Introduction to the Legal Debate
At birth, sex is typically recorded as an observable fact, seemingly straightforward, biologically determined, and legally uncontroversial. Yet in a striking turn, this foundational classification has become the subject of constitutional scrutiny, culminating in a landmark 2025 decision by the UK Supreme Court. Originating in the Scottish courts, the case has reignited discourse surrounding the legal definition of sex and its implications for women’s rights (see paragraph 94 of the judgment). This piece focuses on the 2025 Supreme Court judgment, contextualised by a brief overview of the preceding legal developments. It is then followed by discussion of the Supreme Court’s two distinct ways of understanding “sex” in UK law. One perspective is a strictly biological approach (which might be termed traditional); the other (which might be termed progressive) relies on legal status changes acknowledged by psycho-social construction determined by a “gender recognition” process, as outlined in paragraph 6, page 3. These perspectives give rise to fundamentally different understandings of legal rights under the Equality Act 2010, as succinctly captured at page 57, paragraph 191 of the judgment.
Background: from the Scottish Courts to the UK Supreme Court
Before the landmark 2025 UK Supreme Court judgment, the case followed a trajectory through the Scottish courts spanning over four years. The case was originally about female representation on public sector boards, aimed at achieving gender parity on such boards with the provision that 50% of non-executive members be women. Section 2 included transgender women in the quotas as part of the statutory definition of “woman” for the purposes of the 2018 Act. The campaign group known as For Women Scotland (‘FWS’) demanded clarity, arguing that including transgender women in the statutory 50% female boardroom quotas could still allow male dominance, thereby undermining the legislation’s goal of achieving genuine gender balance. They therefore challenged the Scottish Government’s interpretation of theGender Representation on Public Boards (Scotland) Act 2018.
The case was heard in March 2021, where the court, in paragraphs 66-67 of the judgment, dismissed the challenge by FWS, finding that the 2018 Act did not introduce a new policy but continued existing efforts to promote equal representation on public boards. The 2018 Act was held to be within the Scottish Parliament’s competence, and no breach of equality duties was found. On appeal by FWS, the ruling of Lady Dorrian in the Inner House of the Court of Session in March 2022 was that this was a matter reserved to the UK Parliament because the 2018 Act encroached on protected characteristics (of trans women) in the Equality Act 2010. This meant that the Scottish Parliament could not modify the protected characteristics in the Equality Act 2010, regardless of the intention (that is, gender representation), and that the definition of “woman” in Sect 2 of the 2018 Act was therefore outwith the legislative competence of the Scottish Parliament (see page 23 paragraph 40).
Following this, in April 2022, the Scottish Ministers replied by issuing a revised statutory Guidance aligning the definition of “woman” for the purposes of the 2018 Act with that in the Equality Act 2010, clarifying that individuals with a Gender Recognition Certificate (‘GRC’) would be legally recognised in their acquired gender according to Section 9(1) of the Gender Recognition Act 2004. FWS was dissatisfied with the Scottish government’s revision of the statutory guidance. They then challenged the Scottish Government for the second time, based on the revised Guidance, noting in the petition that the revised Guidance was a misrepresentation of the substance of the Inner House March 2022 judgment.
In this second FWS challenge, the opinion of Lady Haldane at the Outer House of the Court of Session on 13th December 2022 was that under the Equality Act 2010, “sex” includes individuals with a GRC stating their acquired gender. Thus, a person with a GRC indicating they are female falls within the 2010 Act’s definition of “woman.” She rejected the argument that “woman” must mean only a biological female, noting that the statutory meaning depends on context. It was also adduced that while some legislation may explicitly refer to biological sex, this does not dictate a uniform interpretation across all legislation (see para 53). In essence, the Scottish Government guidance was held to be lawful.
FWS appealed this ruling to the Inner House, and Lady Dorrian, on the 1st November 2023, provided the court’s opinion, which held that the Scottish Government Guidance distinguishes between two protected characteristics under the Equality Act 2010: gender reassignment (section 7) and sex (section 11). A person with GRC has gender reassignment protection and is legally recognised as their acquired sex under section 11. Those without a GRC retain their birth sex for section 11 purposes. Thus, a person with a GRC identifying as female is lawfully considered a “woman” under section 11 of the 2010 Act, making the Guidance for the 2018 Act lawful. The FWS appeal was therefore refused (see para 65).
FWS brought judicial review proceedings arguing that the term “woman” under the Equality Act 2010 should be limited to biological females. They appealed to the UK Supreme Court seeking to overturn the Inner House’s 2023 interpretation, which they argued conflicted with previous understandings of how the Equality Act treats sex and gender reassignment for those with GRC. Their goal was to secure a final ruling on this complex and contentious issue. On 16 April 2025, the Supreme Court delivered its landmark judgment in For Women Scotland Ltd v The Scottish Ministers [2025] UKSC 16, representing a defining moment in UK constitutional and equality law.
