How many times did I say it – to myself, out loud alone or out loud to others, throughout my childhood? ‘I wish I were a boy.’

The words were mine, a fervent and frequent wish. They were not born of a feeling of mismatch between external expectations and internal signals. Except for a lifelong tension with society’s mixed messages about what it means to be a woman, I’m comfortable identifying as the gender assigned to me. But I wished for boyness because the boys did so many things I wanted to do and was excluded from doing because I was a girl. My body and my brain mapped to each other just fine, but my body didn’t map at all to what society told these boys – and me – I was allowed to do.

As many a woman can attest, this feeling of belonging in male spaces that lock you out doesn’t end with teenhood, adulthood, careerhood or parenthood. An aficionado of adventure stories, I couldn’t – still can’t – help but notice that the places men can go are often No Women’s Lands for someone like me. Not because I lack the physicality, strength or stamina to traverse them but because the mere presentation of being female is itself dangerous. Realistically, it invites violence, exclusion and violation in too many ways to be considered anything but a liability.

And then there are the less wild places, just boys’ clubs and men’s clubs, de facto or tacit, where being a girl or woman means being viewed as an intruder or, as women have always known, being subject to harassment or worse. Every day, I see men circle their masculinity like musk oxen, protective and exclusionary, in my professions of academia and journalism. Even in the virtual world of social media, they reflexively exclude women who are their peers in expertise and competence while readily engaging men who are neither. I am wryly amused when people committed to the idea that men and women are cognitively different throw women the double-edged bone of being ‘better at verbal expression’. (Look, we’re good at a thing! That you’ll also use to make fun of us chatty, chatty Cathies!) I read that and think of who receives most of the major book awards and other writing accolades. Hint: it’s men. I’ll wager that the social factors involved in the latter contribute to the assumptions underlying the former.

Norm-preserving behaviour is infamous among men in tech. The cork in that pressurised cask of Silicon Valley misogyny blew in the summer of 2017 when one Google employee put into words what so many men like him think and desperately seem to want: women don’t belong in these male spaces because their brains just can’t handle the man-work that these spaces require. The wish-fulfilling justification has remained unchanged for centuries, a facile crutch that well-educated men in particular will philosophise and rationalise, with various ‘cool girls’ trailing in their wake, cheering them on. Of course, once upon a time, the guiding hand was God and now it’s Nature. Never mind that, in the God version, it was women who discovered the space of knowledge and invited men into it, and that Nature herself is a mother who birthed a world of astonishing diversity that defies binaries more often than not.

The unfulfilled wish is, of course, that men’s brains differ from – and by the usual implications are superior to – those of women in just the same way as men’s physiques differ and are superior. Stronger body, stronger mind, as though the brain were not, at a minimum, a pack of neurons with a wide and varied cast of support cells but instead a mass of contractile tissue, easily built up and broken down simply by following the rules dictated in Men’s Health.

And of course, the perceived binary of human anatomy offers thin scaffolding for the facile argument. Penile genitalia must mean a penile brain, evidently, just as a vulva and a vagina must mean a vulvo-vaginal brain. Nature, you see, in building the genitalia, which are much less complex than our minds, apparently defaulted to a super-lazy setting, and created brains on the binary.

So if you’re a Silicon Valley brogrammer who wants to drive women out of your manspace, or a Bernie Bro who thought that ‘Bern the witch’ was the summit of wit, then you’re more likely to rush to the rationale that society and culture – ie, things that we create outside of biology – don’t shape the male spaces that exclude women. No, you desperately want Nature to be responsible.

Appealing to a higher power and cherrypicking ‘evidence’ to support a convenient claim of superiority over others of your species is as human as scratching your butt. In this false iteration of that desperate measure of a failing privileged class, Nature created men to like and do certain man things, and naturally, therefore, men are simply better at these things. In complement, goes this wish-fulfilment rationale, Nature created women to like and do certain lesser things, and women are condescendingly told how great they are at it, especially the talking part, and please stay in the kitchen and make a sandwich because all of this analysis is over your head, Sweetie.

