In her excellent and informed critique of the Supreme Court’s judgment in For Women Scotland v The Scottish Ministers [2025] UKSC 16 (“FWS”), Crash Wigley notes that in its judgment the Court neglects to address several important issues, including the human rights consequences of the “biological sex” interpretation favoured by the Court. My own discussion of the judgment is meant to complement Wigley’s criticism. I will focus on more straightforward interpretive problems with the Court’s judgment, which support the conclusion that the Court could have pursued an interpretation of the statute compatible with Convention rights.

A core aspect of the Court’s reasoning in FWS was its conclusion that a trans-inclusive reading of the Equality Act 2010 (“EA 2010”) would result in an incoherent and unworkable interpretation of the statute (see the Court’s summary at [265]).  Here I will argue that, on the contrary, it is the Court’s preferred “biological sex” interpretation of the EA 2010 that results in incoherence. I will then argue that to the extent that some sections of the statute must be read as referring to the biological characteristics of sex and gender, those sections indicate that sex and gendered terms needed to be assigned a variable meaning in the statute, depending on their context and purpose.

The Legal Question and the Court’s Interpretive Response   

I will refer to the words “sex”, “gender”, “man”, “woman”, “male” and “female” as “gendered terms”. The Supreme Court was asked to interpret the EA 2010 in order to determine whether references to gendered terms were used in a manner that included reference to the sex acquired by a trans person who has obtained a Gender Recognition Certificate (“GRC”) under the Gender Recognition Act 2004 (“GRA 2004”).

Section 9(1) of the GRA provides that where a gender recognition certificate is “issued to a person, the person’s gender becomes for all purposes the acquired gender (so that, if the acquired gender is the male gender the person’s sex becomes that of a man and, if it is the female gender the person’s sex becomes that of a woman)”. On the other hand, the appellants claimed, and the Court accepted, that to read references to “men” and “women” throughout the EA 2010 as referring inclusively to trans men and trans women would result in absurdity or incoherence. The judges highlighted a variety of provisions across the EA 2010 that, they thought, compelled them to adopt a “biological sex” interpretation of gendered terms.  For instance, the provisions protecting pregnancy and maternity (like sections 17 and 18), refer to “a woman” who is pregnant or who has given birth. If these sections were read as referring to the sex acquired under s9(1), this would prevent pregnant trans men from being protected from discrimination.

There is a well-established presumption that words have the same meaning throughout a piece of legislation [13]. This presumption is strengthened when a statutory definition is provided, as it was for the terms “man” and “woman” in s.212 of the EA 2010 [13]. (Section 212 provides that reference to a man “means male of any age” and a reference to a woman “means a female of any age”.) Accordingly, the Court concluded that a “biological sex” reading of gendered terms in some provisions of the EA 2010 required it to adopt the same reading for all provisions of the EA 2010. Section 9(3) of the GRA provides that the effect of section 9(1) is subject to provision made by “any other enactment”. The interpretation of EA2010 adopted by the Court led it to conclude that it was one such legislative exception to s.9(1) of the GRA.

The Incoherence of a “Biological Sex” Reading

According to the Court, the term “biological sex” describes “the sex of a person at birth” [7]. Practically, the Court uses “biological woman” to designate a group of women that excludes trans women, and “biological man” to designate a group of men that excludes trans men. The sex acquired by a trans person upon obtaining a GRC is referred to as “certificated sex” [7].

There is, however, an unfortunate and unresolved ambiguity in the Court’s claim that “biological sex” corresponds to sex at birth. First, the “sex of a person at birth” could be read as a reference to the sex at birth recorded on an individual’s birth certificate as a result of certain perceived biological characteristics.  I will adopt the Court’s language by referring to this understanding of sex at birth as a person’s “certificated sex at birth”. Throughout the judgment, the Court seems to take judicial notice of the fact that certificated sex at birth is associated with certain biological characteristics. Those who are certificated male at birth, for example, will possess, on average, advantages in certain kinds of competitive sport (cf. s. 195 EA 2010). Except perhaps in extremely rare cases involving intersex people (not considered by the Court), only those who are certificated as female at birth are able to become pregnant. According to the Court, these biological associations “are assumed to be self-explanatory and to require no further explanation” [171].  We do not need, however, to deny these biological associations to criticise the judgment.

