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

• Hovedpåstand: i enhver art med to kønsceller af forskellig størrelse findes der præcis to køn, fordi han betyder, at kroppen er bygget til at danne sæd, og hun, at den er bygget til at danne æg, ikke kromosomer, hormoner eller kropstræk.
• Wright adskiller, hvordan køn udvikles, som varierer meget på tværs af arter, fra, hvad køn er, og argumenterer for, at megen forvirring kommer af at blande de to sammen.
• Han besvarer fem udfordringer: svampes ‘parringstyper’, ekstra eller manglende kønskromosomer såsom XXY, variationer i kønsudvikling beskrevet som et spektrum, definitioner baseret på en klynge af træk og modeller, der deler køn op i genetiske, hormonelle og kropslige ‘niveauer’.
• Om variationer i kønsudvikling argumenterer han for, at to-køn-synet ikke kræver, at hver person kan klassificeres klart; uklare tilfælde udgør ikke et tredje køn, fordi de ikke producerer en tredje slags kønscelle.
• Han kalder modeller med klynger af træk og flere niveauer cirkulære, fordi træk kun kaldes ‘typisk mandlige’ eller ‘typisk kvindelige’ ved at se på kroppe, der allerede er sorteret efter sæd eller æg.
• Det blev offentliggjort i november 2025 (bd. 54, s. 3941-3945) i en sektion om ‘How Many Sexes Are There? How Many Genders Are There?’; Dana Mahr offentliggjorde en kritik i januar 2026, og Wright svarede i marts 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.