Testosterone is having a moment. On social media, it’s being lauded for everything from building muscle to gaining wealth and sexual dominance.

United States secretary of war Pete Hegseth has ordered testosterone screening for US service members over 30, as part of “restoring warfighter capability”. Health secretary RFK Jr has promoted testosterone replacement as an answer to a supposed collapse in American manhood. And online, men are encouraged to check their testosterone levels and “Tmaxx”: push their testosterone as high as it will go.

Review: Dad Brain: The New Science of Fatherhood and How it Shapes Men’s Lives – Darby Saxbe (Bodley Head)

But high testosterone isn’t always a good thing. A new book by US clinical psychologist Darby Saxbe offers a refreshing perspective: the life event that changes men most profoundly (becoming a father) naturally drops their testosterone. And the more involved they are, the more it drops.

The book draws on 20 years of research and Saxbe’s own, brain-scan based studies of men crossing into fatherhood. It shows how profoundly, and hormonally, fatherhood changes men.

High testosterone doesn’t equal good fathering

Calls to “remasculinise” men by administering testosterone rest on the same view: that testosterone is the dial of man, and the man with his testosterone set to 11 is the greater.

But a fundamental flaw with the idea of “more testosterone, more man” is that biological processes rarely work in such simple terms. Much more commonly, they are negative feedback loops.

Further, we know that testosterone levels respond to a man’s life and environment. Increasing testosterone from outside the body, via injections or gels, does not reliably lead to more aggressive behaviours or more competitiveness. Nor does it lead to more wins.

Dominant, “successful” men do tend to have higher testosterone: the correlation the manosphere has noticed is........

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