What Is Gender? Biology, Psychology, and Identity Explained

In 1955, the psychologist John Money at Johns Hopkins introduced a distinction that would transform how medicine, psychology, and eventually ordinary language think about human identity: the distinction between biological sex and psychological gender.

Money proposed "gender role" as the psychological and social expression of being male or female - something distinct from chromosomes, hormones, and anatomy.

The concept emerged from his clinical work with intersex patients, people whose biological sex characteristics did not fit neatly into the male/female binary, and who required terminology for the fact that their psychological sense of themselves could develop somewhat independently of their anatomical characteristics.

Within a generation, the concept of gender had escaped from clinical medicine into the center of culture, law, and politics. Robert Stoller's 1968 book "Sex and Gender" popularized the distinction for a broader audience.

Second-wave feminism seized on it: if gender roles were socially constructed rather than biologically determined, then the subordination of women was not natural or inevitable but the product of social arrangements that could be changed.

Judith Butler's "Gender Trouble" (1990) radically theorized how gender itself was performed and constituted through repetition of social norms.

And beginning in the 1990s and accelerating rapidly into the 21st century, the concept of gender was extended to encompass gender identity in the sense developed by transgender communities - the internal psychological experience of one's own gender as potentially distinct from the sex assigned at birth.

Few distinctions have proved more consequential, or more contested. The claim that gender is distinct from biological sex has been foundational to feminist theory, gay and lesbian studies, transgender medicine, and legal anti-discrimination frameworks.

It has also been the target of sustained criticism from those who argue that collapsing sex and gender, or treating gender as fully independent of biological sex, obscures biological realities that matter for medicine, law, and policy.

Understanding what "gender" means, what the evidence shows about its origins and nature, and why the debates are so heated requires engaging carefully with a body of research that spans biology, psychology, anthropology, philosophy, sociology, and medicine.

"Gender is the repeated stylization of the body, a set of repeated acts within a highly rigid regulatory frame that congeal over time to produce the appearance of substance, of a natural sort of being." - Judith Butler, Gender Trouble (1990)

Key Definitions

Biological sex: the cluster of physical characteristics associated with maleness or femaleness: chromosomal sex, gonadal sex, hormonal profiles, and anatomical characteristics of the reproductive system.

Gender identity: a person's internal, psychological sense of their own gender - whether they experience themselves as a man, woman, nonbinary person, or some other gender.

Gender role: the behaviors, activities, and attributes that a given society considers appropriate for people of a given gender.

Gender expression: the external presentation of gender through clothing, hairstyle, behavior, and mannerisms.

Intersex: a range of conditions in which a person is born with biological sex characteristics that do not fit typical definitions of male or female; estimated at 1.7% of the population by broad definition, 0.018% by narrow medical definition.

Binary vs. non-binary gender: whether gender is organized into exactly two categories (man and woman) or whether additional categories are recognized or experienced.

Cisgender vs. transgender: cisgender refers to people whose gender identity aligns with their sex assigned at birth; transgender refers to people whose gender identity differs from their sex assigned at birth.

Gender dysphoria (DSM-5): clinically significant distress arising from the incongruence between a person's experienced gender and their sex assigned at birth.

Gender performativity (Butler): the theoretical claim that gender is not the expression of a prior inner essence but is constituted through the repeated, regulated enactment of gender norms.^[1]

Third gender categories: culturally recognized gender identities beyond the male/female binary, appearing across numerous human societies.

Gender socialization: the process by which individuals learn and internalize the gender norms, roles, and expectations of their society.

Sex assignment: the designation of a person as male or female at birth, typically based on anatomy.

Gender gap (economic): the average difference in earnings, employment, and economic status between men and women.

The Sex/Gender Distinction: Origins and Significance

John Money's 1955 introduction of the gender role concept was not a philosophical intervention but a clinical one.^[2]

He was working with patients born with ambiguous genitalia whose anatomical characteristics were surgically modified in infancy, often to conform to female anatomy because it was technically easier to construct than male anatomy.

The question he confronted was whether these patients would develop a feminine or masculine psychology despite the mismatch between their chromosomal and gonadal sex and their surgically modified anatomy.

