Sex differences in the human brain — long-awaited Edinburgh study

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BrainThe largest single-sample study of structural and functional sex differences in the human brain has been attracting attention for more than a year, but only now has been published in a peer-reviewed journal, reports Quilette. The Edinburgh study’s implications regarding male and female intellectual abilities will be fiercely debated between those who believe gender is a social construct and those who believe it is hard-wired.

The team of researchers from Edinburgh University, led by Stuart Ritchie, author of Intelligence: All That Matters, found that men’s brains are generally larger in volume and surface area, while women’s brains, on average, have thicker cortices.

The largest single-sample study of structural and functional sex differences in the human brain ever undertaken, involving over 5,000 participants (2,466 male and 2,750 female) has been attracting attention for more than a year, but only now has it been published in a peer-reviewed journal, writes Toby Young, associated editor at The Spectator in a Quilette report.

He writes: “For those who believe that gender is a social construct, and there are no differences between men and women’s brains, this paper is something of a reality check. The team of researchers from Edinburgh University, led by Stuart Ritchie, author of Intelligence: All That Matters, found that men’s brains are generally larger in volume and surface area, while women’s brains, on average, have thicker cortices. ‘The differences were substantial: in some cases, such as total brain volume, more than a standard deviation,’ they write.

“This is not a new finding – it has been known for some time that the total volume of men’s brains is, in general, larger than that of women’s, even when adjusted for men’s larger average body size – but all the studies before now have involved much smaller sample sizes.

“Does this paper have any implications when it comes to men and women’s intellectual abilities? The answer is yes, but they’re not clear cut.

“On the one hand, feminists won’t like this confirmation that men, on average, have bigger brains than women because there’s a well-established connection between total brain volume and IQ. That was the conclusion of the authors of a 2015 meta-analysis that looked at 88 studies involving 148 mixed sex samples comparing magnetic resonance images of people’s brains with their cognitive test scores. They found that the association between brain volume and cognitive ability was positive in children and adults, applied across a range of different IQ domains (full-scale, performance and verbal IQ) and was true of both men and women. According to another study led by Richard Haier, author of The Neuroscience of Intelligence, total brain volume accounts for about 16 % of the variance in IQ.

“Remember, we’re just talking about mean differences between men and women’s brains – as Ritchie and his team point out, there is a substantial degree of overlap between the sexes on all their measures. Nonetheless, if there is a positive correlation between total brain volume and intelligence, and men generally have larger brains than women, doesn’t that mean that men are, in aggregate, more intelligent than women?

“Not so fast. Don’t forget that Ritchie’s team also found that women’s brains, on average, have thicker cortices than men’s and there’s some evidence linking intelligence with the thickness of the cerebral cortex. For instance, this 2009 study of 216 children found a positive association between cortical thickness and general cognitive ability, as did this 2013 study. However, this finding is less robust than the link between total brain volume and IQ, with some studies failing to replicate it and others both replicating it and seeming to contradict it at the same time. For instance, this 2015 paper involving 514 subjects found that the association between cognitive ability and cortical thickness was negative for 10-year-olds – that is, the smarter they were, the thinner their cortices – but positive for 42-year-olds.

“It is worth noting that Ritchie et al – who studied more than 5,000 subjects, don’t forget – confirmed the positive association between total brain volume and intelligence. The men in their sample scored, on average, fractionally higher than the women on a test of verbal-numerical reasoning and recorded slightly faster processing speeds on another test. After extensive statistical analysis, they concluded that the modest sex differences in verbal-numerical reasoning were almost entirely due to differences in brain volumetric and surface area measures and the differences in reaction time were partly due to the same.

“Ritchie’s team caution against reading too much into this finding and note that the cognitive tests given to their subjects were fairly rudimentary and their sample may not be representative of the population at large. They also point out that previous, representative studies have found no mean difference between men and women in general cognitive test performance. Back in 2017, before his paper had been peer-reviewed, Ritchie was keener to talk about another of his team’s findings, namely, that the male brains they studied were, on most measures, more variable than the female ones.

“He was excited about the fact that this discovery complemented a 2008 study of male-female IQ differences, also carried out by a team from Edinburgh, which found only negligible differences in the mean scores of men and women on intelligence tests, but that men outnumbered women at either end of the cognitive bell curve. So greater variability among men when it comes to cognitive ability. That was also the conclusion of a 2007 paper which found that among those scoring in the top 2% of the Armed Forces Qualification Test, men outnumbered women by a ratio of 2:1.