The UKSC 2025 Judgment: “Sex and Gender”
In a unanimous judgment (paras 266–267), the UK Supreme Court held that the Scottish Government’s Guidance about the 2018 Act was incorrect in treating trans women with GRC as “women” for the purposes of sex discrimination under sections 11 and 212 of the Equality Act 2010. The Court ruled that, for the Gender Representation on Public Boards (Scotland) Act 2018, which must align with the Equality Act, the term “woman” refers exclusively to biological women. As such, appointing a trans woman with a GRC to a Public Board does not count toward the statutory 50% non-executive female representation target. While the judgment emphasised the value of trans inclusion on Public Boards, it clarified that legal definitions under the Equality Act 2010 must be based on biological sex.
The ruling highlights two distinct legal perspectives central to the case: biological sex as defined under the Equality Act 2010, and legal gender recognition through GRC. We now examine each in more detail.
The Biological Literal Perspective: This perspective adopts a biology-based interpretation of “sex,” as referenced in para 7 of the judgment. It defines sex by immutable physical and genetic traits, aligning with sections 11 and 12 of the Equality Act 2010 and reinforced by paragraph 204. Such a reading preserves longstanding legal protections rooted in biological distinctions, especially in sensitive areas like pregnancy and maternity (para 178) and women-only services (para 190). For instance, single-sex hostels, medical screenings, and rape counselling services are typically designed around biological realities (see para 211). A biologically grounded definition of sex ensures that such services preserve dignity, safety, and autonomy for their intended beneficiaries, thereby preventing sex-based discrimination (see paras. 211–212). It also upholds coherence across key areas of equality law, including equal pay, harassment, and direct sex discrimination, where the effectiveness of legal comparators relies on categorisation by biological sex (see paras 39, 113, 121, and 149).
By contrast, adopting a broader interpretation that includes transgender women with a GRC risks complicating the provision of single-sex services (paras. 8, 208). Service providers may be required to navigate conflicting rights and expectations, potentially undermining privacy, dignity, and comfort in settings such as communal accommodation, women’s healthcare, and competitive sport (paras. 222–225, 232). As the judgment observes, many women reasonably expect such services to be underpinned by shared biological experiences.
While a GRC has legal significance in some contexts, the Court held that it cannot redefine “sex” for the purposes of the Equality Act 2010(para 268). Retaining a biologically grounded definition promotes legal certainty and ensures that the intended beneficiaries of specific protections, particularly biological women, are not inadvertently excluded or disadvantaged by evolving interpretations (para 175). This interpretation now sets a clear precedent for how gender equality policies are to be framed: with sex understood as a fixed, legally coherent category, not one subject to redefinition based on identity or documentation (para 22).
The Legal Recognition Flexible Perspective: This interpretation recognises legal sex as a changeable status, whereby individuals who obtain a GRC under the Gender Recognition Act 2004 are legally recognised in their acquired sex for all purposes (paras 6–7, 157). In line with section 13(1) of the Equality Act 2010 and section 22 of the GRA 2004, this view supports the evolving dignity and rights of trans individuals, situating legal identity within an administrative framework of gender reassignment distinct from fixed biological classification (paras 243–244).
It reflects a broader understanding of sex that adapts to gender identity through legal certification, enabling trans individuals to be treated as their affirmed gender in specific legal contexts. This affirms the law’s capacity to accommodate identity-based changes and recognise trans persons within anti-discrimination frameworks. However, the judgment warns that using a GRC to redefine “sex” may lead to legal and practical inconsistencies (paras 225, 229). For example, including trans women with GRCs (who are biologically male) under “woman” might dilute intended protections for biological women in contexts such as maternity services, sanitary facilities (para 224), or employment decisions involving sex-based perceptions (para 251).
Further challenges arise in data collection, positive action, sports, and single-sex services, where clear group definitions are crucial. Public bodies under the Public Sector Equality Duty (‘PSED’) require sex-disaggregated data to identify underrepresentation and allocate support effectively. Redefining “woman” based on legal status rather than biology risks undermining data accuracy and the consistency of protections for historically disadvantaged groups (paras 230, 232, 237–242, 249–253). This perspective thus raises complex policy questions: how can equality goals be achieved if the category of “woman” becomes fluid? And how can biological disadvantage be addressed if legal recognition obscures the foundational biological distinctions underlying these protections?
Striking a Delicate Legal Balance
In sum, the two perspectives presented in the judgment transcend theory and profoundly influence how equality law is interpreted and applied in practice (para 228). On the one hand, sex as a biological reality provides a stable, objective foundation for safeguarding group-specific protections, particularly those designed to protect women from discrimination in areas such as maternity, equal pay, and single-sex services. On the other hand, the recognition of legal gender through a GRC affirms the rights and dignity of trans individuals within modern legal frameworks (para 182). The Court’s reasoning reveals an ongoing tension between these imperatives: maintaining the original legislative intent of sex-based protections, rooted in physical and reproductive differences, while also acknowledging the evolving understanding of gender identity and its legal recognition (paras. 194, 222). Since the mid-1970s, UK legislation has sought to redress structural inequalities faced by women based on sex. More recently, legal reforms have extended protections to trans individuals, recognising their particular vulnerability to discrimination and social exclusion (para 1).
