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Assigned Sex at Birth

The sex (male, female, intersex) assigned to a child at birth, most often based on the child's external anatomy. Also referred to as birth sex, natal sex, biological sex, or sex.

Other examples: AFAB (assigned female at birth), AMAB (assigned male at birth).

Note: It is not valid to use “biology” or “science” to claim assigned sex at birth and/or gender are a fixed, concrete concept and/or binary. Science favors the fact that sex and gender are more complicated and involve much more than one’s sexual organs.

Adapted from  The Fenway Institute  and informed by  Stop Using Phony Science to Justify Transphobia - Scientific American Blog Network

Previous term

The Glossary for Culture Transformation  is a living document to be revised on an annual basis.

Suggested Citation: Office of Equity, Vitality, and Inclusion, Boston University Chobanian & Avedisian School of Medicine, Boston Medical Center, and Boston University Medical Group.  Glossary for Culture Transformation . 2021.

For questions or suggestions, please email [email protected]

Biological Sex and Gender in the United States

In the United States, most people are assigned both a biological sex and gender at birth based on their chromosomes and reproductive organs. However, there is an important distinction between biological sex and gender. Biological sex, such as male, female, or intersex, commonly refers to physical characteristics. Gender refers to the socially constructed roles, behaviors, and actions people take on, usually in relation to expectations of masculinity or femininity. As of 2022, many different groups continue to debate the specific relationship between sex and gender. No matter the relationship, people’s biological sex and gender greatly influence the way they understand themselves, as well as how others treat them and how they interact with society. Moreover, some people’s gender differs from what they were assigned at birth, and they face discrimination, harassment, and violence. Evolving understandings of gender and sex in the US have created more ways for people to live and express their gender identities.

In the US, most people are assigned a gender at birth according to their biological sex. People who are male at birth are usually assigned and raised according to stereotypical expectations of men, while people who are female at birth are usually assigned and raised to act according to stereotypical expectations of women. People who feel comfortable living as the gender they were assigned at birth are cisgender. People who do not feel comfortable living as the gender they were assigned at birth are transgender. In 2016, the University of California, Los Angeles, in Los Angeles, California, conducted a study showing that up to 1.4 million people in the US, or 0.6 percent of the total population, were transgender. For some people who were born intersex, or people whose bodies do not conform with stereotypical expectations of males or females at birth, a gender that is often chosen for them at birth may not feel accurate for them later in life.

Additionally, many people who were born male or female may also feel that their label as either a man or a woman is not accurate to their identities or experiences, either. As a consequence, they may identify as a different gender than what they were assigned at birth. A person’s gender identity has to do with their internal sense of their own gender, whereas their gender expression has to do with how they present their gender to the world. Many people who identify as or express a gender other than the one they were assigned at birth fall under the umbrella term gender diverse, which encompasses gender identities other than cisgender, such as transgender. Transgender is often defined as an umbrella term similar to gender diverse. Because gender comes from social expectations rather than people’s biology, it is able to change throughout a person’s life. However, in the United States, that conception of gender only began to gain recognition during the second half of the twentieth century.

A person’s biological sex usually refers to their status as female, male, or intersex depending on their chromosomes, reproductive organs, and other characteristics. Chromosomes are tightly packed DNA, or molecules that contain the genetic instructions for the development and functioning of all living things. Humans typically have forty-six chromosomes. Two of those are sex chromosomes that contain instructions for the development and functioning of characteristics related to biological sex, such as reproductive organs. There are two kinds of human sex chromosomes, X and Y. Individuals identified as males tend to have one X and one Y chromosome, while those identified as females tend to have two X chromosomes. However, other people are born with other chromosome combinations, such as XXY, that lead them to develop a mix of characteristics. People who fit that description are often referred to as intersex, a category meaning that they are not born as strictly biologically male or female.

