Gender Mosaic

One summer morning over ten years ago I was home with my three kids when I heard the hissing of a burst water pipe outside my apartment building. Holding my youngest, then still a baby, in my arms, I rushed to the yard, bent the end of the gushing rubber pipe from an automatic garden-watering system to stop the water running, and asked my two older kids to call a friend who lived next door. When my friend arrived, I assumed he'd take care of the situation. But he just stood there, obviously at a loss. Only then did it dawn on me that he was just as clueless about plumbing as I was. I asked him to hold the folded pipe—and the baby—and went looking for the main faucet so I could shut off the water.

It had taken the bewildered look on my friend's face for me to become aware of my own bias. I must admit I was embarrassed. I had always believed in equality between the sexes, and I thought I was running my life accordingly. Yet here I was, expecting a man to handle a technical emergency.

Just around that time, I received an excellent opportunity to explore in depth my own and other people's gender biases: a colleague asked me to take over a course on the psychology of gender she had been teaching at Tel Aviv University. To prepare, I spent a year reading books and scientific articles on the development of women and men from the moment of conception. As a neuroscientist, I was most interested in the relationship between sex and the brain.

I soon learned that many scientists, just like many others, believe that the brains of females and males differ in profound and important ways, and that this is the reason for fundamental differences between women and men in almost every domain, from cognitive and emotional abilities, through interests and preferences, to behaviors. Self-help books that try to teach us how to cope and communicate with the other sex tend to take this belief as a given.

According to a popular version of the story, the female brain has a large communication center and a large emotion center, and is hardwired for empathy. The male brain, on the other hand, has a large sex center and a large aggression center, and is hardwired for building systems.

This story seems to provide us with a neat biological explanation for much of what we encounter in day-to-day life. It explains why women are more sensitive and emotional, whereas men are more aggressive and sexual; why most teachers are women, and most engineers, men.

"It's the hormones, stupid," we are told. In the womb, the story goes, a huge surge of testosterone, secreted by the testes of the male fetus, changes his brain from the default female to male. So girls are born with a female brain and boys with a male brain. The details of the rest of the story vary among different authors, but they all explain why women and men behave in the ways depicted by popular gender stereotypes. Girls are nice and empathic, and boys are active and aggressive—because this or that region in the brains of girls is smaller or larger than in boys, or because they have more or less of this or that hormone.

There are no surprises. No matter what the findings are, they are never interpreted in a way that would run counter to prevalent gender stereotypes. For example, the amygdala, a brain region central to emotions, was for many years considered to be, on average, larger in males than in females, yet no one ever claimed that based on the size of their amygdalae, men were, by nature, the more emotional sex. (Recent statistical analyses have shown that, in fact, there is no difference between the sexes in the average size of this brain region.)1

The concept of a male brain and a female brain fits well the popular view that men and women come from different planets, but does it fit the scientific evidence? My own attempt to answer this question began with a startling study I encountered about a decade ago while preparing to teach the psychology of gender course.

Did you know that thirty minutes of stress is enough to change the "sex" of some brain regions from male to female, or vice versa? I hadn't known this either. Reading about this study led me to several years of extensive research, which completely transformed the way I thought about sex, gender, and the brain.

After analyzing hundreds of brain scans, I realized that sex differences in the brain do not consistently add up in individuals to create "male" and "female" brains. Note that I'm not saying there are no differences between the brains of females and males; on the contrary, my team has documented many such differences, as have numerous other scientists. What I'm arguing is that these differences mix together in each individual brain to create a unique mosaic of features, some of which are more common in females and others are more common in males. This notion goes hand in hand with what I'm sure many people already know: we are all patchworks of "feminine" and "masculine" traits. But it goes further; it suggests that there is no such thing as a "male" or "female" brain—or a "male" or "female" nature.

Before I describe how I arrived at the brain mosaic idea and what its implications might be, let me share with you a few intriguing facts about the brains of females and males, and how the perception of these facts has changed over the past few centuries.

When egalitarian ideas started circulating prominently in Europe in the seventeenth and eighteenth centuries, men faced an embarrassing dilemma. The new principles implied that all humans, women and men, were by nature equal. This notion threatened the existing social order, in which women played subordinate roles. The fear was that equality would undermine the very foundations of society—most important, that given equal status, women would stop serving men.

