Transgender rights discussions often turn on the distinction between “gender identity” and “sex assigned at birth.” Gender identity is a person’s own internal sense of whether they are a man, a woman, or nonbinary. “Sex assigned at birth” means the male or female designation that doctors ascribe to infants based on genitalia and is marked on their birth records. Sex assigned at birth is intended to displace the concept of “biological sex.”

This Article provides an account of the origins of the terms “biological sex” and “sex assigned at birth” and assesses the potential of the shift to sex assigned at birth for transgender rights arguments. The debate is not one over mere nomenclature. This Article’s examination reveals that the term “biological sex” rose to prominence to lend a veneer of scientific support to projects denying the validity of transgender identities and that the unquestioned use of that concept continues to underwrite exclusion. By referring instead to sex assigned at birth, transgender rights advocates convey that “biological sex” is not simple, static, or binary and that gender identity also has biological aspects. Furthermore, the phrase “assigned at birth” invokes philosophical arguments against assigning particular social roles to individuals at birth. It taps into the moral intuition that a person’s genitalia and health data are private matters.

This Article argues that sex assigned at birth is an important concept that clarifies the stakes of disputes over transgender rights. But it cautions that this conceptual shift is not sufficient to secure victories in transgender rights litigation. Ultimately, definitional debates about sex and gender cannot resolve the moral and practical questions at the heart of contemporary controversies over transgender rights. Recent legal victories on transgender rights issues have done more than debate the meanings of sex and gender: They have addressed practical objections to transgender inclusion, cultivated empathy for plaintiffs, and staked claims in the registers of equality, autonomy, and dignity.

The full text of this Article can be found by clicking the PDF link to the left.


In the last few years, the concept of “sex assigned at birth” has appeared with increasing frequency in U.S. case law on discrimination against transgender people. 1 See, e.g., Williams v. Kincaid, No. 21-2030, 2022 WL 3364824, at *1 (4th Cir. Aug. 16, 2022) (“Williams is a transgender woman whose gender identity (female) differs from the gender (male) she was assigned at birth.”); Adams ex rel. Kasper v. Sch. Bd. of St. Johns Cnty., 3 F.4th 1299, 1305 (11th Cir. 2021) (“The sex assigned to Mr. Adams at the time of birth was female, but his consistent, internal sense of gender is male.”), vacating and super­seding 968 F.3d 1286 (11th Cir. 2020), vacated and reh’g en banc granted, 9 F.4th 1369 (11th Cir. 2021) (mem.); Whitaker ex rel. Whitaker v. Kenosha Unified Sch. Dist. No. 1 Bd. of Educ., 858 F.3d 1034, 1048 (7th Cir. 2017) (“By definition, a transgender individual does not conform to the sex-based stereotypes of the sex that he or she was assigned at birth.”); A.M. ex rel. E.M. v. Indianapolis Pub. Schs., No. 1:22-cv-01075-JMS-DLP, 2022 WL 2951430, at *1 (S.D. Ind. July 26, 2022) (“Plaintiff A.M. is a ten-year-old transgender girl whose birth-assigned sex was male.”), appeal docketed, No. 22-2232 (7th Cir. July 27, 2022); B.P.J. v. W. Va. State Bd. of Educ., 550 F. Supp. 3d 347, 351 (S.D. W. Va. 2021) (explaining that “B.P.J. is a transgender girl who, while assigned the sex of male at birth, knew from a young age that she is a girl”); Corbitt v. Taylor, 513 F. Supp. 3d 1309, 1313 (M.D. Ala. 2021), appeal docketed, No. 21-10486 (11th Cir. Feb. 21, 2021) (“For individuals born in Alabama or pre­viously licensed here whose gender identity differs from the sex they were assigned at birth, the policy requires surgery, which results in permanent infertility in ‘almost all cases,’ to be able to obtain a license with a sex designation that matches their gender.”); Hecox v. Little, 479 F. Supp. 3d 930, 957 n.11 (D. Idaho 2020) (stating that it was permissible to refer to the plaintiff as “a person whose sex assigned at birth (male) differs from her gender identity (female)”), appeal docketed, Nos. 20-35813, 20-35815 (9th Cir. Sept. 17, 2020); J.A.W. v. Evansville Vanderburgh Sch. Corp., 396 F. Supp. 3d 833, 836 (S.D. Ind. 2019) (“J.A.W. was assigned the gender of female at birth.”); Evancho v. Pine-Richland Sch. Dist., 237 F. Supp. 3d 267, 272 (W.D. Pa. 2017) (using the term “assigned sexes” and explaining that transgender plaintiffs “had ‘male’ listed on their birth certificates when they were born”). The phrase had been used, at least since the 1960s, to describe an obstetrician’s “casual pronouncement of the newborn as a male or female,” “based upon inspection of the external gen­italia.” 