Early crude measurements in the 19th-century showed the male brains are significantly larger (about 11% larger) than female brains, which is sometimes used as an argument that the average male is more intellectually equipped than the average female. However, this neurosexist viewpoint has been refuted by modern brain imaging and investigations that show there are very little to no functional differences between the male and female brains.

Men are from Mars, women are from Venus. Oh, really?

The invention of magnetic resonance imaging (MRI) in the early 1990s allowed scientists to produce highly detailed 2-D and 3-D images of the brain, unleashing a revolution in neuroscience. Some researchers took advantage of this opportunity to look for differences between men’s and women’s brains, spurred by observable gender-specific differences in terms of personality, as well as dimorphic traits between the sexes (hormone production, reproductive organs, chromosomes).

Over the years, a grand body of studies has amassed in the scientific literature pertaining to sex-linked brain differences between the two sexes. Not all that surprising, these findings have proven extremely controversial, ranging from conclusions that can be interpreted as “women are inferior” to “men and women’s brains are different, but complementary”.

Women’s brains are said to be wired better for empathy and intuition, whereas male brains are better equipped for reason and action. This would explain stereotypes about genders, such as that women are more emotional and better at communicating, while men are more competitive.

But these pop neuroscience notions are based on very thin and shaky research, and forming world views based on them can even be damning. James Damore, a former Google engineer, learned this the hard way. In 2017, Damore wrote a 10-page manifesto that basically argued against workplace diversity since “the distribution of preferences and abilities of men and women differ in part due to biological causes, and that these differences may explain why we don’t see equal representation of women in tech and leadership.”

The Google engineer, who holds a graduate degree in biology, linked to various scientific studies that support his claims, such as research suggesting that women care more about people than things, later concluding that “differences in distributions of traits between men and women may in part explain why we don’t have 50% representation of women in tech and leadership. Discrimination to reach equal representation is unfair, divisive, and bad for business.” Damore was later dismissed from Google following the leaking of the internal memo for violation of the company’s code of conduct.

Although it’s easy to see Damore’s sacking as unfair and political, the harsh truth may be that he was the victim of flawed, gender-biased thinking that is pervasive in all corners of society, academia included. Although Damore’s views have been supported by some noted psychologists such as Debra Soh and Jordan Peterson, the consensus is that the engineer gravely overemphasized the literature. Gina Rippon, the chair of cognitive brain imaging at Aston University, noted that Damore “relied on data that was suspect, outdated, irrelevant, or otherwise flawed.”

The problem with many of these studies is that they can be flawed in methodology because the human brain is inherently complicated to understand and still very much a work in progress, or only approach a small subset of supposed sex differences in cognitive abilities and brain structure that can be easily taken out of context by laypeople. Take for instance one study from the UK that looked at brain scans using MRI for 2750 women and 2466 men and examined the volumes of 68 regions within the brain. On average, scientists found that women tended to have significantly thicker cortices than men while men had higher brain volume than women in subcortical regions. Thicker cortices are associated with higher scores on cognitive and general intelligence tests. Alright, but what does this mean for how the brain works? Very unclear.

Even so, some scientists believe that there have to be some sex-specific differences in the human brain in order to explain significant differences between men’s and women’s cognitive function. For instance, there are many instances where the male-female ratios are unbalanced for cognitive and neuropsychiatric disorders. Women are twice as likely to experience clinical depression and anxiety as men, whereas men are about three times as likely to suffer from autism and twice as likely to have ADHD as women. Boys’ dyslexia rate is perhaps 10 times that of girls and they’re 40% more likely to develop schizophrenia in adulthood.

Gender-binary societies can lead to gender-binary assumptions. Even in science

The fact that there could be biological differences between the sexes that may explain these significant gender differences is not only logical but also seductive.

But Lise Eliot, a professor of neuroscience at the Chicago Medical School, claims that anyone searching for innate differences between the sexes is on a futile journey. Although she acknowledges some slight differences between the male and female brains, Eliot believes the human brain is a unisex organ.

Eliot is the lead author of a 2021 study that conducted a mega-synthesis of hundreds of the largest and most highly-cited brain imaging studies addressing 13 distinct measures of alleged sex differences. The meta-analysis encompasses three decades’ worth of research.

For nearly every measure, Eliot and colleagues found virtually no differences that could be widely reproduced across studies. For instance, the volume or thickness of specific regions in the cerebral cortex is often cited to differ among men and women, as in the UK study listed above in this article. However, the analysis showed that the regions identified differ by a wide margin between studies.

Another red flag when it comes to drawing conclusions from sex-specific brain research is the poor replication between diverse populations. The analysis found wild variations in findings between Chinese versus American populations, for instance, which indicates that we lack a universal brain marker for distinguishing men and women’s brains across the human species, if one even exists.

“Since the dawn of MRI, studies finding statistically significant sex differences have received outsized attention by scientists and the media,” said Dr. Eliot in a statement.