But Nature made me. And I am not rare. Indeed, an entire category of girl and woman exists that is large enough to warrant the now-archaic category of tomboy. We are legion, and most of us likely wished fervently at some point that we could be boys so we could simply gain access to what interested us most. How could nature both create an entire legion of girls whose interests and abilities cross into manworld, yet somehow be capable of producing brains only on a binary?

Nature doesn’t do binaries nearly as often as people think. But she’s a whiz with mosaics.

Mosaics are taking shape as the scientific metaphor of the moment. Genetic mosaics – people with only a proportion of body cells carrying specific mutations – might be far more common than we realised. Habitats are mosaics of features, such as a meadow fading into a forest’s edge. And our brains, rather than fitting neatly into some binary of being ‘male’ or ‘female’, might also be mosaics, quilted together from pieces with varying hues of ‘masculine’ and ‘feminine’ expression. You know, just like human behaviour, the result of these brains.

In his book The Astonishing Hypothesis (1994), Francis Crick echoed Lewis Carroll’s Alice in Wonderland (1865) in asserting about the brain: ‘You’re nothing but a pack of neurons.’ But a ‘pack of neurons’ is just the beginning. The human brain is a structure patterned from billions of parts. These parts create a collection of regions, each with a continuum of form and function, and variable responses to internal signals. That sentence is not pithy, but it’s a lot closer to what you are than either Crick’s quotable quote or society’s artificial dichotomy grounded in deep-rooted misperceptions about biology and anatomy.

Within each of these regions, the character of components will vary from person to person just as two quilts of the same design and even made from the same scraps will never quite be identical.

Currently, we have no way of looking at these brain regions to classify the organ as ‘male’ or ‘female’. But complexity intensifies when you add in the influence of steroid hormones. Like lemon juice acting on invisible ink, these signalling molecules can reveal the hidden capacities of neurons forming different brain regions. And just as no two brains will have the exact same neuron assemblage, no two brains will experience the exact same cocktail of sex steroid hormones. Indeed, no one brain will experience a consistent exposure to these hormones over the course of a lifetime or even over the course of two consecutive days. How could anyone paying attention ever argue that brains and the behaviours that they produce could be reduced to a simple choice of either this or that?

Just look at those steroids, tiny signalling molecules carved from cholesterol, coaxing out the unexpressed potential of neurons. They can slip like quicksilver into cells, and deliver their intense chemical messages, guiding distinct brain regions to develop or communicate along different paths. In the invisible cast of steroid hormones with roles in the human body, the stars of the show are testosterone and oestrogens (a collection of molecules with similar structures but often different effects in different tissues). And what they do to our brains is far more complex than ‘testosterone = masculine; oestrogen = feminine’.

Is it any wonder that, with all these bits and pieces, human brain development defies strict categories?

One of their earliest roles is shaping the brain. As the budding embryonic brain takes on an increasingly complex structure, the future gonad begins making sex steroid hormones under genetic instructions. These hormones in turn make their way to the developing brain, and aid in building its internal parts. As a result, gonadal and brain development are thought to fall under the organising influences of roughly the same hormones in an individual. But gonads aren’t brains, and what happens in the brain involves many more moving parts.

This dual process of brain and gonadal development requires billions of cells and hundreds of molecules guiding, signalling and shaping those cells into their final locations and forms. Now add in the genetic codes for many of these molecules, sequences with thousands of units that also can vary from person to person. If genes vary, the proteins they encode can vary, too. Is it any wonder that, with all these bits, pieces and participants, human brain development defies strict categories? Nature hands us more than 7 billion individual puzzles each made from billions of pieces and, somehow, we think they’ll fit into a two-category system.

Indeed, a simple two-category system seems impossible and almost silly given this underlying complexity, and intriguing scientific evidence points away from a dichotomy and toward a three-dimensional spatial continuum of variations that defy any categorisation.

This view of the human brain is stunning, both in how well it analogises human behaviour – the product of our brains – and in how it distinguishes us as human. Our brains blur the boundaries between the biological and the social just as we do ourselves.