2 See, e.g., Edgar Burns, Albert Segaloff & G.M. Carerra, Reassignment of Sex: Report of 3 Cases, 84 J. Urology 126, 126 (1960) (using the phrase “[a]ssignment of sex at birth”). This pronouncement, then and now, results in a male or female designation on a child’s birth certificate that is sometimes considered the person’s legal sex, unless changed through formal processes. 3 See, e.g., Radtke v. Miscellaneous Drivers & Helpers Union Local #638 Health, Welfare, Eye & Dental Fund, 867 F. Supp. 2d 1023, 1034 (D. Minn. 2012) (holding that the sex on a Minnesota birth certificate is an individual’s “legal sex” for all purposes). Although some legal authorities refer to the birth certificate as establishing “legal sex,” see, e.g., id., administrative practices of sex classification are varied, complex, and contradictory, and birth certificate designations do not control in every legal context. See, e.g., Paisley Currah, Sex Is as Sex Does: Governing Transgender Identity 7–10 (2022) [hereinafter Currah, Sex Is as Sex Does]; Dean Spade, Documenting Gender, 59 Hastings L.J. 731, 734 (2008) [hereinafter Spade, Documenting Gender]. Over the past two decades, the concept of sex assigned at birth has been taken up by transgender rights advocates to replace the troublesome term “biologi­cal sex.” 4 See infra section I.C. Assigned sex is opposed to gender identity—an individual’s own internal sense of whether they are a man, a woman, or nonbinary. 5 GLAAD, Glossary of Terms: Transgender, GLAAD Media Reference Guide (11th ed.), [] (last visited Aug. 13, 2022) (explaining that “infants are assigned a sex at birth, ‘male’ or ‘female,’ based on the appearance of their external anatomy” and defining “gender identity” as “[a] person’s internal, deeply held knowledge of their own gender,” which may or may not “align with the sex they were assigned at birth”). It is the basis for the legal definition of “transgender”: having a gender identity that does not match the one expected for an individual’s sex assigned at birth. 6 See, e.g., Bostock v. Clayton County, 140 S. Ct. 1731, 1756 n.6 (2020) (Alito, J., dissenting) (“The Court does not define what it means by ‘transgender status,’ but the American Psychological Association describes ‘transgender’ as ‘[a]n umbrella term encom­passing those whose gender identities or gender roles differ from those typically associated with the sex they were assigned at birth.’” (quoting A Glossary: Defining Transgender Terms, 49 Monitor on Psych. 32, 32 (2018))); GLAAD, supra note 5. It appeared in 2016 regulations interpreting the Affordable Care Act 7 Nondiscrimination in Health Programs and Activities, 81 Fed. Reg. 31,376, 31,467 (May 18, 2016) (to be codified at 45 C.F.R. pt. 92) (defining “gender identity” as “an individual’s internal sense of gender, which may be male, female, neither, or a combination of male and female, and which may be different from an individual’s sex assigned at birth”). In 2020, the Trump Administration repealed these regulations. Nondiscrimination in Health and Health Education Programs or Activities, Delegation of Authority, 85 Fed. Reg. 37,160, 37,161–62 (June 19, 2020). In 2022, the Biden Administration proposed regulations that would revert to the Obama-era policy. Nondiscrimination in Health Programs and Activities, 87 Fed. Reg. 47,824, 47,828 (Aug. 4, 2022). The proposed regulations also refer to “sex assigned at birth.” Id. at 47,918. and in a proposed 2021 federal law that would bar discrimination on the basis of LGBTQ status in employment, health care, and housing. 8 Equality Act, H.R. 5, 117th Cong. § 1101(a)(2) (2021) (“The term ‘gender identity’ means the gender-related identity, appearance, mannerisms, or other gender-related characteristics of an individual, regardless of the individual’s designated sex at birth.”).