His clinical observations led him to conclude that psychological gender identity was substantially shaped by the social environment in which a child was raised - that it was more malleable than the chromosomal and gonadal facts suggested.

Robert Stoller's 1968 "Sex and Gender" popularized Money's clinical concepts and added the theoretical framework that gender is a psychological overlay on biological sex, capable of diverging from it in certain individuals.

For feminist theorists of the 1970s and 1980s, particularly Gayle Rubin in her influential 1975 essay "The Traffic in Women," the sex/gender distinction was politically essential: it established that the social arrangements that subordinated women were not natural extensions of biological difference but cultural constructions that bore no necessary relationship to biology.

Biology might determine sex; social organization determined gender; and social organization could be changed.

The theoretical clarity of this distinction has been complicated by subsequent developments. First, biological research has shown that hormonal sex - particularly testosterone exposure in utero - has measurable effects on behavior, interests, and psychological characteristics that go beyond the narrow categories of reproductive function.

Second, transgender experience and theory has complicated the feminist framework in which sex was taken as the stable foundation on which gendered oppression was built: if gender identity can diverge not just from social gender assignment but from the phenomenology of one's own sexed body, the relationship between sex and gender becomes more complex than either biological determinism or strong social constructionism captures.

The Biology of Sex

Biological sex is not a single characteristic but a cluster of related characteristics that typically co-occur but need not. Understanding this cluster is essential for evaluating claims about both gender and sex.

Chromosomal sex is typically XX or XY, but variations are more common than many people realize. Klinefelter syndrome (XXY) occurs in approximately 1 to 2 per 1000 male-typical births. XYY syndrome occurs in approximately 1 per 1000 male-typical births.

Turner syndrome (X0) occurs in approximately 1 per 2500 female-typical births. These chromosomal variations produce a range of effects on development, fertility, and physical characteristics, but most people with these conditions identify with conventional gender categories without ambiguity.

Gonadal sex - the presence of ovaries or testes - is typically determined by chromosomal sex through the SRY gene on the Y chromosome, which triggers testicular development in utero. Hormonal sex follows from gonadal development: testes produce testosterone and anti-Mullerian hormone; ovaries produce estradiol and progesterone.

The hormonal environment in utero affects brain development as well as somatic development: testosterone exposure in the second trimester of pregnancy appears to influence the development of hypothalamic structures and has measurable effects on subsequent behavior and cognitive preferences.

The evidence for hormonal effects on gendered psychology comes most clearly from studies of congenital adrenal hyperplasia (CAH), a condition in which XX individuals are exposed to elevated androgens (male sex hormones) in utero due to a deficiency in cortisol synthesis.

Girls with CAH show significantly higher rates of gender-atypical toy preferences, play behavior, and occupational interests compared to unaffected female siblings, and higher rates of transgender identity compared to the general female population.

These effects are not entirely attributable to parental socialization and appear to reflect direct effects of prenatal androgen exposure on developing brain systems. This is among the strongest evidence that hormonal biology affects psychological characteristics associated with gender.

Intersex conditions - differences of sex development - illustrate the complexity of the biological sex cluster. Anne Fausto-Sterling's widely cited estimate of 1.7% of the population being intersex by a broad definition includes conditions like late-onset CAH, Klinefelter syndrome, and polycystic ovary syndrome.^[6]

By narrower definitions limited to conditions involving ambiguous external genitalia, the figure is closer to 0.018%.

What the intersex literature establishes is that the biological sex characteristics do not always cluster in the expected directions - which complicates any simple binary account of biological sex without implying that biological sex is not real or that it cannot be meaningfully defined.

Cross-Cultural Gender Categories

Cross-cultural evidence about gender is one of the most important and most frequently misread bodies of evidence in the gender debates. It is important because it establishes the extent to which gender is culturally variable; it is frequently misread because the implications of that variability are contested.

The existence of third gender categories in many human cultures is well-documented and important.

The hijra of South Asia - a category including transgender women, intersex individuals, and others who take on feminine roles - have been recognized for centuries in South Asian societies and hold a legally recognized status in India, Pakistan, and Bangladesh following court rulings in the 2010s.