Like most people, our brains are mosaic. When the neurobiologist Tom Curran, now at the University of Kansas School of Medicine, wrote an op-ed for The Scientist called ‘Deconstructing the Mosaic Brain’ (2011), he used the term ‘mosaic’ to describe the wide-ranging diversity of human brains, rather than a spectrum of sex-based differences. Genes can change as an embryo develops, he points out, leading to specific brain-related conditions such as epilepsy. The patchwork changes can even lead to differences in brain structure between identical twins. It’s puzzling that this concept is not terribly controversial when it involves twins, yet the suggestion that brains from two people of the same sex might similarly differ fuels fiery debate.

The neuroscientist Daphna Joel at Tel Aviv University and her team entered the fray in 2015 with a seminal and controversial paper on the mosaic brain in the Proceedings of the National Academy of Sciences. Joel analysed the structure of more than 1,400 brains by MRI. She found a huge structural overlap between men and women in the hippocampus, a centre for memory, along with a continuum of structural variation across the spectrum of what we think of as masculine to feminine. Indeed, even in regions with the most significant differences between the sexes, many women fell on the masculine end of the spectrum, while many men landed at the feminine end. The overlaps are so striking, the team reported, that only about two or three out of every 100 brains were wholly at one or other of these extremes. Everyone else tended toward a central average. From person to person, Joel found, our brains are a mixed and unique mosaic of regions with variable ‘maleness’ or ‘femaleness’ in each. ‘Human brains do not belong to one of two distinct categories: male brain/female brain,’ she wrote.

Over in New York, Bruce McEwen, a neuroendocrinologist at Rockefeller University, and Teresa Milner, a neuroscientist at Weill Cornell Medical College, summarised this new perspective on our old brains in a review of the recent evidence, writing in 2017 in the Journal of Neuroscience Research that ‘we are entering a new era of our ability to understand and appreciate the diversity of gender-related behaviours and brain functions’. One focus for them: the uncreatively named ‘sexually dimorphic nucleus of the preoptic region’, or SDN-POA. But their point isn’t that this region shows a sex difference. Instead, they present it as one of the handful of examples from our brains where sexual dimorphism, or a relatively clear average difference between sexes, seems apparent. As for the rest, ‘the vast majority of sex differences are far more subtle’.

Even in the animals we use as scientific stand-ins for ourselves, these patterns can be fuzzy. Male and female rats and mice show differences in neuron structures that are related to their response to stress. Females respond more strongly to acute stress, and are less flexible in adjusting to chronic stress. Yet during puberty, before rats achieve complete sexual maturity, they show no sex-based response to stress in the hippocampus, the central processor of memory. And in this same brain area, compared with unstressed female rats, unstressed males will have fewer neuronal structures called spines. But if they experience stress, the males will increase the number of spines, and the females decrease them. Which structural presentation is masculine and which is feminine – lots of spines or fewer spines? Both sexes can swing either way.

Of course, these are rats. We are a long way from rats in terms of the selection pressures and cultural adaptations we’ve developed in the 80 million years since we and our urban cohabitants went our separate evolutionary ways.

One area where that distinction is quite stark is our expression of gender, a uniquely human social construct. Given the increasing recognition of a diversity of genders that seem to shade into one another and even shift over time in the same person, it certainly makes sense that the system underlying them – our brain – would share some of that uncategorisable subtlety.

Perhaps we’d be better off considering the whole structure that we call ‘the brain’ an organism unto itself, an assemblage of discrete systems or puzzle pieces interacting as the thrumming pink-grey mass inside our skulls. Some of those systems form a mosaic of internally variable pieces to create a whole, the ‘you’ who thinks, feels, perceives, loves, hates, responds and reads this article. And other puzzle pieces are more like our genitalia, with largely predictable internal patterns, an average female effect and an average male effect, depending on the hormones that shape and govern them.

In a mind-governs-heart scenario, for example, sex-based brain differences might interact with the cardiovascular system to produce sex-specific outcomes. Several brain regions operate to control heartrate and blood pressure, and men and women – on average – tend to show differences both in these regions and in how they regulate cardiovascular responses.

At the root of this divergence are proteins that can respond – or not – to the presence of specific sex steroid hormones. The response or lack of response in turn affects molecules that regulate, for example, arterial tension. Tense arteries mean increased blood pressure.