But courts have been reluctant to embrace the concept. In the Supreme Court’s landmark Bostock v. Clayton County decision, which held that Title VII of the Civil Rights Act forbids discrimination against transgender people, the Court declined to use the term sex assigned at birth. 9 Bostock, 140 S. Ct. at 1731. The employees’ briefs in Bostock used the term “sex assigned at birth,” but at oral argument, David Cole, the advocate for Aimee Stephens, a transgender woman, conceded that assigned sex and biological sex were one in the same. Ezra Ishmael Young, What the Supreme Court Could Have Heard in R.G. & G.R. Harris Funeral Homes v. EEOC and Aimee Stephens, 11 Calif. L. Rev. Online 9, 11 (2020) (criticizing Cole’s decision). At one point, Cole resisted Chief Justice John Roberts’s references to “biological sex,” arguing that the issue was discrimination based on “what we think is more accurately referred to as sex assigned at birth,” but he did not press the point. Transcript of Oral Argument at 7, Equal Emp. Opportunity Comm’n v. R.G. & G.R. Harris Funeral Homes, Inc., 139 S. Ct. 1599 (2019) (No. 18-107), 2019 WL 9096155. Instead, it proceeded on the assumption that “sex” refers “only to biological distinctions between male and female.” 10 Bostock, 140 S. Ct. at 1739 (“[B]ecause nothing in our approach to these cases turns on the outcome of the parties’ debate, and because the employees concede the point for argument’s sake, we proceed on the assumption that ‘sex’ signified what the employers sug­gest, referring only to biological distinctions between male and female.”). Bostock assumed that these biological distinctions were synonymous with sex “identified,” not assigned, “at birth.” Id. at 1741. There is a meaningful difference between identification, a term that implies an objective process of classification, and assignment, a term that connotes the imposition of expectations. See infra section II.B. In this respect, Bostock is representative. Many federal court decisions fail to critically consider the differences between sex assigned at birth and “biological sex” or even conflate the two concepts. 11 See, e.g., Adams ex rel. Kasper v. Sch. Bd. of St. Johns Cnty., 3 F.4th 1299, 1304 (11th Cir. 2021) (“Mr. Adams is transgender, meaning when he was born, doctors assessed his sex and wrote ‘female’ on his birth certificate . . . .” (emphasis added)), vacating and superseding 968 F.3d 1286 (11th Cir. 2020), vacated and reh’g en banc granted, 9 F.4th 1369 (11th Cir. 2021) (mem.); Parents for Privacy v. Barr, 949 F.3d 1210, 1217 (9th Cir. 2020) (“This case concerns whether an Oregon public school district may allow transgender students to use school bathrooms, locker rooms, and showers that match their gender iden­tity rather than the biological sex they were assigned at birth.” (emphasis added)); Doe ex rel. Doe v. Boyertown Area Sch. Dist., 897 F.3d 518, 522 (3d Cir. 2018) (“‘Sex’ is defined as the ‘anatomical and physiological processes that lead to or denote male or female.’ Typically, sex is determined at birth based on the appearance of external genitalia.” (emphasis added)); Dodds v. U.S. Dep’t of Educ., 845 F.3d 217, 221 (6th Cir. 2016) (“The crux of this case is whether transgender students are entitled to access restrooms for their identified gender rather than their biological gender at birth.” (emphasis added)); Evancho v. Pine-Richland Sch. Dist., 237 F. Supp. 3d 267, 273 n.3 (W.D. Pa. 2017) (“[T]he Court will use the term ‘assigned sex’ to refer to the physical characteristics of the external sex organs of a person being referenced.”).

This confusion is dangerous. In the wake of Bostock, there has been an unprecedented onslaught of federal and state legislation aimed at curtail­ing transgender rights, 12 See, e.g., Tommy Beer, Latest GOP Trans Ban Fails: Sen. Tuberville’s Effort Is Latest Related to Sports Participation, Forbes (Mar. 6, 2021),‌sites/‌tommybeer/2021/03/06/latest-gop-trans-ban-fails-sen-tubervilles-effort-is-latest-related-to-sports-participation/?sh=5d76491e67ac (on file with the Columbia Law Review) (describing a proposed amendment to federal COVID-19 relief legislation in 2021 that would have barred transgender women and girls from women’s and girls’ athletics but was voted down in the Senate 49-50); Matt Lavietes & Elliott Ramos, Nearly 240 Anti-LGBTQ Bills Filed in 2022 So Far, Most of Them Targeting Trans People, NBC News (Mar. 20, 2022), https://‌ [] (discussing the rise in state anti-LGBTQ bills in response to Bostock, including 238 in the first three months of 2022 alone). almost all of it directly invoking the idea of “biological sex.” 13 See infra notes 144, 147–149 (quoting recent state laws invoking “biological sex” to restrict transgender rights). Federal courts will soon be asked to consider the defini­tion of sex as they resolve challenges to new laws in eighteen states barring transgender women and girls from sports, 14 See infra note 144 (collecting state laws restricting transgender students from participating in sports). Thus far, federal courts have reached