of policy and policing can change that.
The development of sexed traits begins during embryogenesis. Sex chromosomes (actually only one gene, SRY, most commonly found on the Y chromosome) and a cohort of genes found on the other (autosomal) chromosomes contribute to someone’s anatomy at birth. These autosomal genes have just as much influence on sex as SRY.
What’s more, sex chromosomes can be arranged in many combinations, not just XX and XY. Intersex people with alternate chromosome compositions (such as XXY or XXYY) can be born with different reproductive anatomies. Such intersex traits are stigmatized by medicine, and doctors often perform unnecessary operations to modify anatomy and artificially reinforce the sex binary. Notably, bans on health care for trans youth contain exceptions that allow doctors to continue to operate on intersex infants.
After birth, your sex chromosomes have remarkably little impact on your physiology. Instead, sex hormones take over. Sex hormone levels remain low until puberty (regardless of sex assigned at birth), when a cascade of brain-derived hormones initiate the production of androgens and/or estrogens. This hormone cascade can be paused by puberty blocking medicines.
This pubertal spike in sex hormones directs the development of secondary sex characteristics and influence the production of gametes (eggs or sperm). Gametogenesis depends on reproductive anatomy, but anatomy is not sufficient. The incomplete or inefficient production of gametes is a cause of infertility. Unlike gametes, secondary sex characteristics (e.g. body hair, breast growth, fat distribution, etc.) develop in response to whatever sex hormones are circulating regardless of sex assigned at birth.
The relative levels of each sex hormone (five main androgens, including testosterone, and four main estrogens) fluctuate over time depending on life stage. Just consider the steep drop in estradiol levels during menopause. At the molecular level, testosterone can be transformed into estradiol by a single enzyme known as aromatase. Additionally, prescription hormones can shift this balance to align with patient priorities for their own body.
The impact of sex hormones is wide-ranging. Physiologists have long observed differences between cis men and cis women in bodily processes. Pervading cissexism in medicine led researchers to assume that these differences are caused by an essentialized sex (probably) located in the chromosomes.
Yet, the physiology of trans people undergoing hormonal transition suggest that hormones are major drivers of day-to-day functioning. Heartbeats, microbiomes, cardiorespiratory fitness, and immune responses all shift within the first year of HRT to align with hormonal sex. At these physiological levels, transfemmes on estradiol more closely resemble cis women and transmascs on testosterone resemble cis men. Physiology, like sex, is not fixed.
An undercurrent in this biology of sex is the way that biotechnology and medicine enable new ways of shifting sexed characteristics. Hormone prescriptions shift hormonal sex just as surgeries can alter reproductive organs, external genitalia, and secondary sex characteristics. Today, technology and medicine construct the biology of sex as much as genetics.
The conception of sex as purely biological (with gender as a distinct psychosocial phenomenon) was popularized by sexologist Robert Stoller in his 1968 book Sex and Gender.1 In reality, this sex/gender boundary is more blurry than Stoller would care to admit, yet his definitions continue to be regurgitated. But, the phrase “biological sex” was rarely used until the 2010’s when gender critical activists capitalized on Stoller’s distinction to advocate sex-based (rather than gender-based) rights.2
Before 2016, “biological sex” was not found in any legal code in the United States. It’s first appearance was in North Carolina’s 2016 bathroom ban. The phrase was laundered into political discourse for the purpose of banning trans people from using the bathroom under the guise of “protecting women,” a claim for which no evidence exists.
Interestingly, the use of biological sex in scientific literature also skyrockets around the same time (almost 50 years after Stoller’s Sex and Gender is published). According to PubMed, a database of biomedical research, the phrase “biological sex” is found in just 8 papers published in 2005. It appeared in 54 papers published in 2015 and an astronomic 762 papers in 2025.
This eye-popping increase is mirrored by analyzing the works published by a single family of journals. Using RSS news feed data curated by researcher Jessica Kant, I examined mentions of “biological sex” in Nature Portfolio articles from 2023-2025. Nature Portfolio publishes over 150 academic journals including the eponymous (and prestigious) Nature.3 References to biological sex increased 284% during this period - from 108 articles in 2023 to 415 in 2025.
graph by author | data curated by Jessica Kant
Clearly, this unscientific anti-trans dogwhistle is working its way into biomedical research, which (as a broad body of knowledge) has largely excluded trans perspectives. Cissexism dominates the biomedical sciences and thus seeps into its epistemological foundations.
Biomedicine often ignore the diversity of sex in nature, work performed by ecologists. Ecological understandings of “biological sex” also paint a complex picture, and currently there is no consensus about how to define it thanks to the multitude of traits that fall under the umbrella of sex.
For example, both birds and lions display natural variations in sex, including across the life of a single individual. According to some estimates, ~5% of birds have discordance between their sex chromosomes and their reproductive organs, a discrepancy thought to be driven by an acute change in sex hormone levels in adulthood. Similarly, adult lionesses can spontaneously develop manes, perform male-specific social behaviors, and even convert ovaries into proto-testes. This change appears to also be driven by sex hormone levels.
This ecological research on lions as well as genetic evidence from other mammals has led to a new idea about the biology of sex: Sex is somewhat malleable and must be maintained constantly throughout life. Sex hormones actualize this process by triggering genetic programs that tell cells how they should function.
The sex maintenance hypothesis could explain why trans people on HRT see physiological changes in many body systems as well as the “partial rejuvenation” of testicular stem cells in transfemmes on HRT. If the maintenance hypothesis is further substantiated in future studies, sex will only get more complex.
The rhetoric of “biological sex” is fundamentally eugenicist. It is an attempt to essentialize sex as a pure binary, an immutable characteristic in defiance of scientific fact and with the intention of eliminating a class of people from society. To these eugenicists, the concept of biological sex is a weapon used to police the boundaries of sex/gender. They tell you these boundaries are self-evident yet not self-evident enough to prevent invasive genital inspections or genetic testing.
Sex will always describe a collection of characteristics that cannot be reduced to a single descriptor. Eugenicists arguing otherwise are just afraid of your own autonomy. With modern biotechnology, your sex is yours to maintain however you want - whether you take hormones, have received plastic surgery, or just use a little minoxidil to get hair growing where you want it.
Sex is not straightforward and neither are you.
from the archive
1 In Sex and Gender, Stoller proposed turning people seeking transition care into test subjects to dissect the boundaries of sex and gender. Ultimately, he wanted to identify the root cause of gender non-comformity so that he could eradicate it. As a gatekeeper, Stoller operated under the basic assumption that some (if not most) people seeking transition care were either lying or not sufficiently deserving of care. He sought those who he deemed as “true transsexuals,” a label generally reserved for white, middle class people who met his subjective aesthetic standards. For example, he preferred transfemme patients with “pretty faces, with fine hair, lovely complexions, graceful movements, and—especially—big, piercing, liquid eyes.”
2 This ideology has since been turned into policy. A January 2025 executive order from President Donald Trump formalized this trans-exclusionary principle for the purposes of enforcing federal law in the United States. In April 2025, the Supreme Court of the United Kingdom, via their decision in For Women Scotland Ltd v. The Scottish Ministers, declared that "biological sex” should also guide interpretation of the country’s Equality Act of 2010.
3 I filtered out entries from Nature News, the science journalism arm of Nature Portfolio.