In its recent judgement on “biological sex”, the Supreme Court avoided defining what “biological sex” is. It says it is “commonly understood”, however the phrase does not appear in most leading dictionaries. 

This is of particular interest to me. As an intersex person, I am not, in UK legal terms, transgender, and I am comfortable presenting as a man in many contexts. I typically use men’s loos, and play men’s sports. But I would not call myself “biologically male”, except in some very peculiar contexts. Reading the recently released EHRC guidance, this seems to imply that I should not use men’s loos or play men’s sports. Does it?

Wiktionary, the only dictionary I have to hand that attempts to define the phrase, defines biological sex as “assigned sex”, that is, the documented sex one receives at birth from a brief identification of one’s observable sex characteristics. Unfortunately, this is clearly not what the Supreme Court intended, as it does not mention birth certificates or medical identification at all, and it explicitly distinguishes between “biological sex” and “certificated sex”. It also wouldn’t help intersex people, who have all sorts of fantastic claims made on their birth certificates.

It also cannot refer to how “biological sex” is sometimes used in genetics, that is, to chromosomal sex. Chromosomal sex is almost never observed in humans, and when it is, it is observed to be only one of many factors that contributes to the meat (as it were) of what is observable “sex”.  

One must infer, therefore, that the UK Supreme Court intended to define biological sex as “some observable sex characteristic or characteristics at birth, which is typically recorded on a birth certificate”. And perhaps this is what they mean by “commonly understood” – that is, sex at birth is what is observable at birth to some common analysis. 

Now, nobody is a woman or man at birth – these are terms we use for adults – and trans people have all sorts of biological and biochemical characteristics, depending on their medical transition or absence thereof. Unless the Court imagined transgender people as springing like Athena from the mind of a god, one must therefore further infer that what they mean by “biological man” is “a person who was identifiable as male at birth due to some bundle of biological characteristics”, and the same for “woman” and “female”. 

However, science and medicine have observed that what we typically call gender identity – that is, the internal, personal sense of one’s femaleness or maleness – can become fixed at a very young age; around three to four. That is, no sooner than we learn to think in complex sentences, we begin to think of ourselves as “boy”, “girl”, or something else. Remarkably, this is as true for non-binary people as it is for girls and boys – suggesting that both binary and non-binary genders are epigenetic, perhaps even genetic. And gender identity precedes, critically, most of the other observable biological properties of “sex”, which are only emergent at puberty. 

We may never find a “trans gene”, just as we have never found a “gay gene”. But if one’s preferred gender identity is deeply rooted in some biological property, it seems likely that we could identify transgender women as female from birth, and transgender men as male, with a sufficiently sophisticated technological instrument. With sufficient education in the scientific properties of sex and gender, it can and should be commonly understood that at least some trans women are innately women, and trans men are innately men. Conversely, it is impossible to reach a common understanding of “biological sex” which circumvents this basic reality.  

In fact, the way the Supreme Court describes “biological sex” makes it necessarily distinguishable from “sex on a birth certificate”, but practically indistinguishable from “gender identity”. 

As liberals and social democrats, we probably wouldn’t try to identify this birth-observable biological sex or gender to a degree of medical certainty. We believe in personal autonomy and privacy; we would never mandate that infants be subject to biochemical testing, brain scans, or psychiatric analysis. 

We must therefore accept that the instruments of certificated sex will likely often oblige us to record an incongruence between one’s actual biological sex – using the Supreme Court’s definition, the sex that was observable at birth – and one’s recorded biological sex – that is, the sex that was observed at birth and is on one’s original birth certificate. 

Again, science and medicine tell us that it is likely that one’s actual and observable biological sex, whatever it may be, is rooted in epigenetic and/or genetic characteristics. The only logical possibility left to us, therefore, using the definitions the Supreme Court now obliges us to use in law, is that “biological sex” is impossible to identify with any certainty by the means afforded to doctors and midwives at the birth of an infant. For liberals, the only reasonable and ethical way of determining biological sex is to ask an individual when it is appropriate to do so.

While I do not think it was the Supreme Court’s intention to introduce such ambiguity, I can see no other logical possibility – and transgender people and cisgender people alike will have to reckon with what their “biological sex” is.

For myself, I’m not going to hold my pee.

* Em Dean is an intersex English teacher and computer scientist. They are a member of Waltham Forest Lib Dems.