The Spectrum of Sex: The Science of Male, Female, and Intersex by Hida Viloria and Maria Nieto purports to show it is unscientific to believe there are only two biological sexes, that believing this is of a piece with bad things like eugenics, and that the world needs to do much better in how it thinks about and treated intersex people. In the first two of these goals I think the book fails. Oddly, it barely argues for the “spectrum” of its title, and the authors never explain what they think it would mean for biological sex to be a spectrum. While it repeatedly claims intersex constitutes a third sex category, its most impressive parts work against that conclusion. The authors also regularly abandon their project for the much more modest and less controversial goal of showing sexual expression—in physical traits and in gender—falls along a spectrum. For their part, the moral and political parts are mostly unsupported assertions even if its renunciation of mistreatment of intersex people is obviously sound.

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