decisions in three cases challenging these laws. A.M. ex rel. E.M. v. Indianapolis Pub. Schs., No. 1:22-cv-01075-JMS-DLP, 2022 WL 2951430, at *14 (S.D. Ind. July 26, 2022) (granting a preliminary injunction against enforcement of an Indiana law that would prohibit a ten-year-old transgender girl from playing softball on the school’s girls’ team), appeal docketed, No. 22-2332 (7th Cir. July 27, 2022); B.P.J. v. W. Va. State Bd. of Educ., 550 F. Supp. 3d 347, 358 (S.D. W. Va. 2021) (granting a preliminary injunction against enforcement of a West Virginia law that would prohibit an eleven-year-old transgender girl from participating in girls’ track and field); Hecox v. Little, 479 F. Supp. 3d 930, 989 (D. Idaho 2020) (granting a preliminary injunction against the enforcement of an Idaho law that would have barred a transgender woman from competing in women’s track and field at Boise State University), appeal docketed, Nos. 20-35813, 20-35815 (9th Cir. Sept. 17, 2020). two laws banning certain forms of gender-affirming health care for transgender youth, 15 See infra note 147 (discussing laws passed by Alabama and Arkansas). Both of these laws have been preliminarily enjoined. Eknes-Tucker v. Marshall, No. 2:22-CV-184-LCB, 2022 WL 1521889, at *1 (M.D. Ala. May 13, 2022), appeal docketed, No. 22-11707 (11th Cir. May 18, 2022); Brandt v. Rutledge, 551 F. Supp. 3d 882, 894 (E.D. Ark. 2021), aff’d, No. 21-2875, 2022 WL 3652745 (8th Cir. Aug. 25, 2022). In February 2022, government officials in Texas invoked biological sex in support of an executive directive to investigate parents of children undergoing certain forms of gender-affirming health care for child abuse. Tex. Att’y Gen., Opinion Letter on Whether Certain Medical Procedures Performed on Children Constitute Child Abuse 2–3 (2022),‌default/files/global/KP-0401.pdf [] (“[I]t is important to note that it remains medi­cally impossible to truly change the sex of an individual because this is determined biologically at conception.”). In May 2022, the Texas Supreme Court held that those execu­tive officials did not have statutory authority to require the relevant state agency to conduct child abuse investigations. In re Abbott, 645 S.W.3d 276, 281 (Tex. 2022). At the time of this writing, litigation over the investigations is ongoing. See PFLAG National, Lambda Legal, and ACLU File New Lawsuit to Stop Texas From Persecuting Parents With Transgender Kids, Lambda Legal (June 8, 2022),‌abbott_tx_20220608_‌pflag-ll-and-aclu-file-new-lawsuit-to-stop-tx-from-persecuting-parents-with-transgender-kids []. three laws limiting restroom access, 16 See infra note 148 (discussing laws passed by Alabama, Oklahoma, and Tennessee); cf. Bongo Prods., LLC v. Lawrence, No. 3:21-CV-00490, 2022 WL 1557664, at *1 (M.D. Tenn. May 17, 2022) (granting summary judgment in favor of plaintiffs on their First Amendment challenge to a Tennessee law requiring that entities with transinclusive restroom policies post warning signs). and three laws restricting a person’s ability to change the sex designation on their identification documents. 17 See infra note 149 (discussing laws passed by Idaho, Montana, and Oklahoma). A number of recent court decisions have found problems with such laws and policies. See, e.g., Corbitt v. Taylor, 513 F. Supp. 3d 1309, 1323 (M.D. Ala. 2021), appeal docketed, No. 21-10486 (11th Cir. Feb. 21, 2021) (holding that an Alabama rule that requires genital surgery before an individual can change the sex designation on their driver’s license violated the Equal Protection Clause); F.V. v. Jeppesen, 477 F. Supp. 3d 1144, 1150 (D. Idaho 2020) (pro­hibiting Idaho from categorically refusing to change birth certificate sex designations for transgender individuals); Marquez v. Montana, No. DV 21-873, para. 183 (Mont. 13th Jud. Dist. Ct. Apr. 21, 2022) (granting motion for preliminary injunction against enforcement of the Montana statute, because the plaintiffs made out a prima facie case that the statute vio­lated their rights to due process because it was impermissibly vague with respect to which surgeries might be required). Although a “growing consensus” of courts agree that it is impermissible discrimi­nation for schools to refuse to allow transgender students to use restrooms consistent with their gender identities, 18 Grimm v. Gloucester Cnty. Sch. Bd., 972 F.3d 586, 593 (4th Cir. 2020), cert. denied, 141 S. Ct. 2878 (2021) (mem.). Justices Clarence Thomas and Samuel Alito would have granted the petition for certiorari. Gloucester Cnty. Sch. Bd. v. Grimm, 141 S. Ct. 2878, 2878 (2021) (mem.). the Eleventh Circuit is currently reconsidering the issue en banc. 19 Adams ex rel. Kasper v. Sch. Bd. of St. Johns Cnty., 9 F.4th 1369, 1372 (11th Cir. 2021) (mem.) (vacating and granting en banc review of a panel decision holding that ex­clusion of a transgender boy from the boys’ restroom violated the Equal Protection Clause), vacating and granting reh’g en banc 3 F.4th 1299 (11th Cir. 2021). Bostock declined to spell out its implications for restrooms, sports, identity documents, or other