Biological Sex

Different parts of “Biological Sex” 

The phrase "biological sex" as in "biological boys" "biological girls" and such is problematic on multiple levels, not the least of which is that it has become a dog whistle for transphobes.  In political arguments and bills, its users evoke a "scientism" and talk about the differences between "biological males" and women or girls, by which they mean "cisgender women or girls" because in their perspective trans people are forever and only the gender they were assigned at birth. And so the term “biological sex” can wind up being an “othering’ term that anti-trans folks use to deny the validity of the genders of trans folks.  [It's noteworthy that a search of the Legiscan archive of U.S. proposed state and federal legislation from 


But the phrase “biological sex” is also a problem on its face.  When used it’s often both inaccurate and incomplete.

Anti-trans folks make claims that "biological sex" is defined by which gametes a person produces or by the presence of XX or XY chromosomes, which we know is more complex and is certainly not the way we determine what goes on a birth certificate and what gender we presume an infant to be - that's all about external genital morphology.

 1) Natal external genital phenotype.  This is where assigned gender at birth comes from.  You take a look and see if there’s a penis or a vulva.  Some intersex conditions make those external shapes less clear cut.  Still, it’s what things look like when you're born. And it's typically this one that folks rely on when talking about whether someone is cisgender and agrees with what the doctor or midwife says or transgender and doesn't (I'll lump non-binary and agender under the trans umbrella for this).  Many folks make assumptions about sex and gender of a person based on this aspect of sex that are true for most but not for a substantial number of people.  Regardless, this is likely what winds up on a person’s original birth certificate.


2) Contemporary external genital morphology.  Some fraction of folks may change things there - whether by design among some trans folks, or to manage some disease like cancer, or through accident. That anatomy remains biological, so contemporary genital morphology is another aspect of biological sex.  The shape of one's external genitals alone does not determine one’s overall sex, but it can be a component.

3 & 4) Chromosomal and Genetic Sex

K12 biology classes will tell you that this corresponds to whether your chromosomes are XX or XY. Call this chromosomal sex. But that's not quite right. The programming you need to turn on all the male stuff is from a stretch on most Y chromosomes called the SRY gene, and sometimes during meiosis (that's something you _do_ learn about in K12 biology) the SRY might hop over to an X chromosome. So there are XX men where an X has an SRY gene built in and they wouldn't know without genetic testing. You can also be an XY woman if the Y chromosome you wound up with lost its SRY gene. You could have XY/XX mosaicism where varying cells around your body have different sets of genes. And there are a number of intersex conditions that wouldn't reveal themselves until after a person hits puberty, like complete androgen insensitivity syndrome where an XY person will appear externally to be a girl but have undescended testes and no uterus.  Consider all of these ways in which your DNA influences how your body is built genetic sex.


We're barely scratching the surface of the different ways a person can be intersex, which when you add them all together wind up being about 2% of the world population.

But we're talking about trans folks and going too far into comparisons with intersex folks is getting into the weeds a bit - except to show that "biological" can mean genetic, that genetic is more than the chromosomal XX = girl, XY = boy, and can mean how the genes are expressed in terms of a constellation of visible sex characteristics. 


5) Internal Reproductive Anatomical Sex.  

Internal reproductive anatomy, is biological and baring accident or particular illness, is not revealed at all before puberty. The presence or absence of those parts can change by design for various reasons among cis folks as well as trans folks.  


6) Gamete Production Sex: Some legislation seems particularly oriented around whether or not someone can produce ova or produce gametes that fertilize ova.  While production is not likely to switch from making one type of gamete to another or to making both, even among healthy, injury free cisgender folks whether or not one makes gametes at all is something that changes over the course of someone’s lifetime. 

But. even with all of this, I, a trans woman, am a biological female.

OK hang in there a moment.  I am not claiming that I am a cis woman - or we wouldn't need to have different terms here.  But there's also more to biology than natal morphology and genetics.

7 & 8) Endocrinological Sex and Secondary Sex Characteristics Sex

Our cells are happy to go along with whichever hormones running through your system tell them to do. SRY's job is basically done by the time you're born. Cis men and cis women have both testosterone and estrogens and progesterone (and LH and FSH...) and while bimodal, the ranges overlap.

But, have a person's testosterone and estradiol levels sit in the standard female puberty range, they'll develop all the standard female secondary sex characteristics & have them sit in the standard male puberty range, they'll develop all the standard male ones. If it's the first time they've gone through puberty, then that will affect skeletal development as well.

