It’s the first thing we want to know when a newborn arrives. We state it on every form we fill out. We mark it with pronouns, names, clothing, and hairstyle. It’s the first thing we register about a person.

Sex categories – whether you have female or male genitals – are fundamental for reproduction. They are also the principal way we carve up the social world. No surprise then that scientists and the general public alike often assume that sex categories are no less essential to how we think, feel, and behave, taking it for granted that there are female and male natures subserved by a “female brain” and a “male brain”, respectively.

But research published yesterday in the Proceedings of the National Academy of Sciences (PNAS), led by one of us (Daphna Joel), puts another nail in the coffin of this persistent view.

The research was inspired by a study with rats by Tracey Shors and colleagues, published in the Journal of Neuroscience. She found that just fifteen minutes of stress can change the “sex” of some features of the brain from the “male” to “female” form, or from the “female” to “male” form. From this and dozens of similar studies – involving different experiences and brain features – two important principles emerged.

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First, a brain feature that is typical in females in one condition (a stress experience, say) may be what is typical in males under other conditions, and vice versa. And second, these interactions between sex and environment may be different for different brain features, cells, and regions. For example, lab rats that have enjoyed a peaceful, stress-free life show a sex difference in the density of the ‘top end’ dendritic spines (these transmit electrical signals to the neuron cell body) in one region of the hippocampus. (The female dendritic spines are denser.) But look at the same brain region in a group of rats who have been stressed for just fifteen minutes, and now the dendritic spines of the male rats are bushy, like those of unstressed female rats. Conversely, the “top end” dendritic spines of stressed female rats become less dense, like those of unstressed male rats. In other words, brief stress exposure reverses the “sex difference” for that particular brain characteristic. However, this same brief stress has a different effect on the “bottom end” dendrites of these same neurons. Here, male and female dendritic spines are identical, so long as those rats have lived a stress-free life. But what happens if the rats are stressed? There’s no effect on “bottom end” dendritic spines in females, but their density increases in males, resulting in a sex difference.

Imagine, then, these sex-by-environment interactions affecting many different features of every brain, from utero onwards. With each experience, some brain features change their form, others do not, giving rise to unique combinations of forms. Not a “male brain”, or a “female brain”, but a shifting “mosaic” of features, some more common in females compared to males, some more common in males compared to females, and some common in both.



This is exactly what the new study found for the first time, with colleagues from Tel Aviv University, the Max Planck Institute, and the University of Zurich. They tested this prediction by analyzing magnetic resonance images, which directly capture structural properties of the brain, from more than 1,400 human brains from four large data-sets. They identified in each data set the regions showing the largest differences between women and men. Next, they defined a “male-end” (males more prevalent than females) zone and a “female-end” (females more prevalent than males) zone for each of these regions, based on the range of scores of the most extreme third of men and women, respectively. They found that between 23% and 53% of individuals (depending on the sample) had brains with both “male-end” and “female-end” features. In contrast, the percentage of people with only “female-end” or only “male-end” brain features was small, ranging from zero to 8%. Where this differs to past research is that it looks for the first time at whether or not sex differences “add up” in a consistent way. While future studies should investigate whether there is internal consistency in additional types of data, such as histological measures, this study of structural sex differences found that these do not combine consistently to create two categories of brains or a male-female continuum of brains.

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So what is a “female brain” or a “male brain”? Is a “female brain” the type of brain possessed by the very few individuals with consistently “female-end” brain characteristics - some of whom, by the way, are men? And if so, what kind of a brain do the majority of females have?

That brains don’t come in two forms fits with what we know about gendered behaviour. With academics and commentators often arguing over the origins and size of sex differences, an important observation by psychologist Janet Spence is often overlooked. This is that correlations between “masculine” traits are weak or non-existent; so too for “feminine” traits . Having one doesn’t imply you have another. For example, being gentle doesn’t imply that one is also dependent. So do sex differences “add up” in a consistent way to create two types of humans, or, like the brain, create “mosaics” of personality traits, attitudes, interests, and behaviours, some more common in males, others more common in females?

The new PNAS study investigated this in over 5500 youth from three datasets. Most differences between females and males are small, but this study looked only at the largest differences (for example, worries about weight was the personal attribute showing the largest difference, with about 80% chance of correctly guessing someone’s sex on the basis of being above or below average concern; while for sex-stereotyped activities, the largest difference was in the use of cosmetics, for which the chance of guessing sex on the basis of their use was above 90%). Even looking only at such behaviours, between 55% and 70% of people had a “mosaic” of gender characteristics, compared to less than one per cent who had only “masculine” or only “feminine” characteristics.

This makes the notion of female and male natures as unintelligible as that of female and male brains. Which of the many mosaics that males display should be considered the male nature? Is it a profile of pure masculinity that appears to barely exist in reality?

Or is it time to let go of binary thinking and celebrate the fact that there are many different ways to be male, to be female, to be human?

We can see social issues more clearly when we stop viewing them through the distorting lens of sex categories, and start fully appreciating human variability and diversity.

Daphna Joel is a professor in the School of Psychological Sciences and Sagol School of Neuroscience, Tel-Aviv University.

Cordelia Fine is an ARC Future Fellow in the Melbourne School of Psychological Sciences and Melbourne Business School, University of Melbourne, and author of the forthcoming book, Testosterone-Rex, which will be published by WW Norton.