Sex has always meant biological sex

The Supreme Court delivers a seismic ruling on equality law

The Supreme Court of the United Kingdom has delivered judgment in For Women Scotland v The Scottish Ministers [2025] UKSC 16. In a testament to simplicity on the far side of complexity, the Court has cast a piercing light of clarity through a body of law characterised by extraordinary misunderstanding and misinformation.

The headline takeaway: references to sex in the Equality Act are references to biological sex; references to men in the Equality Act are references to biological males; references to women in the Equality Act are references to biological females.

The Supreme Court has ruled that our anti-discrimination law has always understood sex to be biologically determined, from as far back at 1970 when the first equal pay protections for women were introduced.

Nothing has changed that. The introduction of anti-discrimination protections for transgender people did not change that; the enactment of the Gender Recognition Act 2004 did not change that; and the consolidation of our anti-discrimination law into the Equality Act 2010 did not change that. Woman has always meant biological woman. Man has always meant biological man. Sex has always meant biological sex.

That’s the simplicity part. The complexity through which this simplicity traversed was 55 years of anti-discrimination law, complex arguments of statutory interpretation, and a knot of legal arguments presented by the Scottish Government to defend its claim that sex in the Equality Act 2010 is, properly understood, determined by certificates, not biology.

In this post, I hope to unpack the reasoning of the Supreme Court in this case. As with much of my writing on this site, I will try to make this as accessible as possible to non-lawyers, given the extensive public interest in issues relating to sex and gender identity. Some of this is going to be complex law, however, so please feel free to comment below with any questions you have after reading this. I will try to do another post in the future responding to those questions.

Biological Sex

The central question before the Supreme Court was whether the terms “sex”, “man”, and “woman” take on their default meaning within our law, tied to biological sex, or whether they take on a certificated meaning determined by state-issued documentation. Those were the only two options. It was never an option for our law to be based on self-identification.

The Court began its judgment by noting that the expression “biological sex” is used widely to describe the sex of a person at birth [7]. The Court is not taking on a special or technical meaning of the expression but is rather relying on the ordinary meaning of the expression, as it would do as the default in statutory interpretation generally.

In response to this judgment, some academic commentators have critiqued the Supreme Court for not providing a definition of the term “biological sex”. In my view, this criticism is misguided as there is a well established and well defined meaning for biological sex in our law, based on biological facts.

In the seminal case of Corbett v Corbett [1971] p 83, cited by the Supreme Court at [54], the High Court concluded that as far as the common law is concerned, sex is biological, immutable, and fixed at birth at the latest. Determination of what sex an individual is, where it is contested, is a matter of fact to be determined based on the evidence presented, as it was in Corbett. In that case, a male-to-female transgender person argued that medical intervention including genital surgery changes someone’s sex. The High Court rejected this contention, concluding that biological sex can be determined by evidence relating to chromosomes, gonads, and external genitalia. It rejected the contention that hormone levels or psychological factors including what we would now call a gender identity were relevant for determining an individual’s sex.

Obviously, in the vast majority of cases chromosomes, gonads, and external genitalia align as unequivocal evidence of an individual being of either the male sex or the female sex. Even more obviously, the overwhelming majority of transgender people are unequivocally male or female. Having a DSD is not a precondition for having a transgender identity and the law is clear about how to deal with those who do have DSDs: it regards determination of their sex as a matter of evidence to be proven in court where it is contested.

The decision in Corbett has been approved on several occasions over the past five decades. It is unequivocal at this point that our law has a clear and established meaning for the expression “biological sex”, one which the Supreme Court in For Women Scotland felt no need to re-hash for the benefit of academics or campaigners unfamiliar with the last 50 years of law in this area. As the Supreme Court stated at [171]:

The definition of sex in the EA 2010 makes clear that the concept of sex is binary, a person is either a woman or a man. Persons who share that protected characteristic for the purposes of the group-based rights and protections are persons of the same sex and provisions that refer to protection for women necessarily exclude men. Although the word “biological” does not appear in this definition, the ordinary meaning of those plain and unambiguous words corresponds with the biological characteristics that make an individual a man or a woman. These are assumed to be self-explanatory and to require no further explanation. Men and women are on the face of the definition only differentiated as a grouping by the biology they share with their group.

Some may disagree with the fact that our law is both based on biological sex and informed by a detailed consideration of the factors which may (and may not) be relied on as evidence of biological sex. That disagreement should not be presented in the guise of criticism of the Court for not defining its terms. It didn’t need to because the meaning of the expression “biological sex” is well established in our law and correlates to the ordinary meaning that all adults and most children can understand.