Investigators at the University of California, Los Angeles, used functional imaging to show that men and women have, on average, real-time differences in activity in these brain regions as their heartrate and blood pressure respond to the same stimulus. Other researchers counter that a male/female brain dichotomy doesn’t underlie arterial tension differences – rather, height differences are the explanation.

Brain-related conditions such as autism that are thought to be male-biased might be equally common in both sexes

The list continues, and so do the caveats. Men and women are reported to experience pain differently at the group level. But dig deeper into that difference, and you find some strangely mixed results. One meta-analysis found that men and women differ very little in the threshold for pain related to heat, and a great deal for pain related to pressure. A closer look at the numbers for pressure, however, shows a greater range of values for how much sex affected the response. Some studies found almost no effect of sex on heat response; one study found that women had a higher pain threshold for heat, and another study found that men did.

Why all the variability? Many factors are in play, including hormone profile, emotional state, even social expectations: women are more willing to report pain than men.

It’s not possible to test women or men in the absence of hormonal influences, but the underlying factor in some of these differences might be exactly that: hormonal influences. In the meta-analysis of pain studies, the report citing the greatest effect of male sex on pressure-pain threshold involved high-school students, who naturally are awash in hormones. Differences in pain conditions don’t emerge between boys and girls until puberty, when the gonads awaken, suggesting a link to the reappearance of the same hormones that shaped their brains and gonads in the womb. And oestrogens, at least in rats, affect the brain’s endorphin response, which can mitigate pain. Testosterone treatment reduces pain perception in men with low testosterone; and, on the flip side, parenting seems to lead to reduced testosterone and pumped-up attachment hormones in fathers.

Studies showing that sex hormones can dampen or enhance pain sensation or promote parenting might seem like the answer to why so many people think that human behaviour is dichotomous. Hormones make it so. Yet even in animal models, we find that responses can be directly conflicting depending on dose or timing of exposure to the hormone. And of course, if we can shift men’s behaviour toward an attachment phenotype simply through diminishing testosterone, doesn’t that mean that the ability to behave parentally was always there in the brain – it just needed the right hormone to bring it out?

Other perceived differences between the brains of men and women also get murkier the deeper you dig. For example, some brain-related conditions, such as autism, that are thought to be male-biased, might be almost equally common in both sexes. They just tend to manifest differently and go undiagnosed in girls and women. Much of the difference could come down to the hefty dose of social conditioning and differential expectations applied to girls from infancy, and the fact that just as social interactions reach a fever pitch along with hormones at puberty, autistic traits in girls become more apparent then, too.

As these few examples imply, evidence around what’s binary and what’s mosaic in the human brain remains a puzzle. But an emerging narrative echoes that of the blind men and the elephant. In that tale, a group of elders who are blind examine a novel creature in their midst. One ends up examining the elephant’s foot while another examines its trunk, and yet another its ears. Naturally, they all draw wildly different conclusions about the form and function of this strange animal. When it comes to the brain, some researchers focus on the pieces that show discrete averages between men and women whereas others work with measures that suggest continuous variation. In the end, everyone has conflicting conclusions about what they’ve discovered.

That’s why Joel and her team encountered peer pushback following publication. In his letter to the journal, Marek Glezerman, a researcher in gender medicine at Tel Aviv University, declared: ‘Yes, there is a female and a male brain.’ Form aside, he argued: ‘Functionally, brains of women and men are indeed different.’ Adding insult to injury, he suggested that Joel’s analysis might not even be relevant: ‘MRIs are “still images”,’ he pointed out. ‘Looking at these is more akin to examining a road map and drawing conclusions about traffic patterns. Other imaging methods might have yielded different results.’

In their reply, Joel and colleagues wrote that the fact that sex can affect the form and function of cells ‘does not necessarily entail that the form and function … are either “male” or “female”.’

Another group, led by the evolutionary psychologist Marco Del Giudice of the University of New Mexico, offered its own counterpoint, saying that Joel and her team failed to conduct analyses that would have tested how well the different brain features would predict a person’s sex. When they did their own analysis, they found that the brain features predicted sex correctly ‘about 69-77 per cent’ of the time. That also means, of course, that for about a quarter of the population, sex was incorrectly predicted in their analysis.