While gender is often conflated with biological sex, a person is not born with gender. Rather, people learn to act in accordance with the socially constructed expectations of their gender as they grow up. Social constructs are ideas that humans originally invented and continue to perpetuate over time rather than being innate roles that exist in nature. People implicitly and explicitly learn how someone of their gender should act as they grow up by the people around them and by popular media. However, those expectations do not always fit with how people act. Gender-specific expectations often come from stereotypes. Stereotypes include widely held beliefs about a certain group of people based on oversimplified or prejudiced ideas. Some stereotypical expectations of men in the US include that they are supposed to be stoic and competitive, large and muscular, and financially provide for their household. In contrast, some common expectations of women in the US include that they are supposed to be polite and nurturing, slim and petite, and are expected to take on domestic work like raising children and running the household to societal standards.

Expectations of men and women can also vary by culture. The culture of the US may expect women to look and act differently than women in Saudi Arabia or Japan, for example. Moreover, different cultures might even recognize a different number of genders. While many cultures see just man or woman, other cultures have three, five, or more genders. Thus, rather than being universal to biological males, females, or intersex people, each human society uniquely determines the genders and gender roles within it.

Early uses of the word gender in reference to men or women tended to view it as one and the same as biological sex. According to The Oxford English Dictionary, the word gender had been used as early as the 1300s to describe categories of people. The Oxford English Dictionary’s earliest record of using the word to specifically refer to men or women, though, did not occur until 1474, when someone used it in a letter to describe what the writer refers to as the masculine gender. Over the next centuries, when gender was used to refer to men or women, it was often synonymous with biological sex. However, according to the Merriam-Webster Dictionary, during the early twentieth century, the word sex became more associated with sexual intercourse. As discussions of sexual intercourse are largely taboo in the US, people began to use the word gender in its place to refer to a person’s status as a male or female by the end of the twentieth century, a practice that is still largely common as of 2022. However, in the 1950s, gender psychologists who studied differences between the sexes began to reframe gender as something entirely separate from biological sex.

In 1955, John Money, a researcher who studied psychology, gender, and sex differences throughout the twentieth century, was one of the first people to use the word gender to mean something separate from biological sex outside of the context of grammar. In his article “Hermaphroditism, Gender and Precocity in Hyperadrenocorticism: Psychologic findings,” Money first introduced the concept of a gender role. He defined a gender role as all the ways a person discloses themselves as being a man or woman. Money used the term gender role throughout several other papers that he published during the same year. Money asserted that people learn gender roles and that anatomy does not determine a person’s gender, though he conceded that, in most cases, a person’s gender roles align with their biological sex.

Money stated that he used the word gender rather than sex to be able to write about the psychology and behavior of intersex people, but his work was often controversial. For example, the intersex advocacy organization, Intersex Society of North America, or ISNA, has criticized Money’s role in the John/Joan case in the 1960s. In the case, Money encouraged a couple to raise their biologically male son as a girl after he lost his penis in a medical accident as an infant in 1966. Money asserted that learned gender was more important than biology. However, the patient involved in that case, David Reimer, reported that he was uncomfortable with his parents raising him as a girl.

The INSA reported that when Reimer spoke out about the way Money treated him when he grew up, some used Reimer’s story to argue that biology determines gender. Those opponents of dividing gender and sex argued that changing a person’s gender from what they were assigned at birth was unnatural. However, the ISNA maintains that Reimer’s experiences were the result of medical malpractice and Money’s irresponsibility, as Reimer would not have identified as a girl on his own accord. The ISNA maintains that Reimer’s case does not reflect the average transgender person’s relationship with gender and should not justify the condemnation of transgender individuals. Nonetheless, in spite of Money’s controversial tactics, which the ISNA labelled as irresponsible, his introduction of the idea of gender as separate from biological sex pushed other researchers to continue talking about the topic over the following decades.

In 1964, Robert Stoller, who researched psychiatry at the University of California, Los Angeles, or UCLA, in Los Angeles, California, reinforced Money’s concept of gender as something separate from biological sex. Stoller asserted that cultures determine gender rather than biology. Additionally, in 1964, Stoller and Ralph Greenson, who also practiced psychiatry at UCLA, introduced the term gender identity at the 23 rd International Psycho-Analytical Congress in Stockholm, Sweden, where psychiatric researchers from different parts of the world met to discuss their research. Stoller and Greenson defined gender identity as a person’s internal sense that they belong to one biological sex and not the other. In contrast, they defined gender role as the behavior one exhibits in society around other people. Like Money, Stoller’s use of the word gender often stemmed from studying people who were transgender or intersex and who needed distinctions between biological characteristics and psychological ones.

