Men, Women, and Big PNAS Papers

By Neuroskeptic | December 3, 2013 1:34 pm

This morning, the world woke up to the news that

Scientists discover the difference between male and female brains

Britain’s Independent today actually made that their front page. They went on to discuss “the hardwired difference that could explain why men are ‘better at map reading’”. The rest of the world’s media were no less excited.

Well. I don’t have time to get into criticizing the media or decrying gender stereotypes, so let’s just stick to the science. The study in question, published in PNAS, is called Sex differences in the structural connectome of the human brain.

The authors used diffusion tensor imaging (DTI) to estimate the integrity of the white matter tracts going in various directions at each point in the brain.

In a large sample of 428 males and 521 females aged from 8 to 22, they report sex differences in the pattern of white matter connectivity. In general, the female brains were ‘more connected’ than the male, except in the cerebellum: here’s the plot for a summary measure, the Participation Coefficient.

gender_brainI have two issues with this:

Head Motion. A perennial Neuroskeptic favorite, this one. A paper just last week showed convincingly that even modest amounts of head movement during the MRI scan causes changes in DTI.

Various commentators on Twitter and elsewhere swiftly pointed out that it’s not implausible that men and women might move different amounts on average, so that might account for at least some of these results.

I doubt there’s a big difference in motion, but in a sample this large, even a tiny sex bias in average fidgetiness would be detected and come out as statistically significant.

Brain Size. Men have bigger brains than women on average. How this would affect DTI estimates of connectivity, I’m not sure. But it might well do. Here’s a paper reporting that “connection length was negatively correlated with degree of connectivity”. Longer connections – as seen in bigger brains – are weaker (or at any rate, appear weaker on DTI).

I recently blogged about the fact that, relative to overall brain size, the corpus callosum, the major connection in the whole brain, is smaller in larger brains. This is an effect of brain size, not of sex, but it was misidentified as a sex issue for a long time… a cautionary tale?

I’m not saying that motion and brain size are responsible for these results. But, given the previous literature, it’s a clear possibility, which is why it’s rather disappointing that the authors don’t so much as mention either issue in the paper.

It ought to be easy to check. Just re-run these analyses with two added covariates: overall motion, and total brain volume.

If sex has an independent effect, after covarying for the potential confounding factors, then the paper’s conclusions will stand on strong ground. But if it turns out that men and women’s brains differ only in motion and size, well, it would have been better to know that from the start.

ResearchBlogging.orgIngalhalikar, M., & et al (2013). Sex differences in the structural connectome of the human brain PNAS DOI: 10.1073/pnas.1316909110

  • http://petrossa.me/ petrossa

    Going purely by logic it should follow the brains are differently wired. No married man can deny a woman is a totally different species with a totally different behavior, intelligence (different, not less or more), emotional state, way of thinking etc. It’s hardly feasible that the same brain can produce such vastly differing personalities.

    All this goes to show is that indeed neuroscience is in it’s very early stages.

    • Ronald Lett

      Logic utilizing biased input results in biased output (See “The Mismeasure of Man” for basic examples). Have you controlled for bias by including measures of personality differences in couples of two men, and non-sexual partners of myriad gender expressions ?

      • Idrake

        Speak clearly, you are obfuscating your point. Do you mean to say that his logic, although possibly valid, is producing biased results due to a pre-existing bias in the way he is receiving information? What are you saying, exactly?

        • Ronald Lett

          There are no obfuscations present in my language. His logic is perfectly valid, but the result of a chain of logic is only as reliable as its input. In his case, his input is an anecdote of one person of a single gender, while he seeks to apply his conclusion to an entire gender. In addition, the “No true Scotsman” fallacy appears to be in use.

    • PrincePoliblog

      That is amazingly sexist of you. You say someone married you huh? Go figure.

      • ElaineFairston

        I don’t see how this is sexist. He’s not saying men are better than women, just different. Different height, different muscle mass, different hormones, different genitals, different brains.

        • Ronald Lett

          The problem is that any such difference is illusory if men differ from each other within a similar spectrum. The same goes for personality differences within women. Without such a comparison, binning fallacies run rampant.