The Supreme Court, therefore, concluded [209] that:

sex has its biological meaning throughout this legislation: “woman” always and only means a biological female of any age in section 212(1). It follows that a biological male of any age cannot fall within this definition; and “woman” does not mean or sometimes mean or include a male of any age who holds a GRC or exclude a female of any age who holds a GRC. To reach any other conclusion would turn the foundational definition of sex on its head and diminish the protection available to individuals and groups against discrimination on the grounds of sex.

Gender Recognition Certificates

The legal position in the aftermath of Corbett was that sex in our law was determined as a matter of biological fact and that this was immutable and fixed at birth, at the latest. This is the baseline legal position for all individuals, regardless of gender identity or medical intervention. The only way in which this can be changed is via acquiring a Gender Recognition Certificate (GRC) per the requirements set out in the Gender Recognition Act 2004. It was settled law in this case, uncontested by any side, that without a GRC, an individual’s sex can only be their biological sex [26].

To obtain a GRC, one needs to have been diagnosed with gender dysphoria, have lived in the “acquired gender” for two years and intend to live in the acquired gender until death. Where medical treatment has occurred, this must be detailed. The legal requirement to provide details of medical treatment, where it has occurred, before being issued with a GRC is compatible with the human rights of transgender people (Carpenter v Secretary of State for Justice [2015] EWHC 464 (Admin), [2015] 1 WLR 4111). However, while any treatment undergone must be detailed, it is not required. There is no requirement to modify physical appearance to “pass” as a member of the target sex [88].

As far as the requirement to live in the “acquired gender” is concerned, the Supreme Court was clear at [90] that “There is no guidance as to what it means to live in a gender, other than to ensure that the person’s name in certain documents is a name in the acquired gender.” There is no requirement to present as a member of the opposite sex; no requirement to dress a certain way or to alter physical appearance in any way. The core means by which the law determines that someone has lived in the “acquired gender” is by updated gender/sex markers on documentation (AB v Gender Recognition Panel [2024] EWHC 1456 (Fam)).

This means that those with GRCs can, as a matter of law, be visually indistinguishable from other members of their biological sex. That must be accounted for when determining the legal effect of obtaining a GRC on the rights of others.

The GRA operates on a binary concept of legal sex: it is not possible to be issued with a “non-binary” certificate (R (Castellucci) v Gender Recognition Panel [2024] EWHC 54 (Admin), [2024] KB 995).

The question to be decided in this case was therefore whether acquiring a GRC changes an individual’s legally recognised sex for the purposes of the Equality Act 2010 (EA). The EA is a profoundly important statute which sets the baseline for our law in terms of anti-discrimination protections as well as the provision of single-sex services, single-sex associations, schools, sport, associations, and so on. It has always been the case that the law in this area does not operate on a Self-Identification basis.

The clarification provided in this case pertains to those who possess GRCs. However, in clarifying that a GRC is irrelevant of the purposes of the EA, the Supreme Court took the opportunity to provide an extensive breakdown of the law in this area. It did not need to do that and it should be commended for fulfilling its constitutional role so effectively. Other institutions tasked with clarifying and informing the public on the law should see this as a shining example of best practice.

The precise interaction between the complex rules of statutory interpretation that informed the Court’s reasoning is something I will address in a longer form academic publication. The key take away on this point is that the test applied by the Supreme Court was as follows [160]:

If the EA 2010 can only be read coherently to mean biological sex, the same result must follow. The question that must therefore be answered is whether there are provisions in the EA 2010 that indicate that the biological meaning of sex is plainly intended and/or that a “certificated sex” meaning renders these provisions incoherent or as giving rise to absurdity. An interpretation that produces unworkable, impractical, anomalous or illogical results is unlikely to have been intended by the legislature.

The Supreme Court concluded that a biological meaning of sex is plainly what was intended by Parliament when it passed the Equality Act in 2010, partially because [173]:

it makes no sense for conduct under the EA 2010 in relation to sex based rights and protections to be regulated on a practical day-to-day basis by reference to categories that can only be ascertained by knowledge of who possesses a (confidential) certificate.

For this post, I want to focus on the key take-aways and practical implications of this decision, the most significant of which is that GRCs are irrelevant to the EA. Transgender people are not placed into a hierarchy of legal status under our equality law dependent on whether or not one has a confidential government certificate.

In coming to this conclusion, the Supreme Court also concluded that Explanatory Notes which indicated a contrary legal effect were not compatible with the meaning of both the GRA and the EA as enacted by Parliament [104], noting at [81] that

In our view, this is a good illustration of why the use to which the courts should put explanatory notes is limited to the context of the legislation and the mischief to which its provisions are aimed.