such controversies, and so transgender rights may end up back in the Supreme Court again soon. 20 Bostock v. Clayton County, 140 S. Ct. 1731, 1753 (2020) (“[W]e do not purport to address bathrooms, locker rooms, or anything else of the kind . . . . Whether other policies and practices might or might not qualify as unlawful discrimination or find justifications under other provisions of Title VII are questions for future cases, not these.”); see also Tennessee v. U.S. Dep’t of Educ., No. 3:21-cv-308, 2022 WL 2791450, at *3 (E.D. Tenn. July 15, 2022) (granting motion for preliminary injunction barring federal agencies from en­forcing interpretations of Title IX that would bar discrimination on the basis of LGBTQ status and interpretations of Title VII that would apply Bostock in the contexts of “dress codes, bathrooms, locker rooms, showers, and use of preferred pronouns or names”).

This Article attempts to uncover the history of the concept of sex assigned at birth, as well as that of its main competitor, “biological sex,” and to set forth the case in favor of the shift toward sex assigned at birth as an idea that can advance legal protection for transgender, nonbinary, 21 This Article uses the term “nonbinary” to refer to a person who does not exclusively identify as a man or a woman. See, e.g., Jessica A. Clarke, They, Them, and Theirs, 132 Harv. L. Rev. 894, 905–14 (2019) [hereinafter Clarke, They, Them, and Theirs] (discussing the diversity of nonbinary gender identities and reasons for bias and discrimination against them). and gender-nonconforming people. 22 This Article uses the term “gender nonconforming” to refer to those who do not reject the gender identity associated with the sex assigned to them at birth but who deviate from the roles, behaviors, and appearances expected of their sex. Cf. Mary Anne Case, Legal Protections for the “Personal Best” of Each Employee: Title VII’s Prohibition on Sex Discrimination, the Legacy of Price Waterhouse v. Hopkins, and the Prospect of ENDA, 66 Stan. L. Rev. 1333, 1335–36 (2014) (discussing “gender benders” such as men who do not claim to be transgender but “nevertheless engage[] in behavior seen as stereotypically feminine”).

One contribution of this Article is to excavate the histories of the concepts of “biological sex” and “sex assigned at birth.” 23 See infra Part I. Other scholars have delved into the meaning of the term “sex” as it is used in the Civil Rights Act of 1964. See, e.g., William N. Eskridge Jr., Brian G. Slocum & Stefan Th. Gries, The Meaning of Sex: Dynamic Words, Novel Applications, and Original Public Meaning, 119 Mich. L. Rev. 1503, 1549 (2021) (explaining that “[s]ex was a broad, catchall term in 1964, used in circumstances where we would use terms such as gender, sexuality, and sexual orientation”). But they have not inquired into the origins of “biological sex,” a concept now being deployed by state legislators seeking to restrict transgender rights, or its progressive alternative, “sex assigned at birth.” In recent years, many courts and legislatures have taken for granted that there is some simple attribute called “biological sex” that is easily separa­ble from gender identity. 24 See infra notes 144, 147–149 (collecting statutes). In litigation over access to sex-segregated restrooms and sports, opponents of transgender rights lean heavily on “biology” as a simple and scientific basis for excluding transgender indi­viduals from the categories of “male” and “female.” 25 See, e.g., Adams ex rel. Kasper v. Sch. Bd. of St. Johns Cnty., 3 F.4th 1299, 1322 (11th Cir. 2021) (Pryor, C.J., dissenting) (asserting that a school did not violate the Equal Protection Clause by excluding a transgender boy from the boys’ restroom based on its def­inition of “‘sex’ in its ordinary, traditional sense” as synonymous with “biological sex”), vacated and reh’g en banc granted, 9 F.4th 1369 (11th Cir. 2021) (mem.); Intervenors-Appellants Madison Kenyon and Mary Marshall’s Opening Brief at 4, Hecox v. Little, Nos. 20-35813, 20-35815 (9th Cir. filed Nov. 12, 2020) (“Recently . . . women and girls have become bystanders in their own sports as biologically male athletes who identify as female demand to be able to compete against women and girls.”); id. at 1–64 (using the terms “biological male” or “biologically male” sixty-eight times in a sixty-four page brief). The Hecox litigation pertains to the constitutionality of Idaho’s Fairness in Women’s Sports Act, which defines sex as “biological” and requires that it be verified based on “the student’s reproductive anatomy, genetic makeup, or normal endogenously produced testosterone levels.” Idaho Code § 33-6203 (2021). But the idea of “biological sex” as distinct from gender identity is not a time-honored scientific or legal category; it is a contested concept from mid-twentieth-century medicine. In the 1970s, “biological sex” found its way into legal doctrine as a result of an outdated understanding of transgender identity as a mental illness caused by early childhood experiences, along with pol­icy concerns about the need to distinguish the sexes for purposes such as avoiding same-sex marriage. 