Molière satirized these fears in his 1672 comedy Les Femmes savantes, in which the husband rails against his wife and other science-minded women, who neglect their domestic duties: "They want to write and become authors. No science is too deep for them… They know the motions of the moon, the pole star, Venus, Saturn and Mars… and my food, which I need, is neglected."1

Science was called upon to resolve the political debate over the role of women in an egalitarian social order. In The Mind Has No Sex?, Londa Schiebinger, of Stanford University, writes that the mission was to show that it was nature, not men, that was responsible for gender inequality. Schiebinger traces how the scientific study of female and male anatomy, including the brain, turned political. Without abandoning the axiom of equality, she argues, the medical and scientific communities became preoccupied with differences between the sexes. "Women were not to be viewed merely as inferior to men but as fundamentally different from, and thus incomparable to, men," she writes.2

Sexual differences between females and males are all too obvious, but do they extend to the entire body and the brain? A great deal was at stake: answering this question in the affirmative could help justify the different social standing of women and men; a negative answer would suggest that women had been unjustly oppressed for centuries, and that major social changes were needed. A great many philosophers and other thinkers—virtually all of them male—tended to define the scope of the differences between the sexes in the broadest possible terms. Schiebinger quotes one eighteenth-century French physician as saying that "the essence of sex is not confined to a single organ but extends, through more or less perceptible nuances, into every part."3

Science became a legitimate arbitration arena for such disputes. Unlike religion, which had carried the burden of justifying women's inferiority up to the scientific revolution, science was thought to be impartial, and therefore to provide objective evidence in the arguments over women's abilities. "Perhaps the knife of the anatomist could find and define sexual difference once and for all," Schiebinger writes. "Perhaps sexual differences—even in the mind—could be weighed and measured."4

Indeed, wrote Stephanie Shields, of Pennsylvania State University, weighing and measuring the skull and, later, the brain—by then established as the seat of the mind—became of paramount importance.5 In ancient Greece, Galen had deemed the testicles the most noble part of the body—which made perfect sense because they were found only in the "superior" sex. But in the seventeenth century, it was the brain that came to be viewed as the most noble and divine organ: holder of all the senses, intelligence, and wisdom. It was hence essential that men be found to have superior brains.

Initially, this seemed like an easy task. The skull—thought to provide a reliable indication of brain size—was found to be, on average, smaller in women than in men. What could better explain women's inferiority (well, except for the absence of testicles)?

But it was too early to celebrate. After all, quite a few animals have larger skulls than we do. Sperm whales, for one, have skulls that are, by far, larger than those of humans. Scientists who were keen on proving the superiority of men over women—but surely not the superiority of whales over men—searched for a way around this inconvenient fact. They suggested that perhaps it wasn't the size of the skull but the ratio of skull size to body size that mattered.

Yet calculating the ratio failed to produce the desired results. Worse still, a number of scientists actually found that relative to total body weight, women's skulls were larger than men's. These scientists did not for a moment conclude that the relatively larger skulls of women meant greater intelligence. Undeterred in their zeal to produce "scientific" evidence of male superiority, some scientists managed to interpret their findings as a sign of women's lesser intelligence. Women, they said, resembled children, whose skulls were also large relative to body size, which meant that women were less developed, and consequently less intellectually competent than men.

Looking back at the history of brain research, I'm impressed by the creativity that went into twisting scientific facts to serve a social or political agenda. When scientists didn't like what they found, they often either changed the interpretation or simply abandoned the method that led to the undesired result, looking instead for a "better" measure. According to Shields, reams of paper were dedicated to arguing over "appropriate" measures for skull size in women and men. Should it be the ratio of skull weight to body weight? Perhaps it was a matter of bone density in the skull compared to the rest of the skeleton? The issue proved impossible to resolve: applying some measures, the results "favored" men; with others, they "favored" women.