The fact that the GRA does not apply to the EA does not mean that GRCs are now meaningless, however. The Supreme Court was clear that a GRC may apply in other areas of law and that it is not solely concerned with the relationship between the individual and the state. It is concerned with relationships between private parties as well [100]. The most obvious area where a GRC will apply to relationships between private parties is marriage. Effectively, a GRC now has the legal effect that many campaigning for gender recognition reform claimed it has when challenged on the potential impact on the rights of women: it is primarily a confidential document which affirms the gender identity of the individual it pertains to and permits minor administrative changes to records of birth, marriage, and death. That is not trivial, but it is not all-encompassing either.

Separate Protection

The key take-away from this judgment as it pertains to the equality rights of transgender people is that anti-discrimination law protects both biological sex and gender reassignment. Where the law references sex, it means biological sex. Gender reassignment protections will cover a person who, according to s.7 EA, “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 by changing physiological or other attributes of sex”.

The Supreme Court was clear at [200] that this definition relies on a biological understanding of sex:

The critical process on which the section 7 characteristic depends involves a change in physiological or other attributes of what must necessarily be biological sex; but there is nothing to suggest that undergoing such a process changes a person’s sex as a matter of law. It does not. Indeed, a full process of medical transition to the opposite gender without obtaining a GRC has no effect on the person’s sex as a matter of law.

In my view, references to “other attributes” of sex must be to be social or cultural factors which are attributed to sex by society, rather than aspects of biological sex which are inherent but not physiological.

Transgender people can face discrimination on the basis of their biological sex, or on the basis of gender reassignment. A GRC is not relevant for determining whether this discrimination has occurred. For example, “trans men” who are biologically female but identify as men could be discriminated against because of their female sex or because of gender reassignment. This ruling affirms that both protections exist separately in our law, without any differentiation based on GRC status [202]:

Since, as we have explained above, neither possession of a GRC nor the protected characteristic of gender reassignment require any physiological change or even any change in outward appearance, there is no obvious outward means of distinguishing between a person with the protected characteristic of gender reassignment who has a GRC and a person with that characteristic who does not. … in either case, the individual’s biological sex may continue to be readily perceivable and may form the basis of unlawful discrimination. A person has the protected characteristic of gender reassignment as soon as they propose to undergo the process so it may be that at that stage there is no change in outward appearance.

Discrimination by Perception and Association

While there is no requirement for a transgender person to “pass” as a member of the opposite sex in order for them to be protected under gender reassignment, the Supreme Court has clarified in this case that associative and perception based discrimination may still apply.

Discrimination by association occurs where someone is discriminated against because they are associated with a protected characteristic. The classic example is where the parent of a disabled child is discriminated against because they have a disabled child. This is disability discrimination, even though the parent is not themselves disabled (Coleman v Attridge Law [2008] ICR 1128).

Discrimination by perception occurs where someone is discriminated against because they are perceived to have a protected characteristic, regardless of whether they do or don’t. The classic example is where a man who is perceived to be gay when he is not is discriminated against anyway. This is sexual orientation discrimination regardless of the sexual orientation of the claimant (English v Thomas Sanderson Blinds Ltd [2009] ICR 543. See also; Chief Constable of Norfolk Constabulary v Coffey [2020] ICR 145 where discrimination by perception occurred on the basis of disability).

In the context of transgender people, this means that, even though sex means biological sex in the EA, if that person is perceived to be of the opposite biological sex or associated with the opposite biological sex, they can bring a sex discrimination claim. The Supreme Court was clear at [250], however, that this does not mean that the transgender claimant is bringing a claim as a member of the opposite sex: they remain their biological sex”.

Applied in the context of a discrimination claim made by a trans woman (a biological male with or without a GRC), the claimant can claim sex discrimination because she is perceived as a woman and can compare her treatment with that of a person not perceived to be a woman (whether that is a biological male or a trans man perceived to be male). There is no need for her to declare her true biological sex. There is nothing disadvantageous about this approach. Neither a biological woman nor a trans woman “bring a claim of direct sex discrimination as a woman” (as the EHRC suggests). That is not how the EA 2010 operates: a person brings a claim alleging sex discrimination because of a protected characteristic of sex.

