A new theory – Testosterone Rex – is attempting to redefine what we think about gender.

A small but significant victory in the war for gender equality this week, as news emerges that my girlfriend is in line for a promotion. If she gets it, her salary will be slightly more than mine, and the pay gap will have narrowed by a fraction of a fraction of a percentage point. Take that, patriarchy! And yet, even as I celebrate another brick successfully removed from the wall of male privilege, I can’t help but wonder what on earth is taking us so long.

It has been 42 years now since the Equal Pay Act came into force in the UK, a landmark piece of legislation that protected the rights of women to be paid as much as men. Since then, the pay gap has narrowed from around 40 per cent to below 20 per cent. But what of the remaining difference? It’s still very much there, and it’s not shrinking with much haste.

This is highlighted in the UK by “Equal Pay Day”, the annual date on which women effectively start working for free for the remainder of the year. Last year, it fell on 10 November, 52 days before the end of the year. That reflects a gap of around 14 per cent for full-time workers (or 1.7 monthly pay checks, if you prefer to think of it that way). The previous year’s Equal Pay Day fell just one day earlier, on 9 November. The rate of progress – if you can even call it that – is glacial.

Why, nearly half a century after the barriers to sex equality in the workplace were dismantled, are women like my girlfriend still the exception to the rule? Has discrimination simply disappeared underground? Or is the glass ceiling being propped up by something else, something that can’t be so easily defeated by legislation? One controversial theory, the target of a new book by academic psychologist Dr Cordelia Fine, author of the hotly discussed Delusions of Gender, lays the blame on no less potent a force than evolution itself.

Testosterone Rex, as the author dubs it, is the theory that men and women have evolved fundamentally different natures over time, and that a degree of inequality between the sexes is therefore inevitable. As the argument goes, the different sexes have historically taken very different approaches to reproductive success. Behavioural traits that proved advantageous to primitive men, such as competitiveness, promiscuity and aggression, have become hardwired into male brains as a result of natural selection, while women, who are more heavily invested in the development of a child, have been forced down a different evolutionary path, becoming more nurturing and coy.

This simple piece of evolutionary logic is the apparent reason why more men sit at boardroom tables and in the cockpits of Formula One cars than women. If it’s correct, it crushes all hope of equality between the sexes, as it tells us not only that men and women will never reach full parity in the workplace, but that to expect them to do so is to deny human nature.

But, what if it’s wrong? As men, we have been brought up to believe that our male brains, fueled by the testosterone surging through our veins, gives us a natural advantage over women. If none of that is actually true – if the neural pathways honed while defending our territory from woolly mammoth, or clubbing each other to death in competition for a mate, are in fact of no real benefit to us in a business environment – then male privilege, far from being a biological inevitability, will prove to have been nothing more than a self-fulfilling prophecy.

Well, it’s time to stop resting on your laurels in the office, guys, because according to the new book by Dr Cordelia Fine, Testosterone Rex is as extinct as its reptilian namesake. Taking a butcher’s cleaver to its scaly carcass, Dr Fine dissects with precision and wit many of our most basic assumptions about sex and gender, such as the notion that men might be rather more concerned with status and rank than women (“Now might be a good moment to remind ourselves that the expression ‘pecking order’ comes to us courtesy of hens,” she writes), or that testosterone makes men more predisposed to taking risks.

To disprove the latter, she examines the “Lehman Sisters hypothesis”, or the idea that “there’s too much testosterone on Wall Street”. Could the financial crisis have been averted if the Lehman Brothers had been packing an X chromosome in place of a Y, as many suggested in the wake of 2008? Not necessarily, argues Dr Fine. In a meticulously researched chapter dedicated to the topic, she examines a wide range of studies on the effects of testosterone on risk-taking – and finds little compelling evidence to suggest a correlation. While behavioural differences have been shown to exist, they are conditional on social and geographical context: an experiment that might see a modest positive result in North America, for instance, will not necessarily be replicated in China.

These are just two of the preconceived ideas upon which Testosterone Rex is founded – ideas which, according to Dr Fine, are misleading, reductive or just plain wrong. None of which is to suggest that sex differences do not exist, of course, or that “testes are merely a social construct”. Dr Fine’s book does not deny that we are an adapted species; it merely argues that we are far more adaptable than we think.

It’s certainly a persuasive idea, and personally I don’t have to look very far to encounter examples of women who are far better adapted to dealing with the pressures of the working world than I am (see: exhibit A, above). Whether or not this book will affect a significant change on the current status quo remains to be seen. Nevertheless, Testosterone Rex remains essential reading for anybody who professes to care about sex equality, and such matters as whether or not their daughters get paid as much as their sons.

Testosterone Rex: Unmaking The Myths Of Our Gendered Minds is out now

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