26 See infra section I.B. Lawmakers today are unable to agree on any definition of “biological sex” based in anatomy, genetics, hormones, or other such properties, so they often enact laws that define sex as the male or female designation on an individual’s original birth certificate. 27 See infra notes 144–150 and accompanying text (collecting statutes invoking “biological sex” without any consistent definition of that term). Rather than vindicating any biological standard, these laws endeavor to exclude transgender people. Assigned sex terminology also emerged at midcentury in medical research related to people with intersex variations. “Intersex” is “an umbrella term for differences in sex traits or reproductive anatomy,” such as “differences in genitalia, hormones, internal anatomy, or chromosomes, compared to the usual two ways that human bodies develop.” 28 What Is Intersex?, InterACT, [https://‌] (last updated Jan. 26, 2021). Transgender theorists borrowed this terminology in the 1990s to describe the process of assigning sexes to all infants, and it began to appear in legal contexts in the early 2000s. 29 See infra sections I.B–.C. It now competes with the term “biological sex” in legal disputes over transgender rights. 30 See infra section I.C.

Another contribution of this Article is to present the full theoretical case in favor of “sex assigned at birth” as an alternative to “biological sex.” 31 I do not argue that “sex assigned at birth” should be the definition of “sex” whenever that term appears in the law; as I have argued elsewhere, to the extent that legal sex classifications are justified at all, sex determinations should reflect each law’s particular purposes, as well as the values of autonomy, dignity, and equality. Clarke, They, Them, and Theirs, supra note 21, at 933–36. Rather, sex assigned at birth is useful as an explanatory concept that challenges the idea that every person has one true sex. See infra Part II. “Sex assigned at birth” is not a euphemism for “biological sex” but a critique of the very concept. It acknowledges that “sex” can be defined in many ways. To speak of assigned sex is to point out that while adminis­trative “M” and “F” classifications might be simple, the biology of sex is not. “Biological sex” is not binary, stable, or uniform. 32 See, e.g., Claire Ainsworth, Sex Redefined, 518 Nature 288, 288 (2015) (“[N]ew technologies in DNA sequencing and cell biology are revealing that almost everyone is, to varying degrees, a patchwork of genetically distinct cells, some with a sex that might not match that of the rest of their body.”); see also infra notes 170–179 and accompanying text. And it is inconsistent with medical research to assert that gender identity has no biological underpinnings. 33 See, e.g., Wylie C. Hembree et al., Endocrine Treatment of Gender-Dysphoric/‌Gender-Incongruent Persons: An Endocrine Society Clinical Practice Guideline, 102 J. Clinical Endocrinology & Metabolism 3869, 3874 (2017) (“Results of studies from a variety of biomedical disciplines—genetic, endocrine, and neuroanatomic—support the concept that gender identity and/or gender expression likely reflect a complex interplay of biologi­cal, environmental, and cultural factors.”); Joshua D. Safer & Vin Tangpricha, Care of Transgender Persons, 381 New Eng. J. Med. 2451, 2451 (2019) (collecting sources in sup­port of the claim that “[a]lthough the mechanisms that inform gender identity are unknown, current data suggest a biologic underpinning programmed from birth”); cf. Aditi Bhargava et al., Considering Sex as a Biological Variable in Basic and Clinical Studies: An Endocrine Society Scientific Statement, 42 Endocrine Revs. 219, 227 (2021) (concluding “there is ample but incomplete evidence for biological substrates—neuroanatomic, genetic, and hormonal—for gender orientation, making this an important area of ongoing research”). For a plain-language explanation, see Denise Grady, Anatomy Does Not Determine Gender, Experts Say, N.Y. Times (Oct. 22, 2018),‌‌2018/10/22/health/transgender-trump-biology.html (on file with the Columbia Law Review). The claim that sex is assigned at birth, rather than being a self-evident biological property that naturally corresponds with certain gender identities and roles, goes beyond the feminist argu­ment against biology as destiny. 34 See infra section III.A (discussing the feminist argument that while biology may not be changeable, social norms with respect to the roles of men and women can be contested politically). Administrative assignments of identities and social roles threaten liberal principles of autonomy that insist that every person should be, at least in part, the author of their own life story, 35 Cf. Joseph Raz, The Morality of Freedom 370 (1986) (“The autonomous person is part author of his life.”). as well as postmodern sensibilities about the role of creativity and play in constructing the self. 36 Cf. Julie E. Cohen, Configuring the Networked Self: Law, Code, and the Play of Everyday Practice 133–35 (2012) (discussing a dynamic view of the self that is in some ways culturally determined but also evolves through “[p]lay with texts, artifacts, personae, and social conventions”). The idea of assignments at birth evokes the egalitar­ian’s umbrage at lotteries of birth in which roles and opportunities are distributed in infancy. 