In the late 1960s and early 1970s, feminists, or people who advocate for equal rights between men and women, began to adopt the distinction between gender and sex that psychologists like Money and Stoller had put forth in the US. Feminist scholars like Harriet Holter, Jessie Bernard, and Ann Oakley explicitly cited Stoller and Money’s ideas about gender in their works arguing for women to have access to the same opportunities as men. Those scholars used the concept of gender to argue against ideas that women held less power in society due to biological inferiority. Instead, they argued that rather than having biological differences, the differences between men and women were predominantly the result of culture and able to be changed. One scholar who utilized the idea of gender in that way was Gayle Rubin. In 1975, Rubin described gender as a socially imposed division of the sexes used to oppress women. However, since society constructs gender, Rubin argued that it was mutable and could be reformed to end women’s oppression. Thus, many feminists of the time conceded that biological sex differences were real, with some even viewing biological sex as fixed, but framed gender as separate from biological sex and able to change throughout a person’s life.

However, many arguments for gender equality that feminists in the US during the twentieth century put forth largely focused on the experiences of white, middle-class women. In the 1980s, other US feminists, and specifically feminists of color, called for more inclusive accounts of womanhood as feminists began using more complex definitions of gender. For instance, Elizabeth Spelman, who has researched and written on race feminism throughout the twentieth and twenty-first centuries, argued in 1988 that many feminists had mistakenly put forth that all women have relatively similar experiences because of their gender. However, Spelman asserted that women have vastly different notions of what it means to be a woman due to factors like race and class as well, and that putting forth one notion of womanhood mostly based on the experience of white middle class women was exclusionary and harmful to women outside of that group.

Though the 1980s brought some more recognition to experiences of gender from the perspectives of people of different races and socioeconomic classes, mainstream feminists and other activists in the US largely did not consider the experiences of LGBTQ+ people until the 1990s. LGBTQ+ is an acronym that stands for lesbian, gay, bisexual, transgender, and queer or questioning people, among other labels that describe people whose sexuality or gender differ from social norms. LGBTQ+ people had been fighting discrimination for decades, but their stories only became prominent toward the end of the twentieth century. The recognition of LGBTQ+ people at large came with increasing exposure for gender diverse people, including transgender, intersex, and gender non-conforming people, whose recognition pushed society to rethink their concepts of biological sex and gender. But even though recognition of some gender diverse people increased, intolerance against LGBTQ+ people was still pervasive through the end of the twentieth century in the US. A US poll from 1999 showed that sixty-two percent of respondents did not think that marriages between same-sex couples were valid.

In the 1990s, a greater variety of gender diverse experiences further developed the idea that gender also exists as a spectrum rather than a binary, and that a person’s gender is not fixed at birth. Transgender and gender non-conforming authors like Kate Bornstein and Leslie Feinberg brought transgender and gender non-conforming stories to wider audiences through works of fiction like Stone Butch Blues , which Feinberg published in 1993, and Gender Outlaw , which Bornstein published in 1994. Both books followed main characters whose struggles with gender largely defined their experiences. Feinberg also helped popularize the word transgender in her 1996 non-fiction book Transgender Warriors , where she broadly defined transgender as a label that included anyone that crossed the boundaries of gender. According the American Psychological Association, increased recognition of such issues helped push feminists and gender studies scholars to be more inclusive of transgender and gender non-conforming people.

Since the 2000s, people often use the word transgender to specifically refer to people who identify as the opposite gender, though organizations like the National Center for Transgender Equality define it as an umbrella term to describe all gender diverse people. For instance, a person who was assigned to be a man at birth may later identify as a woman, or vice versa. People who fit that description are commonly called transgender men and transgender women. Some transgender men and transgender women undergo procedures called gender affirmation surgeries. Those surgeries can give them body parts that may better align with their gender identity and can make them more comfortable expressing their gender the way they choose. Some transgender men and transgender women who undergo a gender affirmation surgery may choose to identify as transsexual to reflect that their bodies have changed. However, not everyone who fits that description uses the term transsexual, as many consider it outdated. Additionally, not all transgender men and transgender women undergo gender affirmation surgery. They may not be able to afford or have the opportunity to undergo gender affirmation surgery, and many transgender people may not want it. Because gender identity does not come from a person’s body, people can identify as transgender men or transgender women without undergoing gender affirmation surgery.

