Large Scale Study Finds Little Difference Between Male and Female Brains

Are male and female brains really different?

Large Scale Study Finds Little Difference Between Male and Female Brains

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 very thin and shaky research, and forming world views 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 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 ly to experience clinical depression and anxiety as men, whereas men are about three times as ly to suffer from autism and twice as ly to have ADHD as women.

Boys’ dyslexia rate is perhaps 10 times that of girls and they’re 40% more ly to develop schizophrenia in adulthood.

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

Credit: Pixabay.

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 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 lier 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 ly 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.

Источник: https://www.zmescience.com/science/are-male-and-female-brains-really-different/

Neurosexism: the myth that men and women have different brains

Large Scale Study Finds Little Difference Between Male and Female Brains

An artificially coloured 3D magnetic resonance imaging scan of a human brain.Credit: K H Fung/Science Photo Library

The Gendered Brain: The New Neuroscience That Shatters The Myth Of The Female BrainGina Rippon The Bodley Head (2019)

Early in The Gendered Brain, cognitive neuroscientist Gina Rippon describes one of the myriad brain studies heralded as ‘finally’ explaining the difference between men and women.

It was a magnetic resonance imaging (MRI) analysis of 21 men and 27 women by researchers at the University of California, Irvine (R. J. Haier et al. NeuroImage25, 320–327; 2005).

Tiny by today’s standards, this brief communication nonetheless went on quite a publicity tour, from newspapers and blogs to television, books and, eventually, teacher education and corporate leadership conferences.

I woke one morning in 2010 to see an especially bad extrapolation of this study on the Early Show, a programme on US television network CBS.

The presenter, Harry Smith, gushed as medical correspondent Jennifer Ashton declared that men have “six-and-a-half times more grey matter” than women, whereas women have “ten times as much white matter” as men. Next came the obvious quips about men’s talent at mathematics and women’s uncanny ability to multitask.

Never mind that these differences would demand that women’s heads were about 50% larger, or that the Irvine team didn’t even compare brain volumes, but investigated a correlation between IQ and measures of grey or white matter.

Neurosexism

The history of sex-difference research is rife with innumeracy, misinterpretation, publication bias, weak statistical power, inadequate controls and worse.

Rippon, a leading voice against the bad neuroscience of sex differences, uncovers so many examples in this ambitious book that she uses a whack-a-mole metaphor to evoke the eternal cycle.

A brain study purports to discover a difference between men and women; it is publicized as, ‘At last, the truth!’, taunting political correctness; other researchers expose some hyped extrapolation or fatal design flaw; and, with luck, the faulty claim fades away — until the next post hoc analysis produces another ‘Aha!’ moment and the cycle repeats. As Rippon shows, this hunt for brain differences “has been vigorously pursued down the ages with all the techniques that science could muster”. And it has exploded in the past three decades, since MRI research joined the fray.

Yet, as The Gendered Brain reveals, conclusive findings about sex-linked brain differences have failed to materialize. Beyond the “missing five ounces” of female brain — gloated about since the nineteenth century — modern neuroscientists have identified no decisive, category-defining differences between the brains of men and women.

In women’s brains, language-processing is not spread any more evenly across the hemispheres than it is in men’s, as a small 1995 Nature study proclaimed but a large 2008 meta-analysis disproved (B. A. Shaywitz et al. Nature373, 607–609 (1995) and I. E. Sommer et al. Brain Res.1206, 76–88; 2008).

Brain size increases with body size, and certain features, such as the ratio of grey to white matter or the cross-sectional area of a nerve tract called the corpus callosum, scale slightly non-linearly with brain size. But these are differences in degree, not kind.

As Rippon notes, they are not seen when we compare small-headed men to large-headed women, and have no relationship to differences in hobbies or take-home pay.