37 Cf. John Rawls, A Theory of Justice 74–75 (1971) (criticizing a conception of equality that allows the distribution of resources to be “decided by the outcome of the natural lottery”); Elizabeth S. Anderson, What Is the Point of Equality?, 109 Ethics 287, 289–90 (1999) [hereinafter Anderson, Point of Equality] (discussing theories of “luck egalitar­ianism” that appeal because of “the force of the obviously correct claim that no one deserves their genetic endowments or other accidents of birth, such as who their parents are or where they were born”). That these assignments subject individuals to in­tersecting social hierarchies—such as those that elevate men over women, gender conformers over nonconformers, and cisgender over transgender people—offends theories of the purpose of antidiscrimination law  as  undermining  systemic  patterns  of  subordination. 38 See, e.g., Anderson, Point of Equality, supra note 37, at 312 (explaining how egalitarian political movements oppose hierarchical social relationships that “generate, and were thought to justify, inequalities in the distribution of freedoms, resources, and wel­fare”); cf. Tarunabh Khaitan, A Theory of Discrimination Law 91 (2015) (explaining that the purpose of “discrimination law is to secure an aspect of the well-being of persons by reducing the abiding, pervasive, and substantial relative disadvantage faced by members of protected groups”). Moreover, by pointing out that sex is assigned at birth, advocates draw attention to the fact that institutions that exclude transgender people are doing so based on what is essentially a medical record of a doctor’s examination of a person’s genitalia in infancy. Yet genitalia and medical records are quintessentially private.

Thus, the idea that sex is assigned at birth has the potential to disrupt legal invocations of “biological sex” as a simple, natural, neutral, and normatively unproblematic basis for classifying individuals. In practice, however, sex assigned at birth has not lived up to its theoretical potential. Another contribution of this Article is to explain why. 39 See infra Part III. Of course, some oppose the concept due to ideological opposition to transgender rights in general. But the idea has encountered resistance even from those without fixed positions in this particular culture war. One reason is that the idea of “biological sex” as a self-evident essence is an entrenched form of common sense that is difficult to dislodge, even though it has been undermined by advances in science and medicine. The concept is reinforced by the insist­ence of many feminists that there is an important difference between biological sex and social gender. 40 See infra section III.A. Another reason for the persistence of biological sex is dissatisfaction with gender identity as an alternative basis for sex or gender classification. Judges and other decisionmakers are often concerned that gender identity is too subjective and easily manipulated to serve as the basis for sorting individuals into male and female categories.

A final contribution of this Article is to discuss ways to overcome these barriers. 41 See infra Part IV. Sex assigned at birth clarifies what is at stake in disputes over restrooms, sports, and identity documents—these are not debates over bi­ology; rather, they are controversies over how to prioritize conflicting values and whether, as an empirical matter, more inclusive policies will have deleterious effects. While sex assigned at birth can clarify what the stakes are, it cannot, on its own, resolve the moral and practical questions at the heart of contemporary transgender rights controversies. This Article cautions against an approach taken by many courts, which is to attempt to evade moral and practical questions by insisting that an individual’s true sex is their gender identity as a man or a woman, if medical experts verify that they live all aspects of their lives consistently with that gender identity. The result of such an approach may be to limit legal protection to only that subset of the transgender community that can prove the bona fides of their gender identities to medical experts and to base the case for protec­tion on a scientific foundation that may not be able to bear its weight. Moreover, this approach is not necessary. A review of recent litigation demonstrates that advocates have won transgender rights cases not just with appeals to scientific authorities on the validity of transgender people’s gender identities but also with arguments that tap into values like equality, autonomy, and dignity, with stories that cultivate empathy, and with evidence debunking practical objections to transgender inclusion.

