Brain matters

Someone told me recently, "men have seven times more grey matter in their brains than women, women have ten times more white matter than men".

I was sure this absolutely startling statement couldn't be true -- and when I checked, it was not -- but I think it could have both accurately remembered and quoted.  The trouble is, the original paper didn't say that, but it had been mangled in some reports (4).

The actual finding (1) was, when looking for correlations of brain activity with intelligence test results, men were found to have more areas of the brain in grey matter where the activity of the area was associated with higher intelligence, while women have more areas of white matter which were so associated.

The results (from Haier et al. 2005) looked like this:

Percentage of brain regions associated with intelligence, by sex and by tissue type.

In fact, there were lots of bits of the brain that were not associated with intelligence, so the actual results could be shown like this:

Percentage of brain regions associated with intelligence, by sex and by tissue type.

So there are a number of confusions in that original statement -- it's not how much grey or white matter, but how many sites of that type that correlated with intelligence.  And that last chart has a strong 'so what?' feel about it for me.  I certainly didn't come away thinking, as did the lead researcher,

"These findings suggest that human evolution has created two different types of brains designed for equally intelligent behavior".

The brain we know is very plastic and men and women can be socialised very differently, so I'm not reaching for an evolutionary explanation just yet.  The field of sex differences in the brain has had a long but messy history, and we should proceed with caution when hopping all the way from measured activity to the evolution of our species.

"Three years ago, I discovered my son’s kindergarten teacher reading a book that claimed that his brain was incapable of forging the connection between emotion and language. And so I decided to write this book.” 
― Cordelia Fine (2010), Delusions of Gender: How Our Minds, Society, and Neurosexism Create Difference

There is a well-known structural difference between the brains, where men have proportionally more white matter(2), but there is also about a 10% difference in the sizes of men's and women's brains, in keeping with their taller bodies (again about 10% more), and the proportions might change with size (5).  Also, there are parts of the brain where men have more grey matter than women (3), and if you control for size, women may still have more grey matter in some areas (6).

Men have more grey matter?  In the immortal words of Ben Goldacre, I think you'll find it's a bit more complicated than that.

(1) RJ Haier (2005). The neuroanatomy of general intelligence: sex matters. NeuroImage Volume 25, Issue 1, March 2005, Pages 320-327 https://doi.org/10.1016/j.neuroimage.2004.11.019

(2) https://www.theguardian.com/science/neurophilosophy/2013/oct/06/male-bra...

(3) https://en.wikipedia.org/wiki/Neuroscience_of_sex_differences#Male_and_f...

(4) https://www.nature.com/articles/d41586-019-00677-x

(5) https://www.psychologytoday.com/gb/blog/brain-myths/201207/two-myths-and...

(6) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3110817/

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DrDave

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