The social and religious role of hijra is complex and does not map straightforwardly onto Western transgender identity: hijra are associated with specific ritual functions, with particular social marginalization as well as a degree of social power, and with a set of social practices that make them a cultural category of their own rather than simply "transgender women" in Western terms.

Two-Spirit people in Indigenous North American societies occupy roles that differ from tribe to tribe but in many cases involve taking on gender roles associated with both male and female, often with ceremonial or spiritual significance.

The term "Two-Spirit" was adopted by Indigenous communities in 1990 as a pan-Indian term for these roles; traditional tribal terms vary considerably, and some scholars caution against over-generalizing across the diversity of Indigenous practices.

Fa'afafine in Samoa are biological males who take on feminine roles and are well-integrated into Samoan family and social life; evolutionary psychologist Paul Vasey and colleagues have conducted extensive research on their social behavior, finding patterns that are neither simply "male" nor "female" in Western terms.

Kathoey in Thailand and muxe among the Zapotec of Mexico are further examples of culturally recognized gender categories that do not map neatly onto Western binary categories.

Gilbert Herdt's edited volume "Third Sex, Third Gender" (1994) provides the most comprehensive scholarly treatment of this cross-cultural literature.^[7]

What does this evidence establish? It establishes that the specific content of gender roles varies substantially across cultures, that the binary organization of gender into exactly two categories is not a universal feature of human social organization, and that third and additional gender categories can be fully integrated into social systems.

What it does not establish is that biological sex differences have no influence on gender, or that gender is entirely arbitrary social construction: every known human society distinguishes between male and female roles in some way, and the cross-cultural categories that go beyond this binary are present in a minority of societies, not universal.

Butler's Gender Performativity

Judith Butler's "Gender Trouble: Feminism and the Subversion of Identity," published in 1990, is the most influential theoretical text in the academic study of gender of the past several decades.

It introduced the concept of gender performativity, which has been enormously generative for queer theory, transgender studies, and feminist theory, while also being subject to persistent misreading.

Butler's starting point is a critique of the feminist assumption that there is a stable category "women" whose shared interests and identity can ground feminist political action. The problem, Butler argues, is that this assumption reproduces the very gender binary that feminism aims to deconstruct.

More fundamentally, she questions whether gender is an expression of a prior inner essence - a natural identity that one "is" and then expresses through behavior - or whether it is something constituted through repeated acts that produce the appearance of such an essence retroactively.

Her answer is the latter: gender is performative.

Drawing on J.L. Austin's theory of performative speech acts (utterances that do not describe a reality but bring it into existence - as in "I now pronounce you married") and Michel Foucault's account of how norms produce subjects rather than merely regulating pre-existing ones, Butler argues that the repeated enactment of gender norms through gesture, dress, speech, and behavior is what produces the appearance of a stable gender identity.

There is no "doer" behind the deed; the subject is constituted through the repetition of the performance rather than existing prior to it and choosing to perform.

Drag is Butler's key illustration, though it has been persistently misunderstood. She does not say that all gender is like drag in the sense of being theatrical or self-conscious performance.

Rather, drag reveals by exaggeration what is true of all gender performance: that it is imitative, that the masculine and feminine "originals" it appears to imitate are themselves performances without originals.

A feminine drag performance mimics femininity; cisgender women also perform femininity; neither is performing a pre-existing "natural" femininity, because femininity is a set of norms that are performed and thereby reproduced rather than expressed from an inner essence.

The most common misreading of Butler is that she is saying gender is a free choice, that individuals can simply decide to perform a different gender. She denied this consistently: performance in her sense is compelled, constrained by social norms that produce subjects rather than being enacted by pre-existing free subjects.

The performance is not free; the penalties for non-normative gender performance - social exclusion, violence, economic penalty - are real and severe.

Butler's later work, particularly "Bodies That Matter" (1993), engaged more seriously with the materiality of sexed bodies and the question of how her account of performance applies to transgender experience.

Transgender Identities and Gender Dysphoria

Transgender identities - experiences in which gender identity does not align with sex assigned at birth - have become significantly more visible in Western countries over the past two decades, with survey data showing prevalence increasing particularly among younger cohorts.

The Williams Institute estimated in 2022 that approximately 1.6 million adults in the United States identify as transgender, representing about 0.6% of the adult population, with higher rates among adults under 35.