However, the Court expressed concern that expanding the legal definition of “woman” to include trans women with GRCs could inadvertently erode the clarity and efficacy of protections designed specifically for biological women. This caution reflects a judicial effort to safeguard the integrity of long-standing rights without negating legal advancements made for trans communities. This tension is obvious in the context of equal pay. Paragraphs 149 and 150 illustrate how employment protections, such as those related to maternity or pregnancy, were crafted to address specific biological disadvantages. Including individuals without such characteristics solely based on legal status risks blurring the comparator analysis that the law was intended to support. For example, comparing pay between a biological woman and a trans woman, who lacks the physical capacity for pregnancy, may dilute the focus on systemic sex-based disparities in the workplace. From the biological perspective, these risks undermine the protection’s original purpose; from the legal recognition perspective, it affirms inclusion but complicates the assessment of “like work” under equal pay law (paras 243, 249-250).
Ultimately, the judgment reflects a carefully calibrated attempt to balance fairness and clarity. It stresses the importance of recognising both biological distinctions and gender diversity, without allowing one to inadvertently override the legal protections of the other. The Court’s decision arguably embodies a pragmatic approach, upholding legal certainty and safeguarding historically disadvantaged groups, while leaving room for evolving rights in a pluralistic society.
Notable Tensions and Legal Impact
To draw a simple analogy, this legal debate resembles holding a formal certificate stating one belongs to a particular category, even though the body was classified differently at birth (para 179). For example, a person assigned male at birth who now identifies and is legally recognised as female may possess a GRC affirming this status, despite unchanged biological traits (para 244). This distinction between legal status and biological characteristics resonates across employment, healthcare, education, and sports sectors. The biological perspective offers clarity where physical and medical needs are paramount. In contrast, the legal recognition perspective is essential for affirming identity and protecting against identity-based discrimination (paras. 75, 97). This balancing act is especially evident under the PSED, where policymakers must decide whether data collection should rely on legal sex or biological sex. As the Court observed, shifting definitions risk fragmenting the cohesion of protected groups, complicating efforts to identify and address shared disadvantages (para 237). Practically, this is akin to attempting to solve a problem without clearly knowing which group is affected. Broadening the definition of “woman” risks blurring the distinction between those with biologically specific needs, such as pregnancy, and those who may not share those embodied experiences.
This case exposes deeper socio-legal dynamics, raising fundamental questions about who defines identity, whose interests are prioritised, and how legal systems manage contested categories. The decision reaffirms a biologically fixed interpretation of sex under the Equality Act 2010, and was praised by some for preserving legislative clarity and competence (para 142, balanced against 243). However, it has also been sharply criticised by others as a setback for trans rights and an obstacle to gender-inclusive reform. Media portrayals have further polarised the discourse, positioning the case as emblematic of broader cultural and political divides.
Furthermore, this ruling must also be understood in the context of international human rights obligations, particularly under the European Convention on Human Rights (ECHR). Articles 8 (respect for private life), 14 (prohibition of discrimination), and 10 (freedom of expression) continue to bind the UK and inform domestic legal interpretation. In addition, the Yogyakarta Principles, while not legally binding, offer influential normative guidance on gender identity and legal recognition. They affirm that gender identity is integral to human dignity and should be respected through self-identification. In contrast to the Supreme Court’s ruling, the Yogyakarta framework, especially Principles 3 and 31, advocates for legal recognition beyond binary, biologically determined categories. Notably, the UN Special Rapporteur’s statement endorsing the Supreme Court’s interpretation of biological sex does not reference the Yogyakarta Principles and arguably adopts a position that diverges from their inclusive and identity-affirming framework.
Concluding Remarks
In conclusion, the UK Supreme Court has clarified the legal meaning of “woman” within the context of the 2018 Act, but it has not resolved the deeper conceptual or normative debates surrounding sex and gender in UK law. The Court’s decision draws a clear distinction between biological sex and gender identity, affirming that, for sex discrimination under the Equality Act 2010, the term “woman” refers exclusively to biological females, even where a GRC has been issued (see para 267). This judgment is best understood not as a terminus but as a step in an ongoing constitutional, statutory, and societal dialogue. While some may regard it as a reaffirmation of the status quo, the broader implications reveal enduring fractures in how sex and gender are understood and protected in law. These fault lines may yet deepen, and new challenges are likely to emerge. For instance, a challenge is already indicated before the European Court of Human Rights following the Supreme Court’s ruling.
As this tale of two legal perspectives continues to unfold, the judgment invites sustained reflection on how the law might balance unity and diversity, upholding rights without reinforcing division. The Supreme Court’s ruling stands both as a guardian of constitutional order and as a harbinger of the difficult conversations still ahead.
Suggested citation: Odusanya, T and Ilesanmi, F. (2025) The UK Supreme Court on Biological Sex: A Tale of Two Legal Perspectives, SLSS Research Blog (RGU), 2025/05. Available at: https://rgu-slss.blog/?p=2361
