What are the medical and scientific definitions of biological sex and gender identity?
Executive summary
Medical and scientific literature distinguishes biological sex — a set of physical, genetic and reproductive characteristics — from gender identity — a person’s internal sense of being a man, woman, both, neither or another identity — but both concepts are complex, overlapping, and debated across disciplines and policy arenas [1] [2] [3].
1. What medical science means by “biological sex”
Biological sex is conventionally defined in medicine and biology as the classification of organisms as male or female based on physical and physiological characteristics such as chromosomes, gonads, hormones and reproductive anatomy, and is used as a variable in health research because it often correlates with disease risk and treatment response [3] [1] [2]. Major institutions and reviews describe sex as a construct that usually groups people into two categories (male and female) while acknowledging variation: chromosomal patterns are typically XX or XY but other combinations exist, and differences of sex development (intersex) produce a spectrum of biological presentations that do not fit a strict binary [1] [4] [5].
2. What medical and scientific communities mean by “gender identity”
Gender identity is a person’s internal, deeply felt sense of being a woman, man, nonbinary or another identity, and is treated in clinical and public-health contexts as self‑identified, potentially fluid over a lifetime, and distinct from the biological characteristics used to assign sex at birth [2] [6] [7]. Social-science and health research frames gender more broadly as socially constructed roles, norms and expectations that shape behavior and institutional practices, with gender identity being one component of that broader social category [1] [7].
3. Where sex and gender overlap, and where they diverge
In most people the biological markers of sex align with gender identity, but exceptions are clinically and socially significant: transgender people have a gender identity that differs from the sex assigned at birth, and intersex people have biological traits that complicate a simple male/female assignment [2] [1]. Research and policy guidance therefore recommend treating sex and gender as distinct variables — for example, collecting both assigned sex and self‑reported gender in health surveys — because conflating them can obscure important biological and social determinants of health [8] [1].
4. Scientific evidence about origins of gender identity and ongoing debates
Empirical work indicates biological contributions to gender identity — including genetic, hormonal and neuroanatomical factors — but the specific mechanisms are not fully demonstrated and remain a subject of active research and debate; many reviewers emphasize a multifactorial model in which biology interacts with social influences [9] [10]. Authors caution that human gender identity cannot be fully modeled in animals and that samples and methods vary, so conclusions are provisional even as evidence points to a significant biological component for some individuals [9] [8].
5. Policy, medical practice, and contested framings
Policy and advocacy actors interpret the scientific distinction differently: major public‑health bodies and academic institutions treat sex and gender as distinct to improve research and care, while some organizations and recent political actions argue for a strictly biological, immutable definition of sex; these conflicting framings reflect diverging agendas about patient care, data collection, and civil rights and must be evaluated against the underlying evidence and stated goals of each source [1] [11] [12]. Medical guidance also notes practical implications — for example, sex‑based differences in disease outcomes and the need to account for hormone use or gender-affirming care in clinical data — and flags ethical issues around confidentiality and stigmatization when recording sex and gender [1] [2] [8].
6. Conclusion: clean definitions, messy realities
The clearest, evidence‑backed working definitions are that sex denotes biological attributes used in medicine and biology, and gender identity denotes an individual’s self‑identified sense of gender and related social roles, but biology is not always neatly binary and gender identity has both psychological and social dimensions; science recognizes both distinctness and interaction, and contemporary practice favors measuring and addressing both sex and gender to serve health, rights and research integrity [1] [2] [8].