Summary

Wright argues that sex is binary in every species with two different-sized sex cells. 'Male' and 'female' simply name the two kinds: bodies built to make sperm and bodies built to make eggs. Nothing else counts as a sex. He separates how sex develops (through chromosomes, temperature and so on) from what sex is (which sex cell the body is built to make). He then answers five challenges, from fungi and people with an extra X chromosome to differences of sex development and models that treat sex as a mix of traits. He concludes that every sensible alternative definition still relies on sperm and eggs, and that blurring the categories harms science and policy. It clearly states biology's standard definition, but it is an argument, not new evidence, and it asserts rather than works out what this means for individual people, law and policy. A critique and Wright's reply followed in 2026.

Key findings

• Central claim: in every species with two different-sized sex cells there are exactly two sexes, because male means the body is built to make sperm and female means built to make eggs, not chromosomes, hormones or body features.
• Wright separates how sex develops, which varies widely across species, from what sex is, and argues much confusion comes from mixing the two up.
• He answers five challenges: fungal 'mating types', extra or missing sex chromosomes such as XXY, differences of sex development described as a spectrum, definitions based on a cluster of traits, and models that split sex into genetic, hormonal and body 'levels'.
• On differences of sex development, he argues the two-sex view does not require that every person can be clearly classified; unclear cases do not make a third sex, because they do not produce a third kind of sex cell.
• He calls cluster-of-traits and multi-level models circular, because traits are labelled 'male-typical' or 'female-typical' only by looking at bodies already sorted by sperm or eggs.
• It was published in November 2025 (vol. 54, pp. 3941-3945) in a section on 'How Many Sexes Are There? How Many Genders Are There?'; Dana Mahr published a critique in January 2026 and Wright replied in March 2026.

Conclusion

This commentary advances a simple claim with broad consequences: In anisogamous organisms, the sexes—male and female—are functional classes defined by the type of gamete an individual has the biological function to produce. Males have the biological function to produce sperm; females have the biological function to produce ova. That definition is universal across all anisogamous taxa. Much contemporary confusion arises from conflating how sex is determined (i.e., how sex develops) with how sex is defined (what sex is), and from conflating upstream determinant and downstream correlates of sex with sex itself. On this account, aneuploidies and DSDs describe variation in development or function within the two sexes; "mating types" belong to isogamous systems and are compatibility classes, not sexes; and "multivariate" or "spectrum" depictions quantify trait variation within and between the two sexes without altering the number of sexes.