Stage two followed in the 1960s, when paediatrician John Money pronounced that “gender identity” (private experience) and “gender role” (the public manifestation of it) are more important and essential than biological sex. Money had been involved with the “sex reassignment” of a male baby, who, after a badly botched circumcision, became Brenda, and was surgically reconstructed with a vagina. 

Stock is too kind to mention this, but, as a child, Brenda was constantly made to simulate sex in a female receptive position with her identical (male) twin. Money was thereby testing his nurture-over-nature theory – but Brenda failed to prove it. Deeply troubled, she identified as male aged 15 and took her own life in adulthood. (So, not surprisingly, did her twin.)

In the late 1980s, Anne Fausto-Sterling, a professor in biology and gender studies at Brown University, claimed that there are at least five sexes, that each of us is sexed in various layers, and that sex exists along a spectrum. None of these (inconsistent) claims had experimental backing, as Stock explains, while Fausto-Sterling’s estimate that 1.7 per cent of humans are intersex is exaggerated: 1.5 of us have (usually undetectable) chromosomal anomalies and only 0.018 per cent (1.8 in 10,000) are hermaphrodites. 

As Stock points out, neither Judith Butler’s famous contention, in her 1990 book Gender Trouble, that sex is merely performative, nor Julia Serano’s in the following decade that no one is “really” biologically male or female, can have any pretensions to being empirically testable – yet today, they are often taken as gospel. 

The “stages in history” approach may make the gender fracas seem a matter of anthropology and psychology (and perhaps it will turn out to be so), but Stock admirably tries to make scientific and philosophical sense of it. She examines whether sex should be determined by chromosomes or by morphology (genitalia, secondary sexual characteristics, facial and skeletal structure, vocal tone), before expounding the importance of sex for our species.

For, as she gently reminds us, we humans are not omnipotent, and cannot reverse-construct or undo our evolutionary history. For the past 65 million years, we have been a two-sexed mammalian species, and have survived and flourished thanks to sexual selection, which is why heterosexuality has been a “widespread… adaptive trait”. I can imagine some mammalian hackles rising here, but isn’t that an incontestable fact?

Health and disease, Stock writes, “can be directly affected by sex characteristics”; she gives scrupulously detailed explanations and examples that are rather beyond the non-scientist. So too can degrees of strength, muscular power and physical prowess; hence the sex-based division in competitive sports.