“Ritchie and his co-authors note that this finding has been replicated many times – ‘almost universally’ is the phrase they use – but that doesn’t mean it’s universally accepted. Far from it. When Lawrence Summers, then the president of Harvard University, suggested that the higher preponderance of men on the right-hand tail of the IQ distribution curve might help to explain why there are more male than female professors in the maths and sciences at top universities, he was rounded on by almost the entire liberal establishment. Distinguished Harvard alumni withheld donations, the university’s Graduate School of Arts and Sciences passed a motion of no confidence in him and he was forced to apologise – over and over again – like a supplicant at a Chinese show trial.

“In the end, he had no choice but to tender his resignation. This controversy is thought to be the reason he didn’t get the job of Treasury Secretary in the first Obama administration.

“Summers made things worse for himself by using the word ‘intrinsic’ to describe this difference between men and women, suggesting it is genetically hard-wired. ‘Research in behavioural genetics is showing that things people previously attributed to socialisation weren’t due to socialisation after all,’ he is quoted as saying. He wasn’t claiming that all men are cleverer than women, or that the average man is brighter than the average woman, or that the most able women aren’t as intelligent as the most gifted men – although many of his critics understood him to be saying those things, or at least pretended to so they could justify how outraged they were. All he was saying is that the greater variability of men’s IQ – at both tails of the distribution curve – might be rooted in genetic differences between the sexes.

“You can see why such a claim would be controversial. According to most progressives, the fact that only 48 of the almost 900 people awarded Nobel Prizes since 1901 were women – and the Fields Medal has only been won by a woman once – is entirely due to social/cultural factors. If you allow that genetic differences may be a factor, then parity between men and women when it comes to intellectual eminence won’t easily be achieved. Just levelling the playing field – eliminating gender stereotypes, sexual discrimination, implicit bias, and so on – won’t be enough.

“Everyone has an excuse, but let’s just call it as it is. The #NobelPrize selection committees are sexist. There is no other explanation.

“Is Summers’ right to claim that this variability difference is hard-wired? We can’t say for sure, but there are some reasons for thinking so. We know from family studies, twin studies and adoption studies that IQ is about 50% heritable in adolescence, rising to 80% in adulthood. It would be odd if genetic differences accounted for such a large percentage of the variance in IQ, but had no effect on its variability. We also know for certain that some cognitive differences between men and women, such as the fact that rates of Alzheimer’s disease are higher in women than men, are at least partly due to genetic differences. And other psychological differences, such as the higher rates of autism spectrum disorders, schizophrenia and dyslexia among men, are part-biological too. If these phenotypic differences between the sexes are genetically influenced, why not others?

“Another consideration is that explanations of the gender gap in IQ variability that rely entirely on cultural/social factors aren’t very convincing. In Inferior: How Science Got Women Wrong – and the New Research That’s Rewriting the Story, Angela Saini argues that the reason men outnumber women by 2:1 among the top two per cent when it comes to cognitive ability is because intellectually gifted boys receive more praise and encouragement than their female equivalents. She quotes Melissa Hines, a Cambridge University psychologist, who believes this is why there are more highly able boys than girls. ‘I think in some social environments they don’t get encouraged at all, but I think in affluent, educated social environments, there is still a tendency to expect more from boys, to invest more in boys,’ says Hines.

“If that was true, you would expect to see a greater discrepancy between the sexes in the variability of IQ among subjects from rich backgrounds than from poor backgrounds. To date, that has never been detected (although to be fair I don’t think anyone has looked for it). Even if we park that, the evidence that expecting more from children and giving them more encouragement boosts their IQ scores is pretty threadbare. (One oft-cited study that purports to show that IQ can be raised by those kinds of inputs is the Abecedarian Early Intervention Project. But the number of children in that study was only 111, with just 57 in the treatment group.) Then there’s the issue of how this same mechanism could account for the higher preponderance of men in the left-hand tail of the bell curve. Do boys who struggle with basic arithmetic receive less encouragement than girls? How does Hines square that with her claim that we invest more in boys?

“Saini points out that male science professors outnumber female science professors by a higher ratio than 2:1, suggesting that there are other factors at play – the same factors that account for why only 9% of the UK’s engineering workforce is female and why she was the only girl in her A level Chemistry class and the only engineering student in her university class. ‘Girls are told from a young age that physics, maths and technical subjects are not for them.’

“Well, yes, there are other forces at work and some of them may be the ones Saini identifies – such as the view among employers and schoolteachers (although not many these days) that women are, on average, less able than men when it comes to science and maths, which isn’t true. But some of those other factors may also be linked to differences between men and women that don’t, on the face of it, appear to be cultural/social either.