          • triangleman

            It’s not clearly a fallacy (or illusory), it just might be very uninteresting. If all men (and women) have different personalities, we might assume that all men (and women) have different brains. As such, you can draw whatever line in the sand you want and it will be equally valid: all blondes have different brains than brunettes; all Saggitarians have different brains than Aquarians; etc.

          • http://petrossa.me/ petrossa

            Ever heard of majority? Fortunately i have escaped the political correct indoctrination which forced generations into this bizarre obsession with equality. Looks fine on paper, doesn’t work out in reality. Like every ideology. One doesn’t measure an entire population by a minority. In real life most men are men, and most women are women. Feminism Flounders http://petrossa.me/2013/11/30/feminism-flounders/ which currently certain bloggers tend to try to dismiss by questioning results of studies on minor details. In the 90′s a dutch neurosurgeon was ostracized when after slicing and dicing large amounts of brains proved the structural difference between a homosexual brain and a heterosexual brain. People like commented here exploded in righteous indignation how dare he! Still the facts were undeniable so after a decade of trying to shovel him under it is now accepted. So if even such a subtle difference as homosexuality is already expressed in the structure of the brain it is completely laughable to imagine such a major difference as gender isn’t. Denying that is more like religion then science, it shouldn’t be like that in the politcor view so it can’t be like that. Anti-confirmation bias strikes.

          • triangleman

            If I may be so bold, I suspect that you may be falling into the same trap that some of those “indoctrinated” people that you are referring to — and yes, I read your blog post. What it comes down to is addressing different concepts with the same ideas.

            What’s at issue in your blog post (and less so in this post) are at least four questions of “equality”:

            [1] Biological similarity
            [2] Equality in rights
            [3] Equality in opportunity
            [4] Equality in representation

            I believe that you are right that many “politically correct” people feel the need to argue for all four to exist at the same time, but they’re somewhat independent. I think you make the claim in your post that [1] should not be related to [2], [3], and [4] — and that I agree with. I believe your post confounds [2], [3], and [4] but that becomes less of a natural science question and more of a social science question.

            I think most modern feminists (though not all, obviously) argue for [2] and [3] but not necessarily [4]. That is, for example, we don’t need the same number of male and female fire(wo)men [4], but they should be held to the same physical (and mental) standards [2] and [3]. Finally, I think you overdo the hyperbole. Women and men are different — I don’t think they’re completely different.

          • Maybe Not

            Except you would never get a statistically significant difference between Sagittarians and Aquarians, while you do get that difference between men and women.

          • Richard

            You might if you had a large enough N…

          • triangleman

            Plausibly, but that wasn’t the point of this particular thread of discussion, as far as I understood it.

            [1] Petrossa made an argument that by simple observation — male and female behavioral patterns are suggestive of differences in male and female brains.

            [2] Lett made a counterargument that such differences might not exist, because males also display different behavioral patterns from other males.

            [3] I made the counterargument that, males may also have different brains from other males, and as such males may still have different behavioral patterns and brains from women.

            Mostly, my response was based on my sentiment that Lett’s critique of Petrossa’s argument is inaccurate. Petrossa’s claims are coherent, even given Lett’s counterarguments. It’s not to say that his claims are or aren’t well-founded, but there’s nothing “illusory” about the effects Petrossa is arguing, even if Lett is correct in that there is a similar degree of variation within male and female subjects.

            ————————–

            You’re meta-argument on statistical significance is more relevant to the broader discussion. I just simply wasn’t referring to it. Again, there are two layers of analyses that can be argued here [1] statistical significance as relates to the study, and [2] statistical significance as related to Petrossa’s evidence.

            I’m on board with the claims that male and female brains may differ significantly (and not based on random noise). That noted, a lot of neuro-work does involve what I consider specious statistical practices, so I tend to weight these studies less until I have significant replication (and yes, I also recognize neuro work is expensive — I’m a tough crowd to please). If you’re polling my Bayesian beliefs, then I also do believe (moreso than i do not) that male and female brains differ biologically. But, as I would note, these beliefs aren’t strongly influenced by a single neuro study and — to Lett’s earlier arguments — are pretty much uninfluenced by Petrossa’s anecdotal (and not even statistically measured) evidence.

            ——————-

            TLDR; I believe male and female brains are biologically different. I don’t believe any of the arguments presented in this particular thread are very strong. I also only have moderate confidence in neuro studies that are not replicated.

