Are there more then two sexes?
Executive summary
The short, evidence-based answer: under the classical biological definition tied to gamete type—small sperm-producing versus large egg-producing cells—there are two sexes in sexually reproducing species, including humans [1] [2]. At the same time, a substantial body of contemporary research and reporting emphasizes that human sexual biology includes a variety of developmental differences (intersex/DSDs), mosaics, and hormone/receptor variations that make strict binary classification incomplete for some people, and therefore scientists and commentators disagree about whether to count “more than two” sexes or to treat sex as a spectrum [3] [4] [5].
1. The definition that matters: gametes and the two‑sex model
Many evolutionary biologists define sex by gamete type—anisogamy—so where reproduction involves small mobile gametes and large immobile gametes, the evolutionary strategies map to two sexes (male and female); by that operational definition the number of sexes remains two [1] [2]. That definition is widely used because it captures the functional reproductive roles that are comparable across taxa and underpins much of evolutionary theory [1].
2. Why some scientists and journals say “sex is more complicated”
Recent reviews and popular pieces in outlets such as Scientific American argue that human sexual development is multi‑factorial—chromosomes, gonads, hormones, receptors, anatomy and gene expression can point in different directions—so many researchers describe human sex as more complex than a simple binary and sometimes as a spectrum of traits rather than discrete boxes [3] [6]. Those accounts emphasize unresolved questions (for example, effects of cellular mosaics or microchimerism) and call for definitions that reflect biological diversity rather than crude categories [3].
3. Intersex conditions: variation that challenges a strict binary
Medical and research summaries note a wide range of differences of sex development (DSDs) and intersex traits—chromosomal aneuploidies, androgen insensitivity, gonadal variations—that produce bodies not easily assigned to “male” or “female” at birth; CNRS and other scientific sources treat the question as complex and note that intersex traits affect a non‑trivial fraction of people (estimates vary) and cover a spectrum of severity and frequency [4] [5]. These biologically real variations are central to arguments that the binary fails to capture human heterogeneity [4] [5].
4. Strong critiques defending two sexes and the limits of “many sexes” claims
Other observers and commentators push back: critics say proposals to enumerate more than two sexes often conflate social/identity categories with biological sex or treat rare anomalies as separate sexes rather than variations within male and female categories [7] [8]. Scholars caution that acknowledging variation need not collapse the functional two‑sex framework used in many biological contexts—the presence of DSDs or mosaics does not by itself create a set of equivalent, reproductively distinct “third” or “fourth” sexes in the evolutionary sense [7] [2].
5. Where the disagreement really lies: definition, purpose, and context
The clearest source of disagreement is definitional and pragmatic: if the question asks about reproductive categories defined by gametes, the answer is two; if the question asks whether all human bodies neatly fit those two boxes, the empirical answer is “no”—there are many forms of sexual differentiation that resist binary assignment [1] [4]. Public debates often collapse those distinctions, with ideological actors on all sides framing the science to support broader social or political aims [7] [9].
6. Bottom line for readers trying to reconcile competing claims
A defensible synthesis: biologically and evolutionarily, sex in anisogamous species is twofold, but human sexual development includes medically and socially significant variation—intersex conditions, chromosomal differences, and developmental mosaics—that make a strict binary an imperfect descriptive tool for every individual; whether that justifies counting “more than two sexes” depends on which definition one adopts and what purpose the classification serves [1] [3] [4]. Reporting and policy should be explicit about definitions and avoid conflating gamete‑based taxonomy with identity, legal, or social categories [6] [9].