Because of these two forms of discrimination, there is no need to rely on a GRC to be covered from discrimination which arises when a trans person is discriminated against because they are perceived to be or are associated with the sex that they identify with. As such, a GRC is simply not necessary to achieve any relevant equality purpose as far as the EA is concerned [253]:

It follows that a certificated sex reading of sex in the EA 2010 is not necessary to achieve the purposes of either the GRA 2004 or the EA 2010 as regards protection from direct discrimination. A man who identifies as a woman who is treated less favourably because of the protected characteristic of gender reassignment, will be able to claim on that basis. A man who identifies as a woman who is treated less favourably not because of being trans (the protected characteristic of gender reassignment), but because of being perceived as being a woman, will be able to claim for direct sex discrimination on that basis. This does not entail any practical disadvantage.

One important implication of this is that a biological meaning of sex in the EA will avoid the irrational situation of a transgender person who does not “pass” at all being able to rely on sex-based rights in the EA reserved to those of the opposite sex.

Single-sex services

The Supreme Court, in concluding that sex must mean biological sex for the purposes of the EA, explained why the provisions permitting the establishment of single-sex services exist in the first place [211]:

In enacting these exemptions, the intention must have been to allow for the exclusion of those with the protected characteristic of gender reassignment, regardless of the possession of a GRC, in order to maintain the provision of single or separate services for women and men as distinct groups in appropriate circumstances. These provisions are directed at maintaining the availability of separate or single spaces or services for women (or men) as a group – for example changing rooms, homeless hostels, segregated swimming areas (that might be essential for religious reasons or desirable for the protection of a woman’s safety, or the autonomy or privacy and dignity of the two sexes) or medical or counselling services provided only to women (or men) for example cervical cancer screening for women or prostate cancer screening for men, or counselling for women only as victims of rape or domestic violence.

It is clear from this that the Supreme Court understands these provisions to operate on a policy-by-policy basis. This is important because for years people have argued that the rule that single-sex services are only permitted where they are a proportionate means of achieving a legitimate aim requires case-by-case analysis at the level of each individual service user so that every separate exclusion of a male person from a female-only service must be justified as proportionate. That cannot be correct. The Supreme Court has already previously made it clear that proportionality requirements do not require such analysis and that policies can be held to be proportionate at the general level. As the Court said in Reference by the Attorney General for Northern Ireland - Abortion Services (Safe Access Zones) (Northern Ireland) Bill [2022] UKSC 32:

“questions of proportionality, particularly when they concern the compatibility of a rule or policy with Convention rights, are often decided as a matter of general principle, rather than on an evaluation of the circumstances of each individual case. Domestic examples include R (Baiai) v Secretary of State for the Home Department[2008] UKHL 53; [2009] 1 AC 287, the nine-judge decision in R (Nicklinson) v Ministry of State for Justice [2014] UKSC 38; [2015] AC 657, and the seven-judge decisions in R (UNISON) v Lord Chancellor (Equality and Human Rights Commission intervening) [2017] UKSC 51; [2020] AC 869 and R (SC) v Secretary of State for Work and Pensions [2021] UKSC 26; [2022] AC 223.”

It is entirely within the discretion of employers or service providers to adopt general rules and policies. An assessment of these rules and policies can be done with regard to the rule or policy itself without the need to conduct a full proportionality test every time a rule or policy is applied.

In the specific context of single-sex services, the Supreme Court was clear that there is no legal entitlement for anyone to use single-sex services intended for members of the opposite sex [217]. What is more, it concluded [221] that those operating lawful single-sex services are entitled, by virtue of the single-sex nature of the service, to exclude all members of the opposite sex, regardless of whether they have a trans identity or a GRC:

if sex means biological sex, then provided it is proportionate, the female only nature of the service would engage paragraph 27 and would permit the exclusion of all males including males living in the female gender regardless of GRC status.

Indeed, this is the central point of the exceptions. As the Court noted at [224], anything other than a biological reading of these provisions:

would undermine the very considerations of privacy and decency between the sexes both in the availability of communal sleeping accommodation and in the use of sanitary facilities which the legislation plainly intended to provide for.

The Supreme Court also addressed the position of those who “pass” as members of the opposite sex and whether they could be excluded from single-sex services intended for their natal sex, as is the case with those trans men who are biologically female but, having taken testosterone, have masculinised their appearance [221]:

Moreover, women living in the male gender could also be excluded under paragraph 28 without this amounting to gender reassignment discrimination. This might be considered proportionate where reasonable objection is taken to their presence, for example, because the gender reassignment process has given them a masculine appearance or attributes to which reasonable objection might be taken in the context of the women-only service being provided. Their exclusion would amount to unlawful gender reassignment discrimination not sex discrimination absent this exception.

This means that trans men do not have a legal entitlement to use male-only facilities because they are female and may some times be lawfully excluded from female-only facilities because they appear male. The only prudent solution in those situations is for service providers to offer a separate service for transgender people.