If we're talking boys and girls, school aged kids, there's not a lot of difference pre-puberty. No one is prescribing gender affirming hormone therapy for trans kids until they're mid-teens, and if there is any medical treatment for them before that, it would be Gonadotropin Releasing Hormone Analogs - and only after they started puberty, and only if they had shown they were trans for a while, and these puberty blocking meds do just that. They press pause on puberty for a few years and if you stop taking them then whatever hormones you've got - either from what's built in or from what you take as medicine - will start it up again.


Secondary sex characteristics are anatomical and are dependent on growth and development under the influence of hormones.

Also less obvious, but painfully relevant to the whole sports discussion is the effect it has on muscle development and recovery.  Shape-wise muscle atrophy has made my shoulders slightly less broad -  I kind of ditched my swimmers muscles in my twenties, so the change was less extreme than it might have been, but it's also made my arms more slender.  But performance wise it's a real kicker.  Even after my heart attack in my 40s, once I had passed about 3 months of recovery, I had a comparable level of fitness to what I had before. 


From when I was a young adult through early middle age, my exercise habits were very irregular, but it didn't matter how long I had gone without doing regular exercise -- a couple months or, sadly more often, a couple years -- I could start up again, with sit ups, and on my first session do a set of 30, no problem, and often 50.  But I'd definitely be up to 50 by the end of the week.  And with lap swimming I might start out with a 1km workout but by the second week I'd be up to 2km. 


This did not work running on estradiol & low T.   A few years ago, after about 10 months of hormone therapy, I tried starting things up again and was exhausted barely getting to 10 sit ups.  


And I could not recover enough to try again within any reasonable amount of time.  And working at it for two weeks I could, with effort, and maybe one or two pauses, get up to 18 or 20 -- and I couldn't get past that during the month I kept things up before slacking again.  


Trans women who undergo hormone therapy wind up reducing their performance levels by something like 10-15% across a variety of measures.


Trans girls who go on puberty blocking medication, never get the adolescent and adult male blood chemistry and physiology in the first place. 


There's more to gender than shape, and muscles of course.  But, the hormones also interact with your neurology.   Once you run on estradiol rather than testosterone, that triggers other regulating hormones to go into action.  Many trans women report a menstruation free pseudo-period with a cycling of variety of effects often associated with that - for me I seem to have a 3 week cycle that includes phases of libidinousness, depression, gastric challenges and normality. 


9) Neurological Sex
But they do affect other longer term structural aspects of neurology. And since the brain and mind are one and the same, a person’s inner sense of gender is inherently biological, but also the ways in which our behavior and feelings are experienced and regulated are associated with levels of various hormones including testosterone, estrogens, and progesterone and humans respond the same way to those, regardless of your starting package.

So.. natal external physiological sex, chromosomal sex, hormonal sex, there's some evidence for neurological sex differences, but that's also a bimodal thing. It's not a cut and dry "we can see your brain works this way, so you have a male brain" thing, but ok, I'm trans, my brain tells me I'm a woman, that transness at least is also biological.

But, the hormonal chemistry helps control how the genes express themselves, and with my cocktail, they're running my body, including my brain as a woman and not a man.

But, of course, not every trans person has undergone hormone therapy.  Still some studies suggest that even trans women who have not undergone any hormone therapy tend to have brain morphology closer to those of cis women than cis men.  In this article from June 2024, Veronica Esposito gives an overview of "Everything You Ever Wanted to Know About Transgender Brains".


So  wait, where did this come from?

It’s notable that medical literature is not immune from transphobic language.  While neither the 1987 DSM-IIIR, the HBIGDA (now WPATH) 1990 Standards of Care 4 (SOC 4), nor the 1994 DSM IV proscriptions for trans diagnosis and treatment include the terms biological men or biological women, the SOC 5 from 1998, and the SOC 6 from 2001 did but the SOC 7 from 2012 did not, nor does the DSM 5 from 2013 which uses “natal gender” or “designated gender”, nor does the SOC 8 from 2022.

It's pretty straightforward though, to lay the increasing misusage of the phrase "biological sex" at the feet of anti-trans legislators and their advisors.  The site, legiscan.com/, archives State, Federal, and DC legislation back to 2009 (with some entries from 2008).   With a free account, one can search for the phrase in quotation marks and sort the bills it appears in by date of last activity.  While there is one nondiscrimination bill per year using the phrase in 2011 and 2012, the overwhelming number of uses begins with the surge of anti-trans bathroom bans in 2015, many of which are associated with variations on Alliance Defending Freedom's model legislation called the Student Physical Privacy Act.



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