The idea that larger is better continued to be popular when scientists discovered that not only the skull but also the brain was larger, on average, in men than in women. The eminent nineteenth-century neuroscientist Paul Broca was among the more diplomatic, but he still expressed this view in no uncertain terms. "We might ask if the small size of the female brain depends exclusively on the small size of her body," Broca wrote in a scientific journal in 1861. "But we must not forget that women are, on the average, a little less intelligent than men, a difference that we should not exaggerate but which is nonetheless real."6 The prominent evolutionary biologist George Romanes was blunter. The smaller size of women's brains was responsible for female mental inadequacy, he wrote in 1887, which "displays itself most conspicuously in a comparative absence of originality, and this more especially in the higher levels of intellectual work."7 Theodor Bischoff, a distinguished nineteenth-century biologist, even went as far as to claim that women, because of their small brains, did not have the intellectual skills necessary for academic studies, and that too much education might hinder the development of reproductive organs in adolescent girls.8

These older versions of the belief in female and male brains being fundamentally different sound absurd to us now. Today, when women outnumber men at so many levels of academic study, it seems ridiculous that scientists could have believed women to be incapable of attending university because of the size of their brains. Don't get me wrong; women's brains are, on average, still smaller than men's. What has changed is not the size of the brain, but rather the social norms that once prohibited or discouraged women from studying.

While the focus on brain size took on a life of its own, the search for scientific findings in support of men's superiority over women had, in the meantime, moved into a new arena. In the wake of the discovery in the nineteenth century that different brain regions perform different functions, scientists began comparing these regions in women and men. Unsurprisingly, here too they found anatomical support for the superior intelligence of men.9

Much of the attention was directed to the cerebral cortex, as this outer part of the brain is responsible for voluntary action, perception, cognition, language, and thought. It is made of so-called gray matter, which holds the bodies of billions of nerve cells—the neurons. Underneath the cortex there is a layer of white matter, which mainly contains the fibers connecting the neurons. The cortex has traditionally been subdivided into four major lobes, named after the skull bones protecting them.

When the role of the frontal lobes in cognitive function was established, many neuroscientists were quick to point out that these lobes were larger and more developed in men than in women. Then some neuroscientists suggested that the seat of the intellect resided in the parietal lobes, those at the top of the brain, rather than in the frontal lobes. And once the importance shifted to the parietal lobes, certain scholars promptly revised the interpretation of anatomical findings to match the accepted view of male superiority.10 In 1895, for instance, American psychologist George Thomas White Patrick wrote in Popular Science Monthly that "the frontal region is not, as has been supposed, smaller in woman, but rather larger relatively.…But…a preponderance of the frontal region does not imply intellectual superiority…the parietal region is really the more important."11

More than a hundred years have passed since these words were written. In that time, neuroscientists have continued to find differences between the brains of males and females, in animals and humans. I'll discuss them again in the next chapter, but here are a few examples. Most of the cortex is, on average, thinner in men than in women; men have on average a lower proportion of gray matter and a higher proportion of white matter. In addition, men have larger ventricles—big, fluid-filled cavities in the center of the brain (these are the large solid-color spaces you see on medical scans). Readers who were happy to learn that men have larger brains than women may be less happy to read about men's larger ventricles.

If you believe, as did scientists in the nineteenth century, that the size of the brain matters, then it is indeed awkward to learn that your bigger brain comes packaged together with bigger—what should I call them?—empty spaces. But the message I want to convey is that both sexes have nothing to worry about. Men do just fine with their larger ventricles; women do just fine with their smaller brains.

What is worrying is that sex differences are still being used to justify gender inequality. No one today would dare use biological comparisons between races or social classes to justify racism or the economic status of the poor—as was done up to the twentieth century—but sex differences in the brain are still being pulled out to validate women's inferior status. Here's how Schiebinger puts it: "The alleged defect in women's minds has changed over time: in the late eighteenth century, the female cranial cavity was supposed to be too small to hold powerful brains; in the late nineteenth century, the exercise of women's brains was said to shrivel their ovaries. In our own [that is, the twentieth] century, peculiarities in the right hemisphere supposedly make women unable to visualize spatial relations."12 In the twenty-first century, the search for the "essential" difference between the brains of women and men continues, resonating all too often with historic myths about differences between the sexes.

A few years ago, I took part in a scientific discussion, "SeXX and SeXY: A Dialogue on the Question of the Female Brain and the Male Brain," within the framework of a neuroscience symposium at Stanford University.1 My co-discussant, Louann Brizendine, argued that women are women because they have a female brain, and men are men because they have a male brain—a view she has also expressed in her best-selling books. I, for my part, presented my own view: that humans and their brains are composed of unique mosaics of "female" and "male" features. After the debate, I overheard someone tell one of the organizers: "The problem is that Louann has a female brain, and Daphna has a male brain." This person was probably implying that Brizendine and I weren't well matched as public speakers because of our different debating styles.