While theorists and advocates have been deploying the concept of sex assigned at birth for the past two decades, 42 See, e.g., Susan Stryker, Transgender History: The Roots of Today’s Revolution 19 (2d ed. 2017) [hereinafter Stryker, Transgender History (2d ed.)] (employing the term “birth-assigned gender”); Paisley Currah, Transgender Rights Without a Theory of Gender?, 52 Tulsa L. Rev. 441, 450 n.36 (2017) [hereinafter Currah, Transgender Rights Without a Theory of Gender?] (discussing the author’s work to pass a 2002 New York City Human Rights Law amendment using the term “legal sex assigned to that person at birth”). no work of legal scholar­ship has explored the idea’s potential for transgender rights arguments, or unearthed the origins of that term and its main competitor, “biological sex.” 43 Much recent legal scholarship has focused on what it means to ban discrimination “because of sex” for purposes of the Civil Rights Act of 1964, the issue in Bostock. See, e.g., Eskridge, supra note 23. The discrimination question is distinct from the less-explored issue of how the law defines who counts as male or female in those contexts in which distinctions might still be permitted. This Article builds on the important insights of Professor Paisley Currah and Professor Dean Spade on this issue. See Currah, Sex Is as Sex Does, supra note 3, at 7–10 (arguing that how the state defines who is recognized as male or female often depends on the work that a particular arm of the state is doing); Spade, Documenting Gender, supra note 3, at 733 (detailing the “rarely discussed” matrix of rules governing gender reclassification in the United States). Transgender people continue to face uniquely high rates of discrim­ination, harassment, and violence with devastating consequences. 44 Sandy E. James, Jody L. Herman, Susan Rankin, Mara Keisling, Lisa Mottet & Ma’ayan Anafi, The Report of the 2015 U.S. Transgender Survey 2, 5 (2016), [] (surveying 27,715 transgender people and finding they reported “high levels of mistreatment, harassment, and violence in every aspect of life”). The Williams Institute estimates that the number of adults in the United States who identify as transgender is 1.3 million, or 0.5% of the population. Jody L. Herman, Andrew R. Flores & Kathryn K. O’Neill, Williams Inst., How Many Adults and Youth Identify as Transgender in the United States? 4 (2022),‌wp-content/uploads/Trans-Pop-Update-Jun-2022.pdf []. Moreover, rules requiring that people conform with expectations for their assigned sex impact individuals who do not necessarily identify as transgender but are gender nonconforming or nonbinary. 45 See supra notes 21–22 (defining these terms). In addition to its contributions to the study of transgender rights litigation, this Article’s account is of relevance to feminist scholarship on biological concepts of sex and gender. 46 See, e.g., Katrina Karkazis, The Misuses of “Biological Sex”, 394 Lancet 1898, 1898 (2019) [hereinafter Karkazis, Misuses of “Biological Sex”] (discussing the “long history of using—and misusing—discrete biological criteria to determine sex and thereby include or exclude certain people from categories”); Heather Shattuck-Heidorn & Sarah S. Richardson, Neurogenderings: Sex/Gender and the Biosocial Turn, Scholar & Feminist Online (2019), [https://‌‌] (discussing feminist criticisms of the concept of “sex as a biological variable”).

Part I of this Article uncovers the origins of the debate between “biological sex” and “sex assigned at birth” in transgender rights advocacy and public policy. Part II sets out the theoretical argument for the shift to sex assigned at birth and explains why that concept is superior to alterna­tives. It demonstrates that sex assigned at birth does useful work in litigation by disrupting the assumption that sex classifications reflect mere biology and pointing to how those classifications can threaten autonomy, equality, privacy, and dignity. Part III explains why courts have been reluc­tant to abandon biological sex in favor of sex assigned at birth. Part IV offers an analysis of recent transgender rights litigation that concludes that, to overcome this reluctance, the claim that sex is assigned at birth must be accompanied by arguments that speak to values, practicalities, and empathy in particular cases.