The DSM-5 diagnostic category of gender dysphoria applies to people who experience clinically significant distress arising from the incongruence between their experienced gender and their birth-assigned sex.

The diagnosis requires both the incongruence and the distress - a recognition that gender non-conformity and transgender identity per se are not pathological. The diagnostic category serves primarily as the gateway to medical treatments that require clinical justification.

The neurobiological evidence for some degree of biological basis for transgender identity, while not conclusive, is suggestive. Zhou et al.

(1995) published post-mortem findings that the bed nucleus of the stria terminalis (BSTc) - a brain region implicated in sexual behavior and affected by testosterone exposure in utero - was female-typical in size in transgender women, even after controlling for hormone therapy received during life.^[8]

Savic and Arver (2011) replicated aspects of this finding and extended it to additional brain regions and connectivity patterns.^[9]

These findings are consistent with the hypothesis that transgender identity has a neurobiological component related to prenatal hormonal exposure patterns, though the sample sizes in post-mortem studies are necessarily small and replication is incomplete.

The evidence on outcomes for transgender people who receive gender-affirming care is the subject of active clinical and political debate.

Turban and colleagues' 2022 PLOS ONE study (doi: 10.1371/journal.pone.0261039) found that access to gender-affirming hormone treatment during adolescence was significantly associated with lower rates of suicidal ideation and depressive symptoms in a large retrospective survey of transgender adults.^[3]

Coleman et al.'s WPATH Standards of Care 8 (2022) summarize the professional consensus supporting gender-affirming care across the lifespan.^[10]

The contested area involves puberty-suppressing medications in early adolescence.

Critics point to research suggesting that a substantial proportion of gender-dysphoric children who present before puberty do not persist in transgender identity into adulthood; if this is correct, puberty blockers may be administered to some children who would not ultimately require transition.

The UK Cass Review (2024), commissioned by the National Health Service, concluded that the evidence base for puberty blockers in gender-dysphoric youth was insufficiently robust to justify routine clinical use outside of research protocols.

Sweden, Finland, Norway, and Denmark reached similar conclusions. The clinical implications are genuinely contested and not resolvable by simple appeal to authority on either side of the debate.

Gender and Society: Roles, Gaps, and Socialization

The social dimensions of gender - how gender roles are reproduced through socialization, what the economic consequences of gender are, and what the evidence shows about gender differences in behavior - constitute a distinct body of research from the identity and biology questions, though they are entangled with them.

Gender socialization research has established that children are socialized into gender roles from infancy through mechanisms that are pervasive, often subtle, and difficult to disentangle from biological predispositions.

Sandra Bem's gender schema theory (1983) proposed that children develop cognitive schemas for gender early in life and use these schemas to organize their experience of themselves and the world.

Gender typing - the process by which children come to prefer gender-typical toys, activities, and attributes - involves both child-driven processes (children appear to actively seek out gender-typical content once they have developed gender schemas) and adult-driven processes (parents and peers differentially encourage and reward gender-typical behavior).

The economic gender gap is one of the most extensively studied empirical phenomena in social science.

Claudia Goldin, whose research on gender and economic inequality earned the 2023 Nobel Prize in Economics, demonstrated in her 2014 American Economic Review paper (doi: 10.1257/aer.104.4.1091) that the remaining gender pay gap in advanced economies is primarily explained not by direct discrimination against women doing identical work but by the structure of compensation in high-paying occupations.

Jobs requiring long, inflexible, immediately available work - what Goldin calls "greedy jobs" - pay disproportionately more per hour than jobs with flexible, predictable schedules.^[4]

Women are underrepresented in greedy jobs primarily because they bear a disproportionate share of caregiving responsibilities that are incompatible with greedy job demands.

The implication is that closing the gender pay gap requires not primarily antidiscrimination enforcement but restructuring how high-paying jobs are organized and how caregiving is distributed.

The "motherhood penalty" documents a related asymmetry. Correll, Benard, and Paik's 2007 American Journal of Sociology study (doi: 10.1086/511799) used audit methodology - sending fictitious resumes that varied only in whether they indicated parenthood - and found that mothers were offered lower starting salaries and rated as less competent and less committed than otherwise identical non-mothers.^[5]

Fathers received a slight premium rather than a penalty.

