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 fatheringCalls 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 real enough. But the reason for it is situational. Winning a contest, or facing a challenge to your standing, is what pushes testosterone up; losing pushes it down. Testosterone is closer to a gauge that reads a man’s circumstances than a lever that changes them.The largest review of the human research into testosterone synthesises studies of more than 15,000 people. It found a man’s testosterone level barely predicts whether he’s aggressive and giving a man extra testosterone makes no measurable difference to how aggressively he behaves. Testosterone really starts to matter when a man is already in a contest. It drives status-seeking – but our social context decides what status looks like. One fascinating and inconvenient truth the manosphere fails to mention is that too much injected testosterone causes the brain to stop sending the signal that tells the testes to produce it. The result is a huge decrease in the production of sperm. The effect is so strong that the World Health Organization even trialled it as a male contraceptive. Of 271 men given weekly testosterone injections, close to two thirds stopped producing sperm altogether – and among those men’s partners, there was a single pregnancy across more than a thousand months.The hormone sold to young men as the essence of manhood is, when administered exogenously and in excess, a fairly reliable way of stopping them becoming fathers.Across many species, male testosterone rises during territorial and mating competition, and drops back during the parental phase. High testosterone across the whole male life course doesn’t help a man stick around and be a good dad.Bonds with our children change our brainsDad Brain offers insight into the male life course, allowing us to contemplate the whole man. It shows that with some physiological changes, men gain a whole lot of meaning and purpose that extra testosterone can’t replace.With collaborators in Madrid, Saxbe scanned the brains of 20 first-time fathers in Barcelona before their partners conceived and again two months after the birth. She compared their brains with those of 17 childless men. The fathers lost cortical grey matter volume; the deeper brain structures were spared. A second cohort in Los Angeles showed the same pattern. Darby Saxbe scanned the brains of 20 first-time fathers. USC Dornsife A later study of 38 first-time fathers, scanned throughout their partners’ pregnancy and again six months after the birth, went further: the men who had bonded most strongly with the unborn child and who planned to take more time off work, lost the most brain volume.Analyses found new fathers lost grey matter in many of the same regions that shrink in women’s brains. Saxbe explains the changes similarly to maternal changes: a “slimmed-down brain” may process information more efficiently once the hard work of parenting starts. “We found that the strength of the bond fathers shared with their children – even while their partners were pregnant – predicted their brain changes,” she writes: Fathers-to-be who felt more bonded with their babies in utero and who hoped to take more parental leave showed more brain volume loss in the cortex […] the downsized paternal brain reflected more readiness to parent.Along with reductions in grey matter, men experience drops in testosterone before the baby comes along. In one study, Saxbe and psychologist Robin Edelstein followed 27 expectant couples across a first pregnancy and found fathers’ testosterone declined steadily – it was around 17% lower at 36 weeks than at 12. The men whose levels fell furthest reported feeling more invested and committed, and their partners described them as more supportive after birth. Sleep deprivation cannot explain a change that arrives months before the sleepless nights. The men whose testosterone levels fell furthest reported feeling more invested and committed. Dylan Sauerwein/Unsplash ‘Best’ dads change with society idealsThese processes occurring among men preparing for fatherhood, then becoming fathers, show that a man doesn’t “optimise” his way to fatherhood by getting hormone tests or testosterone replacement therapy.In fact, Saxbe also shows that some of the best fathers have the lowest testosterone of all. Among the BaYaka, an egalitarian foraging people of the Congo Basin who prize touch and connection – and who judge a good father partly by his willingness to share with the wider community – the men rated the best dads have the lowest testosterone. In neighbouring villages, the Bandongo – a fishing and farming society anthropologist Lee Gettler describes as very patriarchal, very competitive and very hierarchical, where providing means dangerous physical work and fathers teach older children to fish, hunt and clear land – the best-rated dads have the highest testosterone. Same hormone but opposite ideal, set by what each society asks of its men. Saxbe is careful not to simply flip the argument. In a study of 149 couples, fathers with lower testosterone at nine months postpartum reported more symptoms of depression. But their partners reported fewer symptoms. Mothers, meanwhile, whose partners had higher evening levels of testosterone reported more depressive symptoms at nine and 15 months postpartum and, in turn, less satisfaction with the relationship.Saxbe’s book’s own conclusion is clear: interventions that supplement or alter men’s testosterone may carry unintended consequences for the whole family. As Saxbe puts it, “what is good for the goose is not good for the gander when it comes to testosterone after birth.”If there’s one thing we can be sure of in this messy hormonal world, it’s that testosterone does not make the man – and certainly not the father.Samuel Cornell receives funding from Movember for research consultancy.