When a baby is born, their sex is usually the first thing we want to know about them. The last thing you’re ever likely to forget about a person is whether they are male or female. We often think of biological sex as a fundamental force in human development that creates not just two kinds of reproductive system, but two kinds of people.

At the core of this way of thinking is a familiar evolutionary story. Men’s much smaller minimum investment in a baby means that they can reap huge reproductive benefits from having sex with many different women; preferably young, fertile ones. Not so for a woman. What most constrains her is access to resources, to help her care for her biologically expensive offspring.

Men evolved to be risk-taking and competitive, since these were the qualities that best enabled them to accrue the material and social resources attractive to women, and to have a promiscuous streak, to turn that sexual interest into a reproductive return.

It’s intuitive to look for a single cause for this divide between sexes, and if you’re thinking of a hormone beginning with “t”, you’re not alone. Neuroscientist Joe Herbert’s recent book Testosterone: Sex, Power, and the Will to Win, leaves readers at no risk of underestimating its potency: “At the end of any discussion of the impact of testosterone on the history of mankind… a simple fact remains: without testosterone there would be no humans to have a history.”

Now there’s a conclusion to inspire the reverence the testicle deserves… at least until you realise that the same fact applies to oestrogen, carbon or even nitrogen. But still – sex! Power! The will to win! As Herbert explains, these are exactly the masculine qualities that, according to received wisdom regarding our evolutionary past, were so necessary for male reproductive success.

This is Testosterone Rex: that familiar, plausible, pervasive and powerful story of sex and society. Whenever we discuss sex inequalities and what to do about them, it is the giant elephant testicles in the room. What about our evolved differences, the dissimilarities between the male and the female brain? But Testosterone Rex gets it wrong, wrong and wrong again. Contemporary scientific understanding of the dynamics of sexual selection and of the effects of sex on the brain undermines its view.

Research in evolutionary biology has destabilised the key tenets once thought to apply across the animal kingdom, whereby arduous, low-investing males compete for coy, caring, high-investing females. The sexual natural order turns out to be diverse, and we also bring our own uniquely human characteristics to the sexual selection story.

There are no essential male or female characteristics – not even when it comes to risk-taking and competitiveness, the traits so often called on to explain why men are more likely to rise to the top.

Testosterone affects our brain, body and behaviour. But it is neither the king nor the king maker – the hormonal essence of competitive, risk-taking masculinity – it’s often assumed to be. So while it’s probably fair to say that it was mostly men who brought about the financial crisis, the fashionable contention that “testosterone did it” is an excellent example of what happens when flawed thinking is applied to public debate.

Testosterone Rex is extinct. It misrepresents our past, present and future, and it reinforces an unequal status quo. It’s time to say goodbye, and move on.

Testosterone Rex: Unmaking the Myths of our Gendered Mind by Cordelia Fine is published by Icon Books at £14.99. To order a copy for £12.74, go to bookshop.theguardian.com