Cordelia Fine is an optimistic writer. In her two earlier books of popular neuroscience (A Mind of Its Own and Delusions of Gender), the psychologist established a reputation for exemplary clarity on complex topics, pleasing wit, feminist principle – and beneath it all, the animating faith that people can be improved through knowledge. Testosterone Rex starts with a quote from Chimamanda Ngozi Adichie’s We Should All Be Feminists that establishes the Fine approach perfectly: “But in addition to being angry, I am also hopeful, because I believe deeply in the ability of humans to make and remake themselves for the better.”

“Testosterone Rex”, Fine’s target, is the name she gives to “that familiar, plausible, pervasive and powerful story of sex and society”, which holds that inequality of the sexes is natural, not cultural. After all, testosterone makes men tall, hairy and deep-voiced; it makes a certain superficial sense to imagine it also produces other characteristics we think of as masculine, such as leadership, violence and horniness. For example, neuroscientist Simon Baron-Cohen (the Alien to Fine’s Ripley in the dispute over brain sex) calls the hormone “that special substance”, and credits it with inducing all manner of adaptive qualities in those creatures fortunate enough to produce large amounts of it. T is the king.

This is an explanation that’s really a justification. If Baron-Cohen was correct that hormones make the man or woman, and we are what we secrete, then efforts to end male dominance would be futile at best and possibly downright harmful. But this, of course, assumes that “Testosterone Rex” is fact when, as Fine compellingly argues, it’s actually fiction. It’s a powerful fiction that shapes our society and our bodies in profound ways, yes – but it’s still a fiction, and one that in no way deserves to be enthroned in our understanding of ourselves.

Here’s one example Fine offers of Testosterone Rex mangling the way we think about sex. In the 1940s, biologist Angus Bateman conducted a series of experiments on fruit flies that appeared to show conclusively that competition between males for “fertile female vessels” was the driving force of evolution. The hypothesis goes something like this: laying eggs is a more substantial physical investment than producing sperm. Therefore, to maximise reproductive success, females should be selective and cautious while males should be promiscuous and competitive; therefore, women are domestic and monogamous, while men are thrusting away both in the public sphere and in as many beds as possible.

Facebook Twitter Pinterest Exemplary clarity … Cordelia Fine

It’s elegant, it’s intuitive, and it’s wrong. Bateman’s experiments were biased by design and by his unexplained exclusion of data that, when included in a recent reanalysis, actually showed that males and females both produced more offspring when they had more mates. But there are limits to promiscuity as a strategy: taking into account female fertility, a man has more chance of being hit by a meteor than fathering 100 children with 100 different women in a year. The player who says it’s in his genes is missing a vital part of the story.

Fine is a sure guide to the science, building up complexity without sacrificing clarity. By the time she’s finished, any lingering confidence that hormones exert a simple dose-response influence on our behaviour is thoroughly done for. Instead, testosterone works in intimate concert with relationship structures – a blow to its dignified reputation as the singular, commanding “male hormone”. Even something as incontrovertibly binary as our male and female genitals is shown to be part of a complex cultural system. As Fine says, “it’s the genitalia – and the gender socialisation this kicks off – that provides the most obvious indirect developmental system route by which biological sex affects human brains”.

Being male or female won't make you into your society’s version of a man or a woman. There's no 'male' or 'female' brain

In other words, being male or female isn’t enough to make you into your society’s version of a man or a woman. There is no “male brain” or “female brain”. But as soon as your maleness or femaleness is recognised, other people start to treat you in ways that form you into a man or a woman, with the support of toys, books, role models and a million other subtle nudges. To then pretend, as the story of Testosterone Rex does, that outcomes such as the wage gap or rape are the unavoidable manifestations of our nature is to sorely misunderstand our species.

We could remake ourselves. But we have not done so thus far, and it’s hard not to wonder whether optimistic rationalism is really a strong enough prescription against the entrenched interest in keeping women down and men up. Fine wonders the same as she gets to her conclusion: “maybe it’s time to be less polite and more disruptive; like the first- and second-wave feminists”. In addition to being hopeful, Fine is also angry. We should all be angry. Testosterone Rex is a debunking rumble that ought to inspire a roar.

• Testosterone Rex: Unmaking the Myths of Our Gendered Minds is published by Icon. To order a copy for £12.74 (RRP £14.99) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.