On a related but distinct interpretation, “sex at birth” could be interpreted as referring to an immutable biological characteristic of individuals that can be determined at birth through an infallible medical or scientific process. An individual’s certificated sex at birth would track these biological criteria closely but there would be exceptions, particularly in the case of intersex people. Given that the Court was concerned with the ordinary meaning of gendered terms in the EA 2010, and did not accept any evidence from biologists or medical experts, it would be surprising if they rested their judgment on the highly contested idea that biological sex is binary, immutable, and determined no later than birth. Nonetheless, there are times in the judgment when the notion of “biological sex” invoked by the Court seems closer to this understanding. At one point the judges refer to “the biological characteristics that make an individual a man or a woman” [171]. For the purposes of discussion, I will refer to this second understanding of biological sex as a “criterial” approach to sex.

Irrespective of this ambiguity in the judgment, it needs to be stressed that on either understanding the univocal “biological sex” account of gendered terms favoured by the Court in FWS results in an incoherent and unworkable interpretation of the EA 2010. Here I will focus on three aspects of the judgment that highlight the incoherence and absurdity in the Court’s interpretation.

The Position of Intersex People

There is no mention in the judgment of intersex people. When the interests of intersex people are considered more fully, however, absurdities in the Court’s preferred interpretation are revealed. The Court argues that a “certificated sex interpretation would cut across the definition of the protected characteristic of sex in an incoherent way” [171]. But the position of intersex people demonstrates similar points of incoherence in their preferred “biological sex” interpretation.

Consider, to begin with, the position of intersex people whose gender identity does not match their “biological sex” (whether that is their certificated sex at birth or their criterial sex). As a result of this ruling, they will be afforded the same protections and be subject to the same exclusions as transgender people. But not all intersex people are transgender, and it seems very unlikely that in passing the EA 2010 parliament intended to allow for all intersex people to be permanently excluded from protection according to their chosen gender. Many individuals with a variety of intersex conditions decide that their gender identity does not match their certificated sex at birth. (The assignment of birth sex to individuals born with 5 -Reductase 2 deficiency, to provide one example, is notoriously difficult.) When seeking to have their birth certificates corrected, intersex people often seek to obtain a GRC. (In England and Wales, they have the option of having their birth certificate corrected as an “error” only if they can provide medical evidence that their “gonads, genitalia and chromosomes” are congruent and the opposite of the sex assigned at birth ). But under the Court’s interpretation those intersex individuals who obtain a GRC will have the same status, and be subject to the same permanent exclusions, as trans people.  It is one thing to offer both trans and intersex people the same legal route to recognition in their acquired gender. It is another thing entirely to condemn them both to the same zone of permanent non-recognition under equalities law.

The recognition of the separate interests of intersex people also places considerable pressure on the coherence of the Court’s argument that the various protections provided on the basis of sex can only be understood on the basis of “shared biology” [172]. Consider, to begin with, a criterial approach to “biological sex” (which brings out the incoherence in the Court’s approach most forcefully). On a plausible criterial approach, which focuses only on chromosomal and gonadal sex, a child with complete androgen insensitivity syndrome, who will usually be certificated as female at birth and raised as a girl, will be a “biological male” (since she has internal testes and  XY sex chromosomes).  She could therefore be excluded from facilities and services provided to other women. This means that a woman—raised as a woman since birth, with the same visible sex-characteristics as any woman, and in practice indistinguishable from any other woman—could be denied access to a women’s changing room because of her chromosomes or gametes.  If, on the other hand, she is offered access to these facilities and services on the basis of her certificated sex of birth, then we can legitimately ask on what rational basis the same access can be denied to a trans woman who has completed her gender reassignment. She too, we may suppose, has been socialised as a woman, has the same visible sex characteristics as any other woman, and is in practice indistinguishable from any other woman. On what rational basis would we deny her entry to facilities of her acquired gender while allowing it to another woman with the same biological characteristics?

Implications for “Same-Sex Orientation”

In what I found to be the most perplexing passages of the judgment, the Court develops a “biological sex” interpretation of the protections offered to people on the basis of “sexual orientation” under s 12 of the EA 2010 [205-209]. Section 12 defines sexual orientation in terms of a person’s “orientation towards” the “same”, “opposite”, or “either” sex. The Court held that references to “sex” in this section were references to “biological sex”.  The implications of this conclusion are very poorly theorised by the Court, who seem to have rather credulously taken the combined submission provided by the “lesbian interveners” as representing the views of a broad range of gay and lesbian people.

