When a baby was born with ambiguous genitals a generation or so ago, it was pretty much standard medical practice to chop off whatever male-looking appendages might be there and declare the hapless infant a girl.

Clinicians saw the bringing of such unacceptable anatomy into line as an urgent task and generally punted for the female sex because, as the saying went: 'It’s easier to dig a hole than to construct a pole.'

Doctors are a lot more cautious about surgical interventions in intersex children these days – largely thanks to the vocal campaigning of some very angry adult survivors of such medical attentions.

But a lot of scientific inquiry still seems aimed at dividing us all into two neat and tidy categories, two 'opposite' sexes, ignoring the evidence that we humans are actually a great deal more interesting than that.

If you look at the research, you’ll hear a lot of claims made for a fundamental, even irreconcilable, divide between men and women in fields as varied as neuroscience, evolutionary biology and psychology.

You may hear that female brains are innately more suited to multi-tasking, to language use, to empathising. That male brains are designed for narrow focus, for better spatial perception, or for understanding systems.

Or that evolutionary imperatives make men naturally promiscuous, while women are driven to find a single mate who will be a good provider for them and their children.

Often, all of these so-called essential differences between men and women will be traced back to the Pleistocene, when the first humans evolved to meet the demands of the African savannah. 

Inside every anxious male bank worker on the morning commuter train is a primal hunter, whose instincts would have him ranging the land in all his physical splendour, while the marketing executive toting her briefcase beside him is really designed to be back in her cave making everything agreeable for his triumphant return.

(Yes, the 1950s-style single breadwinner family may have pretty much disappeared in our Western societies, but it’s still alive and well among some of the evolutionary psychologists.)

When you really look into these sorts of claims about hard-wired characteristics of men and women, it’s hard to find much in the way of quality evidence to support them. Even those differences that can be established between the sexes as a whole tend to be small, subject to change over time, and pretty much irrelevant when it comes to individuals.

When you think about it, there is not a single generalisation about males or females that can be applied to every member of a particular sex.  Women can be tall, hairy, aggressive or good at map reading. Men may gossip, lactate, shrivel at the sight of a spider or stay awake after sex.

After spending much of the last two years delving into the research while writing my book, Making Girls and Boys: Inside the science of sex, I’ve come to the conclusion that it makes more sense to think of sex as a spectrum than as a pair of opposites. All of us – male, female, those who resist easy classification – have to find our own place, conforming perhaps to some of the stereotypes associated with our biological sex and not at all with others.

I’m not alone in thinking like this. Some scientists too have turned their backs on the seductive lure of the binary to give us more complex and dynamic descriptions of men and women.

The tired old nature versus nurture debate is giving way in the face of evidence that biology and environment are actually inextricably entwined, that they constantly affect each other in a myriad of unpredictable ways.

When our hormone levels fluctuate in response to life events, they change us on a biological level. When we learn new behaviours, when we fall in love, neural connections rewire, changing the actual anatomy of the soft tissue within our skulls.

The new technologies that are allowing us to look inside our living brains as they think, grieve and desire are also showing us how astonishingly ‘plastic' (capable of change) they can be. Far from being stuck with the behavioural patterns of a Pleistocene hunter, it seems we modern humans actually have some capacity to create our own brains in concert with our ever-changing world.

Over the course of human history, those plastic brains have allowed us to adapt to environments from the Arctic to the Pacific Islands, to set in motion and then grapple with the demands of urbanisation, industrialisation and digitalisation. They have helped us to create new societies, new ways of relating to each other and new ways of being men and women.

I'm not saying sex is irrelevant. For most of us, it's an important part of who we are, and we wouldn't want it any other way. But that doesn't mean we want to be imprisoned by our gender, told that our particular brand of anatomy means we have to behave, feel or think in a particular way.

Male, female, or however we like to define ourselves, we humans are complex creatures, infuriating, entrancing, and most of all unpredictable. Science may be drawn to nice, neat categories but, if we’ve proved anything about ourselves, it’s that we just don’t fit into boxes very well.

This article was originally published on ABC Unleashed. Jane McCredie is the author of Making Girls and Boys: Inside the science of sex, published by NewSouth.