But here's the irony. This remark undermined the view held by Brizendine and many others—that the male brain is a product of exposure to high levels of testosterone in the womb and later in life, whereas the female brain develops as a default, in the presence of low testosterone levels in the womb, and is later further shaped by "female" hormones. If that were the case, how could I, born a typically developed female, who later on had been exposed to high levels of "female" hormones in the course of three pregnancies and a total of about three years of breast-feeding, possibly have a male brain?

Irony apart, the belief that "men's brains are like this, and women's brains are like that," is still immensely popular, among scientists and the general public alike. Today—as in previous centuries—the common belief is that differences between the brains of men and women lie at the root of the fundamental differences assumed to exist between the sexes. It is little wonder, then, that this area of research is so intense. A review of scientific literature published in Neuroscience & Biobehavioral Reviews in 2014 yielded some 5,600 studies in which the volume and density of various brain regions in men and women had been compared in the preceding quarter of a century.2

By now scientists have reported on hundreds of sex differences in the brain. Women and men differ in the size of the entire brain and in the size of specific brain regions. (Many of the latter differences disappear when overall brain size is taken into account; others are reduced or even reversed—that is, a region may be smaller, on average, in women, but relatively larger compared to their brain size.) 3 With the advance of technologies enabling scientists to peer into the brain with increasing depth and detail, sex differences have also been found in several systems of chemical messengers called neurotransmitters. Moreover, differences have been found in the microanatomy of the brain—the structure of neurons and the density of receptors, the molecules to which the neurotransmitters bind.

Note that these are all average differences—they emerge when women and men are compared as two groups, but not necessarily when they are compared individually. We may, for example, find that a particular brain region is, on average, larger in men taken as a group than in women as a group. But if we make individual comparisons, we'll discover a great deal of overlap—namely, that this region is the same size in many women and men; furthermore, in some women it will be large, whereas in some men it will be small. That's true for most known sex differences in the structure of the human brain. The average differences are small, and there is a great deal of overlap between the sexes.4

There are scientists, however, who argue that although average sex differences in brain structure are small, they may underlie greater differences in brain function—in other words, that the brains of women and men, even if similar in structure, might work differently. That's the rationale for studies aimed at discovering sex differences in patterns of brain activity during the performance of various mental tasks.

But in reality, in most tasks, the patterns of brain activation are similar in women and men; lots of studies have looked for such differences but failed to find any. When studies do find a difference, it is typically present in the functioning of only some of the brain regions involved in performing a certain task, while others are similarly activated in the two sexes. The problem is that the sameness, for the most part, goes unreported, whereas the differences get plenty of play in both the scientific and the popular press.

That was just what happened with one popular theory: that when processing language, women tend to use both cerebral hemispheres to a greater extent than men. For instance, in a study published in 1995 in Nature, Yale University researchers used a method called functional magnetic resonance imaging to scan brain activity in nineteen women and nineteen men who were asked to perform three types of language-related tasks.5 In their scientific paper, the researchers devoted little attention to the two tasks—letter recognition and grouping words by meaning—in which they found no difference between the sexes. Instead, they reported in great detail on the third task—rhyming—for which they did find a sex difference: when performing this task, men activated a number of regions on the left side of their brain; women activated these same regions on both sides of the brain. The brain scans for rhyming, which lit up in different patterns in the two sexes, were included in the paper; the scans for the other two tasks were not.

The study got plenty of media attention—it fit in nicely with the stereotype of the sexes being different to the point of applying their brains to a given task in different fashions. Some of you may even recall those newspaper headlines and stories on TV. Here's a fairly dramatic one from the New York Times: "Men and Women Use Brain Differently, Study Discovers."

Then a number of other studies showed no consistent difference in brain activity between women and men performing various language-related tasks. (How such disparate results come about is an interesting issue in itself; I'll discuss it in chapter 8.) In an attempt to resolve the controversy, scientists from the University Medical Center Utrecht pulled together the results of twenty-six studies on this topic using a statistical method called "meta-analysis." Their conclusion, published in 2008 in a journal called Brain Research, was that no difference in language processing could be proven to exist between the sexes.6 Do you recall any major media stories about these findings? I don't either.