One implication of the ruling is to restrict the freedom of association of trans-inclusive gay and lesbian groups, who will no longer be afforded the protections offered to same-sex oriented people groups under the EA 2010. There will be other “absurd or irrational results” from the Court’s preferred interpretation (cf. [32]). Prima facie, a straight man who is oriented towards trans women and any straight woman who is oriented towards trans men can now join an exclusive association for gays or lesbians. They can benefit from the same “group-based rights” afforded to other gay and lesbian people [243]. They would, for example, be able to join a group or association designed to offer career opportunities to gay and lesbian people. It is hard not to see the results of this interpretation as having a similar “chilling effect” on the rights of association of same-sex orientation that the Court was determined to avoid [207].  

The judgment also results in a tension between legal and de facto sexual orientation. Although the landmark case of Bellinger v Bellinger [2003] UKHL 21 records Mr and Mrs Bellinger’s fight for the same recognition as any other heterosexual couple, the clear implication of the Supreme Court’s judgment is that anyone in their position will be regarded as a homosexual couple under the EA 2010. A lesbian woman who is in a relationship with a trans woman, who is understood to be a lesbian by her friends, has been told by the Supreme Court of the United Kingdom that she is not “legally lesbian” for the purposes of the EA 2010 but rather “legally bisexual”. Prima facie, she can be excluded from a lesbian-only association.

Once again, the implications for intersex individuals are not even considered by the Court—is a lesbian woman with 5 -Reductase 2 deficiency, for example, to be treated as a lesbian for the purposes of the EA 2010? Does her status as a “legal lesbian” depend entirely on her birth certificate?  The Court’s rather dismissive assertion that “people are not attracted to a certificate” can be generalised to demonstrate the incoherence of their own interpretation [204]. People are not attracted to birth certificates either, nor are they attracted to gametes or chromosomes.  

What is most perplexing about these passages is not so much the direct implications of the Court’s interpretation as the fact that the Court felt obliged to take such a rigid view on the matter of sexual orientation in the first place. The phenomenon identified by “sex orientation” may well have biological associations (most plausibly, people of a certain “sex orientation” might be thought to be oriented towards primary and secondary sex characteristics), but it is not associated exclusively with the features that determine an individual’s “biological sex”. Sexual orientation, as it is commonly understood, is at least a weakly social phenomenon. In practice it is usually determined by something that resembles self-identification.  The “biological sex” interpretation of same-sex orientation pressed by the Court implies a legislative intention to strictly regulate what counts as same-sex orientation for the purposes of the statute. It is highly implausible to think that it was Parliament’s intention to force a biological straitjacket on sexual orientation in this way.

When I first read the judgment, I took the Court’s claims about sexual orientation to be misguided attempts to buttress the coherence of the interpretation that the Court was already legally compelled to favour. But I have come to the view that they are essential to the judgment. If the judges formed the view that the “biological” reading of same sex attraction was less coherent than an alternative reading that allowed for the criteria for same-sex attraction to be (wholly or partly) socially determined, then this would provide evidence against the presumption in favour of a unified “biological sex” meaning throughout the EA 2010, and in favour of an interpretation that assigned a variable, non-“biological” meaning to the term “sex” in s 12.

The Court’s Interpretation of “Gender Reassignment”

Finally, it is important to note that interpreting gendered terms as having a contextually insensitive meaning that refers in all cases to the “biological sex” of a person, would, in particular, result in an absurd interpretation of the provisions of the EA 2010 dealing with “gender reassignment”. Section 7 reads that “a person has the protected characteristic of gender reassignment if the person is proposing to undergo, is undergoing, or has undergone a process (or part of a process) for the purpose of reassigning the person’s sex”. As I have argued in more detail elsewhere, this passage of the provision clearly and unambiguously speaks of sex as something that is capable of being “reassigned”. But, if “sex” is throughout the act a reference to “biological sex” determined “at birth”, the procedure of reassignment contemplated by s 7 would be oxymoronic and absurd. Sex would, quite simply, not be capable of reassignment. The interpretation of the provision offered by the Court involves, in essence, assignment of a variable meaning to “sex” in section 7, according to which the sex which is being reassigned is not the person’s “legal sex” [200]. But this amounts to a concession that the terms “sex” and “gender” do not have a uniform meaning across the statute, in violation of the Court’s own interpretive principles.

