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

• 核心主張：在每一個有兩種不同大小性細胞的物種中，恰好有兩種性別，因為雄性意指身體被構造來製造精子，雌性意指被構造來製造卵子，而不是染色體、荷爾蒙或身體特徵。
• Wright 區分了性別如何發育（在不同物種間差異很大）與性別是什麼，並主張許多混淆來自把兩者混為一談。
• 他回應了五項挑戰：真菌的「交配型」、額外或缺少的性染色體（如 XXY）、被描述為連續光譜的性發展差異、基於一組特徵的定義，以及把性別分為基因、荷爾蒙和身體「層次」的模型。
• 關於性發展差異，他主張兩性觀點並不要求每個人都能被清楚分類；不明確的情況不會構成第三種性別，因為它們不會產生第三種性細胞。
• 他稱特徵叢集和多層次模型是循環論證，因為特徵之所以被標為「典型男性」或「典型女性」，只是透過觀察已經依精子或卵子分類好的身體。
• 本文於 2025 年 11 月發表（第 54 卷，第 3941-3945 頁），刊登在「有多少種性別？有多少種性別認同？」專欄；Dana Mahr 於 2026 年 1 月發表批評，Wright 於 2026 年 3 月回覆。

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.