Transgender stories in the 1990s also paved the way for other gender diverse people to speak up, find communities, and create new vocabulary to capture their experiences. In 1995, the activist Riki Anne Wilchins was one of the first people to use the word genderqueer in their 1995 newsletter "In Your Face" as an umbrella term to describe all people who struggled with the gender they were assigned at birth. Zines, activist flyers, and publications that LGBTQ+ communities ran quickly adopted the word and spread its use over the coming years.

The 2000s and 2010s saw the creation of such terms as non-binary, agender, and gender fluid to capture different ways people can experience and interact with gender. As of 2022, non-binary is generally used as an umbrella term that describes people who may not feel comfortable strictly identifying as a man or woman. Many non-binary people choose for people to use "they/them" pronouns to refer to them, as opposed to "he/him" or "she/her" to reflect their identity. Non-binary people can further identify as agender, meaning they do not feel comfortable as either a man or a woman at all, and instead feel genderless. On the other hand, some non-binary people are gender fluid. Gender fluid people may identify as a man, woman, or even both at the same time at different points in their life. Since gender relates to a person’s identity rather than their biology, it is possible for people to adopt elements of different genders simultaneously or express themselves in ways that carry no gender. Many other labels, or gender identities, exist, and new ones constantly come about to better describe how a person feels about themselves.

As of 2022, more adults than ever before identify as LGBTQ+ and acceptance of LGBTQ+ people is on the rise but only after centuries of fighting against discrimination. LGBTQ+ people at large have been denied rights and actively oppressed in the US for centuries. Sodomy laws enacted in the 1600s that made engaging in homosexual acts illegal, even in the privacy of a person’s home, were still in place up until the early 2000s in some states. Same-sex marriage only became legal on a national level in the US in 2015, and though more Americans are accepting of gay rights than ever before, up to thirty percent of gay, lesbian, and bisexual people still face discrimination and harassment every year. Additionally, expressing oneself in ways outside of gender norms was commonly illegal and heavily denounced as immoral. One way it was made illegal was through what have historically been referred to as cross-dressing laws. Those laws were commonly passed in the late nineteenth and early twentieth century and made it illegal for people to wear clothing associated with the gender opposite to what they were assigned at birth. Cross-dressing laws enabled police to specifically target and harass transgender people under the guise of protecting against public-indecency. Transgender men and transgender women, like other LGBTQ+ people, have often been the targets of harassment, violence and discrimination, but laws actually protecting transgender people from actions like discrimination and hate-crimes still do not exist in many US states as of 2022.

Legal recognition of people’s gender identities is still lacking in the US as of 2022. It remains difficult in many states for gender diverse people to change their gender identity on legal documents like drivers’ licenses or birth certificates. Doing so remains especially difficult in states like Kentucky and Alabama that require a person to undergo gender affirmation surgery and have proof of identity change on their birth certificate before correcting a person’s gender identity on their driver’s license, or in states like Tennessee and Ohio where it still is not possible for a person to change their sex on a birth certificate. Additionally, in many states, there is no option to identify as a gender other than man or woman.

Beyond restrictions on changing their legal identities, gender diverse people face higher levels of discrimination than any other LGBTQ+ group. A 2020 report showed that over sixty percent of transgender and non-binary people report having faced discrimination in the previous year. Those rates increase for people who are also a part of other oppressed social groups such as women, people of lower socioeconomic status, and people of color, with transgender women of color being the most at risk. Discrimination makes it more difficult for gender diverse people to secure employment. Fifteen percent of transgender people faced unemployment in 2015, three times the national average unemployment rate of five percent at the time. Nearly thirty percent of transgender people also reported having experienced poverty in 2015, which was over twice the national average at the time. In 2020, the US Supreme Court case Bostock v. Clayton County (2020) was one of the first cases to rule that discrimination based on gender identity in employment is illegal across the US.