History of bias

Rippon’s central message is that “a gendered world will produce a gendered brain”. Her book stands with Angela Saini’s 2017 Inferior and Cordelia Fine’s 2010 Delusions of Gender in rooting out the “neurosexism” that has pervaded attempts to understand difference at the brain level.

It’s a juicy history that would make for super-fun reading, if it were all truly in the past. Sadly, the moles keep surfacing. Rippon begins with an 1895 quote from social psychologist Gustave Le Bon, who used his portable cephalometer to declare that women “represent the most inferior forms of human evolution”.

She ends in 2017, with Google engineer James Damore blogging to co-workers about “biological causes” for the dearth of women in tech and leadership roles.

As Rippon shows, the hunt for proof of women’s inferiority has more recently elided into the hunt for proof of male–female ‘complementarity’.

So, this line goes, women are not really less intelligent than men, just ‘different’ in a way that happens to coincide with biblical teachings and the status quo of gender roles.

Thus, women’s brains are said to be wired for empathy and intuition, whereas male brains are supposed to be optimized for reason and action.

This was how researchers at the University of Pennsylvania in Philadelphia framed a highly touted 2014 MRI study that seared into the public imagination a picture of men’s and women’s brains as diametrically opposed subway maps: the connections in women are mostly between hemispheres, and those in men within them (M.

Ingalhalikar et al. Proc. Natl Acad. Sci. USA111, 823–828; 2014).

However, the map omits the vast majority of connections that did not differ between the study’s adolescent participants; nor did it control for puberty-related maturation or, once again, for brain size, all of which reduces apparent male–female difference.

Cultural paths

So if it’s not brain hard-wiring, how do we explain the often stark differences in behaviour and interests between men and women? Here is where we get to Rippon’s thesis on the impact of a gendered world on the human brain.

She builds her case in four loosely defined parts, from the sordid history of sex-difference research through modern brain-imaging methods, the emergence of social cognitive neuroscience and the surprisingly weak evidence for brain sex differences in newborns.

Rippon shows how children’s “cerebral sponges” probably differentiate thanks to the starkly pink-versus-blue cultures in which they are soaked from the moment of prenatal sex reveal.

Part 4 brings us into the twenty-first century, although not to any happy ending. It focuses on women in science and technology, and how the gendered world — including the professionalization of science and a masculine stereotype of “brilliance” — has impeded their entry into, and advancement across, this high-status realm.

Talented women are regarded as “workhorses”, men as “feral geniuses”, a distinction that children internalize by the age of six, according to research by Lin Bian, Sarah-Jane Leslie and Andrei Cimpian (L. Bian et al. Am. Psychol. 73, 1139–1153; 2018).

And all of this factors into the brain-building cycle of differential expectations, self-confidence and risk-taking that drives boys and girls down different trajectories of career and success.

Changing minds

This final focus explains the book’s subtitle, ‘The New Neuroscience that Shatters the Myth of the Female Brain’.

For a volume about debunking brain difference, why narrow it to women? At first, I thought it was a stab at Louann Brizendine’s 2006 The Female Brain, skewered in these very pages (R. M. Young and E. Balaban Nature443, 634; 2006).

Or perhaps it’s to underscore how ‘the female brain’ has been sized up as a strange variant of the real thing, much as we refer to a ‘female physicist’ or ‘female surgeon’.

Whatever the subtitle, the book accomplishes its goal of debunking the concept of a gendered brain. The brain is no more gendered than the liver or kidneys or heart.

Towards the end, Rippon flirts with the implications of this finding for the growing number of people transitioning or living between current binary gender categories.

But for now, she concludes, most of us remain strapped in the “biosocial straitjackets” that divert a basically unisex brain down one culturally gendered pathway or another.

Nature 566, 453-454 (2019)

  • Correction 06 March 2019: An earlier version of this article erroneously stated that non-linear scaling of some brain features can be seen in comparisons of small-headed men and large-headed women.

Источник: https://www.nature.com/articles/d41586-019-00677-x

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