          • Maybe Not

            What you’re saying applies to random samples, not averages.

            To find out whether a difference of averages is statistically significant, you have to divide the standard deviation (i.e. how much men differ from each other) by your study’s sample size.

            What this means is: A difference between men and women can be perfectly valid and significant, even if it is smaller than the standard deviation among men. You just have to test enough people to exclude random chance.

          • Richard

            Valid and significant statistically yes, but the effect size (not really an effect size in this case) could be tiny. The relevance of the small difference is not known given that there is no theory mapping connectivity->behaviour.

          • http://petrossa.me/ petrossa

            Or current methods and understanding just aren’t refined enough to see the difference. Which is more likely then just because ‘we’ can’t see it doesn’t exist. As i mentioned earlier, it took Professor Swaab http://en.wikipedia.org/wiki/Dick_Swaab years of dissecting brains to discover the difference between homosexual and heterosexuals. Perhaps this method is better suited for such delicate research then machines with the resolution of a 48 pixel camera.

          • PrincePoliblog

            Yes, people are different, but that doesn’t mean they are different based upon gender.

  • Hj Hornbeck

    Only two? I have two more of my own.

    1. Are there legitimate gender differences in the first place? As an example, Woolley’s 1914 survey found no gender difference in math, while Jacklin and Maccoby’s much-cited 1974 book found math-based gender differences, and yet studies less than a decade old demonstrate no gender difference. The scientific literature cannot come to a consensus on whether or not there are actual gender differences in the first place (see “The Gender Similarities Hypothesis,” 2004 I believe).

    So if we haven’t established there are gender differences, what good is a study which presumes there are gender differences then goes looking for them?

    2. Even if we grant that legitimate gender differences exist, and that this study found legitimate brain structure differences, that still doesn’t prove these differences are inherent to our biology. We can also find differences between the brains of musicians and non-musicians; does this mean there’s a set of genes granting musical ability, or is this an example of the brain rewiring itself based on environmental input? I see no evidence they considered an alternative hypothesis: different social gender norms resulted in different environments, and our brains rewired themselves to suit our environment.

    This study presumes brain structure is independent of environment, which is absurd. Yet without also studying the environment these children are within, they cannot declare these differences to be biological.

    • PrincePoliblog

      Well said!

  • carolannie

    It still would be on shaky ground if the differences were due to nurture, not nature. When you scan adult brains, you see brains that have endured years of nurture, which would change the wiring (and you know that “wiring is changed by experience as well as by proclivity). That being the case, this is interesting research, but not enlightening. Also, the notion of “left brain, right brain” is brought up even though that has been debunked repeatedly. Which shows that psychologically satisfying memes endure even among scientists.

    • Maybe Not

      I guess whether or not neurological differences as big as these can be caused by nurture factors depends on what exactly you mean by “nurture”.
      Permanent medication from infant days on? Sure.
      Slight differences in diet, social activities and other habits? Uhh… let’s just say, I would love to see a study which supports that idea.

      Also, as it turns out, the notion that nature is an insignificant factor compared to nurture is a meme as well, and it’s definitely “psychologically satisfying” if you have an emotional attachment to the idea that people are equal in a “tabula rasa” fashion.
      And it seems many people have that attachment…

  • Lisa Moro Northover

    This article explains the brain study in much more detail: http://t.co/zViYh9vZ7G http://t.co/C1Q06VSIGH

  • ROBERTO COLOM
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  • Richard

    Regarding IQ tests: females might have performed worse (references?) for all sorts of psychological reasons unrelated to their innate ability – in fact, I would argue that if we did “hold women to the same mental standards” as a society, then any discrepancies in IQ/cognitive ability would disappear (IQ is not a culturally blind measure).

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  • Asgard

    It’s interesting to see so many misinformed comments on a sceptic blog.

    The scientific literature has already come to a consensus on the existence of cognitive sex differences, especially with regard to spatial ability. Both neural/hormonal and social factors contribute to those differences. See Diane Halpern’s “Sex Differences in Cognitive Abilities”.

    Stephen Jay Gould’s “The Mismeasure of Man” is an extremely biased work itself. See “The Mismeasure of Science: Stephen Jay Gould versus Samuel George Morton on Skulls and Bias”
    http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1001071

    Between-group differences can be real and worthy of note even if within-group differences are greater than between-group differences.