“For instance, on average women are more interested in people and men more interested in things – a gender difference that remains constant across cultures and across time, suggesting it’s at least partly biological. (See this 2010 paper by Richard Lippa.) In his now famous debate with Cathy Newman on Channel 4 News, Jordan Peterson suggested it was this that explained why men outnumber women in professions dealing with things, such as computer science, and women outnumber men in fields dealing with people, such as nursing. Additional evidence for the same point has come from several international studies showing that the more gender equality there is in a society, the lower the percentage of women going into engineering and tech, implying it’s the result of women exercising their free will rather than misogyny, patriarchy or even low-level sexism.

“Women are eschewing those fields in favour of professions like health-care (82% of obstetrics and gynaecology medical residents in the US in 2016 were female) because of population-level gender differences, not because they’re victims of oppression.

“One final point: women who score in the top 2% or higher for general cognitive ability are more likely than men to have strong verbal scores, meaning they have more career options than their male counterparts. Could that be why the ratio of male to female professors in science and maths is higher than 2:1? Perhaps women capable of landing chairs in STEM subjects at top universities – like Lady Gaga, who has a genius level IQ – are more interested in other ways of using their talent. Ironically, writers like Saini who are so eager to ascribe the low numbers of female professors in science and maths to sexism are guilty of something like sexism themselves – namely, under-estimating the agency of the women who could go into these fields but choose not to.

“If you’re a conservative male, making these points can result in you being depicted as a ‘custodian of the patriarchy’, as Peterson was recently. To be clear, I think the likelihood that there are genetically-based differences in men and women’s personalities – at an aggregate, population level, not to be confused with essentialist claims about every man and every woman – and that these are linked to average differences in men and women’s brains is pretty high; but that doesn’t mean I’m opposed to equal rights.

“Saying that women have certain population-level characteristics is not the same as saying all women have those characteristics, so it would be irrational for an employer to discriminate against a woman, or a teacher against a female student, by citing these average differences. In any case, women are morally entitled to equal rights, regardless of their characteristics. So please don’t confuse this article with a defence of sexual discrimination. That remains wrong whether or not psychological gender differences are, in part, biological. And it follows that defenders of equal rights don’t need to continually deny the scientific evidence backing up that hypothesis. As countless others have pointed out, to maintain that equal rights are contingent upon behavioural differences being reducible to social/cultural factors is to commit the naturalistic fallacy.

“The difficulty this evidence presents is not for believers in equality of opportunity, but equality of outcome. The differences between men and women are such that gender parity in STEM fields, particularly at the top of those professions, is unlikely to be achieved without some highly intrusive state interventions. And I don’t mean equal pay or paternity leave legislation which, as we’ve seen in Scandinavia, has resulted in fewer women going into engineering and tech, not more. What this data tells us is that hard gender equality of the kind favoured by intersectional feminists can only be achieved at a huge cost to human freedom, particularly the freedom of women.

“Back in 2017, Stuart Ritchie cautioned against ascribing any of the differences between male and female brains to genetic differences. ‘Our manuscript is just about describing the differences, and we can’t say anything about the causes of those differences,’ he is quoted as saying. But he added that it won’t be long before we’re in a position to start talking about the genetic and environmental causes of those differences – he is hoping to get his hands-on imaging data for 100,000 brains soon. I have little doubt that future studies of this type involving huge sample sizes will reveal the biological underpinnings of human nature, like the genetic research looking at the DNA of hundreds of thousands of people which I’ve also written about. Whether it’s the new genetics or cutting-edge neuroscience, the egalitarian left is on a collision course with science.”

Abstract
Sex differences in the human brain are of interest for many reasons: for example, there are sex differences in the observed prevalence of psychiatric disorders and in some psychological traits that brain differences might help to explain. We report the largest single-sample study of structural and functional sex differences in the human brain (2750 female, 2466 male participants; mean age 61.7 years, range 44–77 years). Males had higher raw volumes, raw surface areas, and white matter fractional anisotropy; females had higher raw cortical thickness and higher white matter tract complexity. There was considerable distributional overlap between the sexes. Subregional differences were not fully attributable to differences in total volume, total surface area, mean cortical thickness, or height. There was generally greater male variance across the raw structural measures. Functional connectome organization showed stronger connectivity for males in unimodal sensorimotor cortices, and stronger connectivity for females in the default mode network. This large-scale study provides a foundation for attempts to understand the causes and consequences of sex differences in adult brain structure and function.

Authors
Stuart J Ritchie, Simon R Cox, Xueyi Shen, Michael V Lombardo, Lianne M Reus, Clara Alloza, Mathew A Harris, Helen L Alderson, Stuart Hunter, Emma Neilson, David C M Liewald, Bonnie Auyeung, Heather C Whalley, Stephen M Lawrie, Catharine R Gale, Mark E Bastin, Andrew M McIntosh, Ian J Deary

Quilette report
Cerebral Cortex abstract


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