“The handful of features that do differ most reliably are quite small in magnitude,” Dr. Eliot said. “The volume of the amygdala, an olive-sized part of the temporal lobe that is important for social-emotional behaviors, is a mere 1% larger in men across studies.”

This study, titled “Dump the Dimorphism”, debunks the idea that the human brain is sexually dimorphic. This is science-speak for biological structures that come in two distinct forms in males and females, such as how only male deers have antlers or the genitalia of men and women. That’s not the case for the human brain though, the authors claim.

Concerning brain size, when overall body size and mass are controlled for, no individual brain regions varied by more than about 1% between men and women. These differences, tiny as they are, are not reported consistently across geographically or ethnically diverse human populations. Furthermore, the nominal brain difference in the size of the brain between sexes is actually smaller than those seen in other internal organs. For instance, the heart, lungs, and kidneys are between 17% and 25% larger in men.

A highly-cited 2014 study from the University of Pennsylvania found females’ brains show more coordinated activity between the left and right hemispheres, while males’ brain activity was more tightly coordinated at local brain regions.

However, the notion that men’s brains are more lateralized, whereas women’s two hemispheres are better connected operating more in sync with each other has been rebutted by Eliot et al. Other studies have found that the actual difference in both accounts is even less than 1% across populations. If men’s and women’s brains were indeed connected significantly differently, we’d see much more disabilities in men following brain injuries such as stroke. Large-scale datasets show that there is no gender difference in aphasia (loss of language) following a stroke in the left hemisphere.

One 2018 study, which summarized the last 40 years of research, found “cognitive sex differences often emerge in the absence of sex differences in hemispheric asymmetry (and vice versa), implying the two phenomena are at least partly independent of each other.” So assumptions that explain sex differences in cognitive abilities such as mental rotation or verbal memory based on studies that reported sex-specific differences in brain asymmetry mistook correlation for causation.

Another point of contention is supposed cognitive differences revealed by studies employing functional MRI, which shows which brain regions ‘light up” during language, spatial, and emotional tasks. Across hundreds of studies compiled by the researchers, the research that reported different activities between men and women also exhibited poor reproducibility.

Another explanation for the large number of contradictory studies in this field is a phenomenon known as publication bias. Smaller, early studies in the late ’90s and early 2000s that found sex differences in the brain were likelier to get published, whereas those that found no male-female brain difference were left unpublished. This file drawer effect is pervasive across all scientific fields, not just neuroscience, and as a result, many studies skew towards those that report “novel” findings or “discover” something. But without complementary studies that find negligible effects, we lack the proper context on how to frame novel findings, and science is left poorer and less reliable as a result.

“Sex comparisons are super easy for researchers to conduct after an experiment is already done. If they find something, it gets another publication. If not, it gets ignored,” Dr. Eliot said. Publication bias is common in sex-difference research, she added, because the topic garners high interest.

Sex-difference research is rife with not just publication bias, but also flawed methodologies (inadequate controls and weak statistical significance). This is why you see a lot of brain studies published in the media that expose differences between men and women. But when peers highlight the hyped extrapolation or all too common design flaw, these rebuttals receive abysmal attention.

But if there are no inherent, hard-wired sex-specific brain differences at birth between the sexes, what then could explain the significant and sometimes obvious gender dissimilarities seen in things ranging from cognitive tests to personality traits. One possible explanation is that the human brain is extremely plastic, meaning its neural circuits morph with practicing certain skills, so socialization and upbringing may play a grander role than we thought. Sex hormones also affect the brain, but the idea that these effects add up to create two distinct types of brain, male and female, has never been proven.

Another reason why there are many inconclusive and controversial studies in this field may have to do with a lot of individual variabilities. One 2015 study that compared the brains of 1,400 men and women, analyzing their volume, connections, and other physical structures, found the human brain is actually a tangled mix of both sex-congruent and sex-incongruent features. The left hippocampus, a region of the brain associated with memory, was found to be generally larger in males, but women with a large left hippocampus were common. Up to 53% of individual brains included in this study had a mix of both “typically male” and “typically female” features, and only 8% had “very male” or “very female” brains.

These findings are corroborated by a similar analysis of personality traits, attitudes, interests, and behaviors of more than 5,500 individuals, which reveals that internal consistency is extremely rare.

“This extensive overlap undermines any attempt to distinguish between a ‘male’ and a ‘female’ form for specific brain features,” said Daphna Joel, a psychologist at Tel Aviv University and lead author. These findings have “important implications for social debates on long-standing issues such as the desirability of single-sex education and the meaning of sex/gender as a social category.”

“We separate girls and boys, men and women all the time,” she says. “It’s wrong, not just politically, but scientifically – everyone is different,” Joel told New Scientist.

Whether or not male and female brains may be no more different from each other than male and female hearts or livers will likely remain a point of contention for years to come, but modern research is showing that, if anything, early studies in this field have been greatly exaggerated. The male and female brains are much more similar than they are different.

“Sex differences are sexy, but this false impression that there is such a thing as a ‘male brain’ and a ‘female brain’ has had wide impact on how we treat boys and girls, men and women,” Dr. Eliot said.