The estimated wage penalty of approximately 5% per child for mothers, and the 6% bonus per child for fathers, represents a substantial effect not explained by productivity differences.

Data from the Bureau of Labor Statistics consistently shows that women do substantially more unpaid domestic labor than men even when both partners are employed full-time.

Alice Eagly's social role theory argues that gender differences in behavior and self-presentation arise primarily from the differential roles that men and women occupy in society, including the caregiving role: the traits required for successful caregiving (warmth, nurturance, communality) become associated with femininity because women disproportionately perform caregiving work.

Contemporary Debates

The contemporary cultural and political debates about gender are often more heated than the underlying empirical and philosophical questions would seem to warrant, partly because the debates involve real stakes for real people - including transgender people who face discrimination and violence, children undergoing medical decisions with long-term consequences, and women whose interests in sex-based protections are genuine.

And partly because the different questions involved (about biology, identity, clinical medicine, and social policy) are frequently conflated.

The debate between gender-critical feminism and trans-inclusive feminism involves a genuine philosophical disagreement about the basis of the category "woman" and its relationship to biological sex.

Gender-critical feminists, drawing on a tradition of feminist political theory that treats the subordination of women as grounded in biological sex, argue that the legal and social category "woman" must remain defined by biological sex to protect the rights and safety of female people.

Trans-inclusive feminism argues that this position excludes transgender women from protections and recognition they require, and that the political alliance between women as a class should include rather than exclude trans women.

The philosophical disagreement is real and not fully resolvable by appeal to science: it involves normative questions about which interests and harms should be weighted how heavily in designing legal categories.

Healthcare for gender-dysphoric children involves different considerations from the philosophical debate about identity: the weight of clinical evidence, the adequacy of informed consent for minors, the significance of desistance data, and the appropriate role of parental, medical, and governmental authority in treatment decisions.

Sports policy, prison policy, and the organization of single-sex spaces involve yet further distinct considerations about physiological differences, safety, and inclusion.

None of these questions has a simple answer derivable from first principles about gender identity, and the quality of discourse is not improved by treating them as though they do.

Connections

For a broader account of feminist theory and the history of women's movements, see what is feminism. For philosophical questions about personal identity that intersect with gender identity debates, see what is the self.

For a broader account of how cultural frameworks shape cognition and identity, see why cultures think differently.

Sources & Further Reading

1. Butler, J. (1990). Gender Trouble: Feminism and the Subversion of Identity. Routledge.
2. Money, J., Hampson, J. G., & Hampson, J. L. (1955). An examination of some basic sexual concepts: The evidence of human hermaphroditism. Bulletin of the Johns Hopkins Hospital, 97(4), 301-319.
3. Turban, J. L., King, D., Reisner, S. L., & Keuroghlian, A. S. (2022). Access to gender-affirming hormones during adolescence and mental health outcomes among transgender adults. PLOS ONE, 17(1). DOI: 10.1371/journal.pone.0261039
4. Goldin, C. (2014). A grand gender convergence: Its last chapter. American Economic Review, 104(4), 1091-1119. DOI: 10.1257/aer.104.4.1091
5. Correll, S. J., Benard, S., & Paik, I. (2007). Getting a job: Is there a motherhood penalty? American Journal of Sociology, 112(5), 1297-1338. DOI: 10.1086/511799
6. Fausto-Sterling, A. (2000). Sexing the Body: Gender Politics and the Construction of Sexuality. Basic Books.
7. Herdt, G. (Ed.). (1994). Third Sex, Third Gender: Beyond Sexual Dimorphism in Culture and History. Zone Books.
8. Zhou, J. N., Hofman, M. A., Gooren, L. J., & Swaab, D. F. (1995). A sex difference in the human brain and its relation to transsexuality. Nature, 378(6552), 68-70.
9. Savic, I., & Arver, S. (2011). Sex dimorphism of the brain in male-to-female transsexuals. Cerebral Cortex, 21(11), 2525-2533.
10. Coleman, E., et al. (2022). Standards of care for the health of transgender and gender diverse people, version 8. International Journal of Transgender Health, 23(S1), S1-S259.