In the end, what we think about our brains might depend on what we’re seeing through the prism of our senses and experiences

Del Giudice and team applied the Joel approach to sexual facial markers on three species of monkeys: crab-eating macaques, grivets and tufted capuchins. Looking at measures for 20 different monkey facial landmarks, they wanted to see how frequently the method would consistently construe individual monkeys of each species as ‘species-typical’. They found what they say was a low rate of correct species identification. Given that the method cannot distinguish even among monkey species with internal consistency, Del Giudice argued, how could it distinguish male and female brains?

Joel and her group responded by calling the critique an ‘elegant validation of our method of analysis’. What it really demonstrates, they said, is that human brains are far more mosaic than other primate species.

The debate rages on. In the end, what we think about our brains might very well depend on which part of the elephant – or article, or research paper – we examine, and how we interpret what we’re seeing through the prism of our senses and experiences.

Humans want tidy patterns, to have things link up neatly and make sense. Our brains strain to make these connections whether they are genuine or not. What’s more difficult is looking past illusory patterns and thinking more deeply about what we’re really seeing. As tempting as it is to collapse a human’s entire being, including the brain, into a single term – male, female – an honest look at how we really behave makes such reductionism look shallow, at best.

The most observant among us manage this in-depth examination. These acute observers are not the scientists, who can be remarkably myopic and rigid within their corners of research, but the storytellers. You can’t tell a good story about people if you’re not a keen observer of human behaviour, and it’s in our storytelling traditions that we find example after example of an inherent if unconscious understanding of the mosaic brain.

The oldest folk stories from every culture demonstrate this insight. Somehow able to look past the presumed anatomical binary of male parts and female parts, and instead at the people, these storytellers knew that women could be brave, courageous, resourceful and strong, just as men could be helpless, fearful, weak and timid. They knew that some people – most people – are a mix of all of it, with more intense expressions of some traits than others, on some days more so than others. Perhaps they used these contrasts to highlight the specialness of their protagonists, but the very acknowledgment of the possibility was significant. Not because it was unusual but because of how the heroes and heroines often pulled it off.

Consider the cross-dressing characters of some of the most famous western stories. From the Greek poets to William Shakespeare, women managed to pass as men and vice versa as long as they donned the expected clothing. Nothing about their behaviour, looks, inclinations or temperaments gave them away in these tales. Do clothes make the man, as they did for Rosalind in As You Like It; or, as in the case of Achilles on Skyros, the woman?

They certainly helped to make women such as Rosalind passably a man, and Achilles passably a woman. These gambits have been taken at face value across millennia. Listeners and readers accepted these characters as human, with traits and behaviours of no specific gender valence but able to assume the identity of a binary through mere clothes.

Even the most sexually dichotomised and repressive era in British history couldn’t keep the mosaic brain hidden. The prolific male novelists of the Victorian era, Charles Dickens and Anthony Trollope, used frank descriptions of men with feminine traits and women with masculine traits. In Dickens’s A Tale of Two Cities (1859), two women – Madame Defarge, representing the vengeful soul of revolutionary France, and Miss Pross as the indomitable spirit of England – both manifest putatively ‘masculine’ physical traits and behaviours, and ultimately face off in a violent scene in which vengeance is the loser. Nothing about them signals a woman who’s all or even mostly ‘feminine’.

Trollope created several female characters with ‘masculine’ traits of independence, insouciance about social norms, and a scorning of the pretence of femininity-defining dress. And he devoted not one but two novels of his Palliser sextet (1864-79) to his hero Phineas Finn, attractive to all women, conventionally ‘manly’ in many ways, yet repeatedly depicted as having a ‘feminine’ and ‘soft’ nature, always ready with a tear or two in a difficult moment. Trollope repeatedly describes his most famous female protagonist, Lady Glencora Palliser, as a woman who, ‘had Fate so willed it … would have been a thorough gentleman’ because of some of her less-than-feminine ways.