Rebutting the Presumption of a Unified Meaning

I am not at all convinced that the “certificated sex” reading of the EA 2010 favoured by the Inner and Outer Houses of the Court of Session was unworkable. But for the sake of argument, I will work on the assumption that at least some sections—especially the pregnancy and maternity provisions and the provisions dealing with discrimination in sport (section 195)—require us to interpret the gendered terms in those sections with reference to biological characteristics.

The Court placed great weight on the well-established presumption that words have the same meaning across a statute, which is strengthened when a term is defined in the statute [13]. But the presumption is rebuttable, and the weight to be placed on it will vary depending on the contexts in which the word or phrase appears, as the Court conceded [13], [176]. In the case of the EA 2010, I think the presumption that sex and gendered terms have the same “uniform meaning” is rebutted by a variety of pieces of textual evidence. First, gendered terms demonstrate a latent and unresolved ambiguity. The Court correctly declined to apply the presumption that the different words “sex” and “gender” had different meanings throughout the Act, noting that they were used interchangeably “across the statute book” [97]. Noting the ambiguities that inhere in a system of expression that treats “sex” (which as a matter of ordinary meaning has biological connotations) and “gender” (which does not) as synonyms, it is unclear why we should place much weight on the correlative presumption that those words have a uniform meaning throughout the EA 2010.

Second, it is worth noting that even on the Court’s interpretation the definitions of “man” and “woman” offered in s 212 are circular, referring only to “males of any age” and “females of any age”. Since the pairs “man” and “male” and “woman” and “female” are both synonymous in law, the definitions do little to fix the meaning of sex and gendered terms across various sections of the EA. Section 212 should be read as saying no more than that “a man means a man of any age” and “a woman means a woman of any age”. The Court’s argument that the definition would have to be read as “saving for context” to allow for these terms to have a context-sensitive application is facile (cf. [176]). If “man” and “male” both have the same context-sensitive meaning, then definition of one in terms of the other will not eliminate the variability in their reference. Imagine that a statute dealing with the rights and responsibilities of parents across a variety of diverse areas of social importance defined a “parent” as a “mother or father of any age”. This would tell us little other than that the term can be used to refer to parents of any age, including children. It would not provide good evidence that the word “parent” had the same meaning in a provision dealing with the rights of adoptive parents as it did in a provision dealing with the rights of egg and sperm donors.

Third, and I think most importantly, we have already seen that the Court’s preferred interpretation already requires it to assign a variable, non-biological meaning to “sex” in s 7 of the EA 2010. The need to avoid this and other absurdities in the “biological sex” reading leads me to the conclusion that the meaning of sex and other gendered terms in the EA 2010 must demonstrate a kind of context-sensitivity across the statute.  As Lord Nicholls noted, well over twenty years ago, in Bellinger v Bellinger, “the distinction between male and female is material in widely differing contexts. The criteria appropriate for recognising self-perceived gender in one context…may not be appropriate in another” ([2003] UKHL 21, [32]). If biology is relevant at all to the interpretation of the Act, then the interpretive problems created by the assumption of a single “biological” meaning of gendered terms vindicate treating their meaning as context sensitive. The different gendered terms in the EA 2010 draw their meanings from the context of their use.

Conclusion
It is true, as the Court notes, that statutory interpretation should strive for a situation in which “citizens, with the assistance of their advisers” are “able to understand parliamentary enactments” ([192], quoting Lord Nicholls in Spath Holme [2001] 2 AC 349, at 397).  But ordinary people understand context very well; context-sensitivity is a basic feature of linguistic expression. We should not suppose that the idea that a trans man might count as a woman for the purposes of a provision that protects his rights during pregnancy is beyond the grasp of ordinary people.  On the other hand, quite a few citizens would be surprised to learn that as recently as 2010 Parliament had in effect classified some of their gay and lesbian friends as “bisexual” for the purposes of a statute. The assignment of a rigid “biological” meaning to gendered terms results in a distorted and incoherent interpretation of the EA 2010—one that violates the Court’s own interpretive principles.       

Dr Robert Mullins is Senior Lecturer in Law, TC Beirne School of Law, University of Queensland.

The author would like to thank Paolo Sandro for his insightful comments on an earlier version of this blog post, and for suggesting the title.

(Suggested citation: R. Mullins, ‘For Women Scotland: Fastening the “Biological” Straitjacket ’, U.K. Const. L. Blog (22nd May 2025) (available at https://ukconstitutionallaw.org/))