It is not my intention to go into great detail critiquing the UK Supreme Court judgement of Weds 16^th April 2025 on sex/gender. I do not intend to fight a legal battle that I lack the resources, or skill, to even begin but rather, given the now increasingly muddled and discriminatory nature of the law resulting from this ‘clarification’ I hope to lay the ground for a better attempt at getting trans rights properly established in UK law next time (whenever that is!) by attending to the underlying philosophical issues in as even-handed a way as I can (whilst acknowledging my strong investment in the issues).

Having said that there are certain aspects of the judgment which are clearly highly problematic and the one that is most obviously so regards biological sex and the decision to treat it as entirely binary. To be fair to the Supreme Court they were mainly trying to make laws work that were/are themselves quite binary but I am not sure that completely justifies their approach.

Specifically, it is important to note that human sex is bimodal and not binary. This means that there are two highly common sets of sex characteristics and then a number of much rarer conditions that do not neatly fit. We should recall that these conditions are experienced by people of worth and dignity who should not be treated as collateral damage in society’s seemingly growing desire to curtail and control transgender people. Intersex people are often overlooked, even by trans people, but however inconvenient it might be for some, their existence is clearly indicating that we have not got our sex (and thus gender) classification right, as a society, and that justice demands greater flexibility. The existence of intersex people has been perhaps most visible in the sporting arena but I am pretty repelled by a culture that tries to make villains of Imane Khelif and Caster Semenya who have clearly been raised as women.

The bio-essentialism that underpins the Supreme Court’s approach is highly problematic as there is no single characteristic that uniquely and unerringly determines biological sex. There is no characteristic that is both necessary and sufficient. This applies even before we admit trans people into consideration or consider those properties that can be changed by human means. This is obviously demonstrated through intersex conditions. To understand this firstly note that the kinds of things that we typically might wish to consider to indicate biological sex include both genotype (i.e. genes and specifically chromosomes) and phenotype (which simply means the physical characteristics of the organism that can be observed). The latter can be further broken down into sub-categories (sex organs, secondary characteristics e.g. breasts or facial hair, and sex hormone production and response) and all of these have more than two different human instantiations.

There is a body of opinion that is keen to make chromosomes the arbiter of sex. However in going down that route consider firstly, without making any cross-gender/sex claims, that not everyone has XX or XY chromosomes. For example, people with Klinefelter syndrome have XXY chromosomes, those with Jacob’s syndrome has XYY chromosomes and that there are also people with three or more X chromosomes (trisomy). Still there is only one gene on the Y chromosome (called the sex determining region of the Y chromosome or SRY gene) that is relevant to sex. Having one of these could cause one to be view oneself as genetically male. But a Y chromosome can lack the SRY gene which would mean that physically (phenotype-wise) the individual is female but chromosomally they are male and genetically (as no SRY) arguably female!

Also some women (including some contentious sportswomen) are likely to have XY chromosomes (the Y presumably with SRY gene intact) and naturally elevated testosterone levels due to internal testes, but will generally be viewed as being a woman due to secondary and primary sex characteristics. This is likely to be caused by androgen insensitivity syndrome (i.e. her body has not responded to the androgens it produces). Then there is the case of the Guevedoces (meaning ‘penis at twelve’), mainly originating from a small town in the Dominican Republic where there have been multiple cases of children seemingly born as girls (having no visible penis or testes) and being brought up as such, then developing a penis and having testicles descend at puberty. It seems these children have XY chromosomes who due to an enzyme deficiency did not respond to the pre-birth testosterone surge but do so with the second surge at puberty. Most of these go on to live as males but some have an operation to remain female. Nature is messy!

In thinking about sex hormone quantities it is also worth noting that some females have more male hormones than some males, and some males have more female hormones than some females!

And in the search for a definitive biological sex we can delve ever deeper or wider and still not find one that perfectly matches our social needs, perceptions and prejudices, meaning that decisions become necessary (for those who really must classify – though I ask do we really have to?) for ambiguous cases.