Gender diverse people in the US also face much higher rates of violence throughout their lives than the rest of the population. Forty-six percent of respondents in the 2015 US Transgender Survey reported having been verbally harassed in the year prior to taking the survey, as well as nine percent reporting being physically attacked because they were transgender. During that same period, ten percent of respondents reported having been sexually assaulted, and nearly half of the participants reported having been sexually assaulted at some point in their lifetime. Additionally, since 2013, the number of hate crimes committed against transgender and non-binary people has steadily risen. Forty-four transgender and non-binary people died by violent means in the US in 2020, the highest number of fatal deaths since 2013 according to the Human Rights Campaign, with the majority being Black and Latinx transgender women. Fifty-seven transgender and non-binary people were killed in 2021. Additionally, many deaths likely go unreported.

Thus, even as recognition of gender diverse people is rising, the fear of violence and discrimination is still pervasive among most gender diverse people and can take a serious toll on their mental health according to the Center for American Progress. Nearly forty percent of transgender people surveyed in 2015 reported having attempted suicide at some point in their life, a rate nine times higher than the attempted suicide rate in the US population at large. However, seeking treatment for mental health is difficult for many gender diverse people. Even those who can afford healthcare face other barriers, as over thirty percent of transgender people in 2015 reported mistreatment when seeking healthcare because of their identity. Additionally, as of April 2022, there are twenty-five bills under consideration in US state legislatures that restrict gender-affirming healthcare for transgender youth that has been shown to be vital to many transgender kids’ mental health.

Despite so many challenges, gender diverse activists are increasingly gaining recognition and using their platforms to support gender diverse communities and challenge stereotypical expectations of binary gender roles. Actresses like Angelica Ross and Laverne Cox have increased visibility of transgender people through their roles playing transgender characters on popular television shows, as well as working as transgender activists outside of their on-screen performances. Ross also founded an organization called TransTech Social Enterprises dedicated to training transgender and other marginalized people with the skills that empower them to overcome poverty. Writers like Alok-Vaid Menon and Tyler Ford challenge binary gender expectations through the stories and perspectives they share. Menon writes extensively about the history of gender norms and sexism through their poetry and book, Beyond the Gender Binary , and Ford writes for the popular LGBTQ+ publication Them . Organizers like AC Dumlao, who runs cultural competency workshops at the Transgender Legal Defense & Education Fund, an organization dedicated to ending discrimination, share resources to educate gender diverse people and their families, as well as works to connect gender diverse people with lawyers who can help get their name changed. Those are just the names of a few prominent organizers and activists as of 2022, but there are countless more people advocating for the recognition, acceptance, and equality of all people no matter their gender.

Beliefs that gender and biological sex were both binary categories fixed at birth have contributed to the oppression of women and gender diverse people in the US for centuries. However, feminists and LGBTQ+ activists have reframed those concepts, allowing gender and biological sex to be tools for self-expression and defining an individual’s identity. As of 2022, biological sex is widely seen as referring to a person’s physical characteristics, and various LGBTQ+ activists, medical professionals, and anatomists are pushing for society to understand biological sex itself as a spectrum rather than a binary. Gender is increasingly put forth as a category a person can choose to identify and express themselves as, often in line with adopting certain societal expectations of a gender or rejecting gendered expectations of behaviors and appearances altogether. The definitions of both biological sex and gender remain widely contested as of 2022, though, and likely will continue to evolve into the future.

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INFORMATION FOR

• Residents & Fellows
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What Do We Mean By Sex and Gender?

Words matter.

At Women’s Health Research at Yale, we are committed to advancing the health of a diverse society. We do this in large measure by studying the health of women and the similarities and differences in health outcomes between and among women and men. As we pursue our work, it is particularly important to use language that captures the different concepts of sex and gender so that our science and our findings can be more precise and better serve everyone.

What do we mean by sex and gender? Aren’t these terms interchangeable? Perhaps at some point in time they were used as synonyms, but this is no longer true in science.

In 2001, a committee convened by the Institute of Medicine (IOM), a nonprofit think tank that took on issues of importance to the national health, addressed the question of whether it mattered to study the biology of women as well as men .

The IOM, now embedded within the National Academies of Science, Engineering, and Medicine (NASEM), concluded there was more than sufficient evidence that, beyond reproductive biology, there were major differences in the biology of women and men that greatly affected their health and influenced treatment and prevention strategies.