    Stereotypes, including gender stereotypes, are mostly accurate. So saying that some view or conclusion is a gender stereotype is not a real criticism. See Lee Jussim’s “Social Perception and Social Reality: Why Accuracy Dominates Bias and Self-Fulfilling Prophecy”.

    Finally, let’s assume the real reason why women have bigger corpus callosums is that they have smaller brains. Does the relation between sex and corpus callosum size cease to exist? No. Women on average will still have larger corpus callosums and all the (presumed) behavioural/cognitive characteristics associated with it.

    Extreme scepticism towards one direction is really a bias towards the other direction. One bias is not necessarily less pernicious than the other.

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  • http://thefloatinglantern.wordpress.com/ Tim Martin

    What surprised me was the “complementarity” nonsense mentioned in the article. There’s no evidence that the tiny average differences in men’s and women’s cognitive abilities are a result of adaptive evolution, nor does that possibility make much sense. The first woman to have a mutation for “better social cognition” would have passed that mutation onto her offspring, male AND female. There’s no reason to think that evolution would act to sequester these beneficial traits in one sex only.

    • Cahokia

      Why wouldn’t evolution “act to sequester” neurological and behavioral adaptations in one sex only when it has been shown to do so in other animals, including other primates?

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  • lifeafterphd

    If you look at the sexual dimorphism from an evolutionary perspective, this makes perfect sense. In early human societies, men went out to hunt while women stayed in the village (with very few exceptions).

    The traits that made men more successful were those that allowed them to be better hunters and warriors: so coordinated action, perception of environmental opportunities and dangers. For the women in the village, their success is more dependent on “politics” and relationships. Instead of organizing hunts, the successful woman is one who is well liked by people and use strong communication skills and wits to solve problems (and not braun since they will lose to the stronger males).

    Thus over time, as the traits that marks a successful man and woman differs, it makes sense that there should be sexual dimorphism in brain structures.

    But as I mention in my blog, yes, men and women are physiologically and perhaps mentally different. But this does not mean that we can’t understand each other. We still share the same experiences. We all can work to improve our reasoning and leadership skills. What we are born with helps gets us going, but as our brain continues to grow as adults, it’s our experiences that makes us who we are in the end.

    More here: http://lifeafterphd.com/whats-difference-men-womens-brains/

    • http://echidneofthesnakes.blogspot.com Echidne of the snakes

      How do you know that early human societies (of the assumed evolutionarily important period) had villages? The references to the Area of Evolutionary Adaptation I have seen all assume that the people were nomadic and lived in small kin-based tribe. Our own thoughts about that time are not the same as actual evidence obtained by time-traveling there.

      Likewise, the usual argument, these days, is not that the women stayed in the villages but that the women did more gathering activities and men more hunting activities. This is based on more recent tribes which some assume can be used as soft evidence.

      Given that people now think women did gathering, they would also have roamed around.

      On the men-as-hunters hypothesis. You argue here that it would have made men better at coordinated action, but I just read, elsewhere, that the reason men are so common in coding is that they like to focus on only one thing, all alone, because the prehistoric hunters did that.

      The problems with these types of hypotheses are pretty severe, in my opinion, because they are ultimately not testable.

      • lifeafterphd

        I totally agree that evolutionary hypotheses are mostly untestable. But it’s interesting to think about as a thought experiment. I’m not an evolutionary biologist myself, I’m trained as a systems neuroscientist. And looking at it from a systems perspective, there are many environmental, developmental, and various other causes real or just correlative.

        In the end, most biology hypothesis can not truly be tested to the point where we can say that “this is the absolute truth”. It’s kind of like a black box that we poke at and give our best guess in the discussion section of papers so as to make sense of what the results mean.

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  • daniel8

    another one here, this time including also age.

    http://cercor.oxfordjournals.org/content/early/2013/12/13/cercor.bht333.full?sid=b8a9802a-1b9e-479b-8a74-967ed04f3615

    and this time brain size is taken into account (not head motion)

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No brain. No gain.

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Neuroskeptic is a British neuroscientist who takes a skeptical look at his own field, and beyond. His blog offers a look at the latest developments in neuroscience, psychiatry and psychology through a critical lens.

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