Sexual Orientation

The Supreme Court made swift work [204] of the idea that the protected characteristic of sexual orientation could mean, in law, anything other than orientation towards members of the same or opposite biological sex:

The protected characteristic of sexual orientation … is framed by reference to orientation towards persons of the same sex, the opposite sex, or either sex. Read fairly, references to sex in this provision can only mean biological sex. People are not sexually oriented towards those in possession of a certificate.

The Court was clear [206] that “lesbians as a group are females who share the characteristic of being sexually oriented to females”. A meaning of sexual orientation in the EA which was tied to GRC status would erase lesbians from our equality law protection and would profoundly interfere with the human rights of lesbians to associate together free from the presence of biological males. That would be an irrational result. The Supreme Court at [206]-[208] was clear that lesbians have a right under our law to associate with each other free from the presence of biological males and gay men have a right to associate with each other free from the presence of biological females. A meaning of sex which was dependent on GRC status would create a chilling effect on the ability of lesbians and gay men to exercise this right. That was one reason why the Supreme Court concluded that the only fair reading of sex in the EA was one based on biology.

Sport

The Supreme Court then addressed what it referred to as “Women’s fair participation in sport”, noting that s.195 of the EA permits the establishment of female-only sporting categories “where it is necessary to do so to secure in relation to the activity- (a) fair competition, or (b) the safety of competitors”. Having concluded that sex means biological sex in the EA, the Court then concluded at [236] that:

a biological definition of sex would mean that a women’s boxing competition organiser could refuse to admit all men, including trans women regardless of their GRC status. This would be covered by the sex discrimination exception in section 195(1). But if, in addition, the providers of the boxing competition were concerned that fair competition or safety necessitates the exclusion of trans men (biological females living in the male gender, irrespective of GRC status) who have taken testosterone to give them more masculine attributes, their exclusion would amount to gender reassignment discrimination, not sex discrimination, but would be permitted by section 195(2).

It is clear from the Courts reasoning that trans women, regardless of whether they possess a GRC “could simply be excluded as male” from a sport covered by s.195 of the EA.

Forstater

A careful reader of this judgment will notice the fact that the Supreme Court has approved the reasoning of the Employment Appeal Tribunal in Forstater v CGD Europe. At [110] the Supreme Court referred to the “comprehensive and impressive judgment” of Choudhury P, the Judge who held that gender critical views are worthy of respect in a democratic society and protected under our anti-discrimination law. While this was not an issue before the Supreme Court in this case, this comment in passing should put to rest any suggestion that an appeal of the Forstater judgment will succeed on the basic point that gender critical views are protected in our law. Competent lawyers will take this reference to Forstater as the endorsement of Choudhury P’s reasoning that it clearly is.

Recognition

It would be remiss of me not to mention the fact that the Supreme Court expressed its gratitude to the interveners:

We are particularly grateful to Ben Cooper KC for his written and oral submissions on behalf of Sex Matters, which gave focus and structure to the argument that “sex”, “man” and “woman” should be given a biological meaning, and who was able effectively to address the questions posed by members of the court in the hour he had to make his submissions.

Ben Cooper KC deserves high praise for his advocacy during the hearing. I was lucky to be in the courtroom when he was on his feet and it was a masterclass in condensing extraordinarily complex legal material into focused and easily understandable arguments in a very tight time frame.

It seems surreal to note that, additionally, I was quoted and cited by the Court in this judgment at [142]:

The EA 2010 is also concerned to prohibit disguised discrimination which operates at a group level. This is important as Michael Foran explains (in an article entitled “Defining Sex in Law” (2025) 141 LQR 76, 91–92: “Arguments concerning the definition of a protected characteristic are never simply manifestations of individual claims. They are always group orientated. The claim that one is a woman is a claim to be included within a particular category of persons and to be excluded from another. It is also a claim to include some persons and to exclude other persons within the group that one is a part of. This matters especially for aspects of the Equality Act 2010 which require duty-bearers to be cognisant of how their conduct might affect those who share a protected characteristic or where there is an obligation to account for the distinct needs and interests of those who share a particular characteristic.”

I won’t say much about my own citation, except to thank Aidan O’Neill KC for including my paper within the bundle of authorities and for devoting time to it in his oral submissions.

This has been an intense week and the next promises to be even more intense for me. Once I have a moment to breathe, I’ll write something about the misinformation we are already seeing about this judgment. For now, I just want to send my heartfelt thanks to those who have supported me in this project of public information. It means a great deal to me.