Importantly, the committee emphasized that neither the health of women nor men is simply a product of biology but is also influenced by sociocultural and psychological experience. To differentiate between these broad areas of investigation, the members created working definitions of “sex” — when referring to biology — and “gender” — when referring to self-representation influenced by social, cultural, and personal experience.

The committee advised that scientists use these definitions in the following ways:

• In the study of human subjects, the term sex should be used as a classification, generally as male or female, according to the reproductive organs and functions that derive from the chromosomal complement [generally XX for female and XY for male].
• In the study of human subjects, the term gender should be used to refer to a person's self-representation as male or female, or how that person is responded to by social institutions on the basis of the individual's gender presentation.
• In most studies of nonhuman animals, the term sex should be used.

These working definitions were a good start in recognizing the value of studying sex and gender and their interactions, yet they were always meant to evolve. Now, we are learning more about ourselves and so must adapt our terminology to be inclusive, respectful, and more accurate.

For example, while most people are born biologically female or male, rare biological syndromes can result in genital ambiguity. Or a resistance to a sex hormone can result in traits typical of the opposite biological sex.

Moreover, while an individual’s internal sense of gender can be female or male, some people identify as nonbinary — neither female nor male. Other individuals can identify as a gender that is the same as (cisgender) or different from (transgender) the one assigned at birth. These terms are separate from an individual’s sexual orientation, which describes a person’s emotional, romantic and/or physical attachments (such as straight, lesbian, gay, asexual, bisexual, and more).

In science, as our understanding grows, so must the precision of our language in communicating what we know.

• Women's Health

Featured in this article

• Carolyn M Mazure, PhD Norma Weinberg Spungen and Joan Lebson Bildner Professor in Women's Health Research and Professor of Psychiatry and of Psychology

Terminology

Here are some current terms defined in Yale’s “Guide to Gender Identity and Affirmation in the Workplace” that can help us all be more precise and respectful of everyone.

Cisgender : A term used to describe an individual whose gender identity aligns with the one typically associated with the sex assigned to them at birth. This is a term that is preferable to “non-trans,” “biological,” or “natal” man or woman.

Gender nonconforming : A person who views their gender identity as one of many possible genders beyond strictly female or male.

Transgender : A term that may be used to describe people whose gender expression does not conform to the cultural norms and/or whose gender identity is different from their sex assigned at birth. Transgender is also considered by some to be an “umbrella term” that encompasses a number of identities which transcend the conventional expectations of gender identity and expression, including transgender man, transgender woman, genderqueer, and gender expansive. People who identify as transgender may or may not decide to alter their bodies hormonally and/or surgically to match their gender identity. Sometimes shortened to the term “trans.”

Intersex : Describing a person whose biological sex is ambiguous. There are genetic, hormonal or anatomical variations that can make a person’s sex ambiguous (e.g., Klinefelter Syndrome, Adrenal Hyperplasia).

SEX ASSIGNED AT BIRTH

Jessica a. clarke*.

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.

* Cornelius Vanderbilt Chair in Law, Vanderbilt University Law School. I am grateful to Anne Alstott, Lisa Bressman, June Carbone, David Cruz, Robin Dembroff, Bill Eskridge, Katie Eyer, Marie-Amélie George, Sasha Gombar, Katrina Karkazis, Ido Katri, Anna Lvovsky, Laura Portuondo, Austin Reagan, Vicki Schultz, Meredith Severtson, Naomi Schoenbaum, Maayan Sudai, Ezra Young, and workshop participants at Boston College Law School, Boston University School of Law, University of Kansas School of Law, Temple University Beasley School of Law, and Vanderbilt University Law School for their feedback on this project. Thanks to Katie Hanschke and Ashli Thomas Wells of the Vanderbilt Law Library for research assistance.