The people who pieced together these stories from the closely observed human experience around them probably had no idea that the unique patterns they devised for their protagonists neatly reflected what researchers are now revealing about our brains. Perhaps in Victorian society in particular, weighted by illusory trappings from interminable widows’ weeds to age-defying hair dyes for men, these novelists and the people who read their stories were better able to look past the window dressing that defines feminine and masculine, and simply describe – and appreciate – individuals as they were.

Trollope and Dickens were quirky fellows who created some quirky and memorable characters. Not coincidentally, they were both men of inviolable habit, living restlessly but rigidly within boundaries that they created. Trollope woke up every day before dawn, writing a precise number of words for a precise time period before going out to work; Dickens tramped for miles through his beloved London, pacing as his characters came to life in his mind. Their natures were unconventional, and it might be that unconventionality is a prerequisite for intuiting what Joel and her colleagues now say about each unique mosaic brain.

There I was, ‘passing’ as male in this online culture, all because I had not declared my own identity

Perhaps no group today is better at looking past conventional trappings than autistic people. There’s the perception that autism arises more often in boys than in girls, and that when it is present in girls, those girls are more ‘masculinised’. This perception is so prevalent that a controversial hypothesis to explain the development of the autistic brain relies on a premise of greater-than-usual exposure to testosterone during development. Explaining autism through the ‘extreme male brain’ is an attempt to triangulate androgenisation in the womb with putatively broadly masculine interests of autistic people (They love trains! And engineering!) and a penchant for being hyperanalytical. Take one developing human brain, add a walloping dose of testosterone at an unexpected time, and – Voila! We get a person obsessed with trains and logical enough to make Mr Spock weep with despair, and so emotionally disengaged he will never master the default human art of empathy.

Except.

Plenty of autistic people don’t share these qualities, and none of these traits that ‘extreme male brain’ adherents cite define autism. At its core, being autistic involves what non-autistic people view as ‘communication difficulties’, with some repetitive behaviours to aid in sensory regulation and expression, and struggles with executive function – planning your day-to-day life to meet your needs. The communication difficulties are a matter of where you stand. From the neurotypical gaze, autistic people can seem brusque, blunt and indiscreet, and unable, goes the canard, to read facial expressions or share emotions, unable to apply appropriate filters in social interactions.

But in my experience, none of that is accurate. Rather, autistic people fail to be fooled by facial expressions, which – let’s face it – often lie by design. And they are by and large immune to that neurotypical sponge-like absorption of social messaging telling them how they should behave. So they speak their minds and don’t fall for false smiles. And their empathy is so powerful that many autistic adults report that they find it overwhelming.

Given that so much of the social messaging that children receive is relentlessly gender-based, perhaps autistic features aren’t manifestations of being masculinised. Rather, they are the result of an innate resistance to, or uninterest in, gender-based social demands of any kind. Indeed, autistic people are far more likely to view gender as largely a secondary consideration, and to express gender that varies from what was assigned them at birth.

In other words, at least some autistic people might well represent the mosaic brain as untouched by gender expectations and other social influences as a human brain can be. And they are on average far less likely than neurotypicals to consider gender as an important factor in … anything.

For a few years in the early 2000s, I spent time on a university message board. Not considering possible repercussions, I signed up to participate with a username that carried no gender identification. Many women tend to self-identify gender in their online handles, appending ‘babe’ or ‘Ms’ or ‘grrl’ and so on to whatever modifier they think fits them. But my choice, without intent but probably out of habit and self-image, had zero gender valence whatsoever, and my identity was otherwise anonymous.

I found that for years, everyone on that board thought I was male, and interacted with me that way. There I was, a woman, mother of two, married to a man, with a history of being really, really into football and the Disney princesses of my youth, ‘passing’ as male in this online culture, all because I had not declared my own identity one way or the other.

Like Rosalind from As You Like It, I found that without the outward trappings of femininity, the visual signals that communicate conformity with society’s gender expectations about me, no one knew from any other clues what gender I was. I was just me, my brain, for once inhabiting a space where my childhood wish had, in a sense, come true: at long last, I was a girl being received as a welcome participant in a largely male-occupied space.