Introduction

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 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”). ... Close 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 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”). ... Close 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 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]. ... Close 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 4 See infra section I.C. ... Close Assigned sex is opposed to gender identity—an individual’s own internal sense of whether they are a man, a woman, or nonbinary. 5 5 GLAAD, Glossary of Terms: Transgender, GLAAD Media Reference Guide (11th ed.), https://www.glaad.org/reference/trans-terms [https://perma.cc/99U2-TMLC] (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”). ... Close 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 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. ... Close It appeared in 2016 regulations interpreting the Affordable Care Act 7 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. ... Close and in a proposed 2021 federal law that would bar discrimination on the basis of LGBTQ status in employment, health care, and housing. 8 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.”). ... Close

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 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. ... Close Instead, it proceeded on the assumption that “sex” refers “only to biological distinctions between male and female.” 10 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. ... Close 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 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.”). ... Close

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 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), https://www.forbes.com/‌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://‌www.nbcnews.com/nbc-out/out-politics-and-policy/nearly-240-anti-lgbtq-bills-filed-2022-far-targeting-trans-people-rcna20418 [https://perma.cc/8KBX-3V9K] (discussing the rise in state anti-LGBTQ bills in response to Bostock , including 238 in the first three months of 2022 alone). ... Close almost all of it directly invoking the idea of “biological sex.” 13 13 See infra notes 144, 147–149 (quoting recent state laws invoking “biological sex” to restrict transgender rights). ... Close 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 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). ... Close two laws banning certain forms of gender-affirming health care for transgender youth, 15 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), https://texasattorneygeneral.gov/sites/‌default/files/global/KP-0401.pdf [https://perma.cc/BBM2-GY9P] (“[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), https://www.lambdalegal.org/blog/‌abbott_tx_20220608_‌pflag-ll-and-aclu-file-new-lawsuit-to-stop-tx-from-persecuting-parents-with-transgender-kids [https://perma.cc/K27V-RSPC]. ... Close three laws limiting restroom access, 16 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). ... Close and three laws restricting a person’s ability to change the sex designation on their identification documents. 17 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). ... Close 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 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.). ... Close the Eleventh Circuit is currently reconsidering the issue en banc. 19 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). ... Close 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 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”). ... Close

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 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). ... Close and gender-nonconforming people. 22 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”). ... Close

One contribution of this Article is to excavate the histories of the concepts of “biological sex” and “sex assigned at birth.” 23 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.” ... Close 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 24 See infra notes 144, 147–149 (collecting statutes). ... Close 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 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). ... Close 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 26 See infra section I.B. ... Close 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 27 See infra notes 144–150 and accompanying text (collecting statutes invoking “biological sex” without any consistent definition of that term). ... Close 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 28 What Is Intersex?, InterACT, https://interactadvocates.org/faq/#definition [https://‌perma.cc/NAK2-93G9] (last updated Jan. 26, 2021). ... Close 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 29 See infra sections I.B–.C. ... Close It now competes with the term “biological sex” in legal disputes over transgender rights. 30 30 See infra section I.C. ... Close

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 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. ... Close “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 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. ... Close And it is inconsistent with medical research to assert that gender identity has no biological underpinnings. 33 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), https://www.nytimes.com/‌‌2018/10/22/health/transgender-trump-biology.html (on file with the Columbia Law Review ). ... Close 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 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). ... Close 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 35 Cf. Joseph Raz, The Morality of Freedom 370 (1986) (“The autonomous person is part author of his life.”). ... Close as well as postmodern sensibilities about the role of creativity and play in constructing the self. 36 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”). ... Close 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 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”). ... Close 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 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”). ... Close 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 39 See infra Part III. ... Close 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 40 See infra section III.A. ... Close 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 41 See infra Part IV. ... Close 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 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”). ... Close 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 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). ... Close Transgender people continue to face uniquely high rates of discrim­ination, harassment, and violence with devastating consequences. 44 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), http://www.transequality.org/sites/default/files/docs/usts/USTS%20Full%20Report%20-%20FINAL%201.6.17.pdf [https://perma.cc/2MKH-MN2C] (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), https://williamsinstitute.law.ucla.edu/‌wp-content/uploads/Trans-Pop-Update-Jun-2022.pdf [https://perma.cc/KE9F-RBSF]. ... Close 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 45 See supra notes 21–22 (defining these terms). ... Close 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 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://sfonline.barnard.edu/sex-gender-and-the-biosocial-turn/ [https://‌‌perma.cc/3Q8H-84ZC] (discussing feminist criticisms of the concept of “sex as a biological variable”). ... Close

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.