Why My Brain Size is Different from Yours: Biological Sex as a Factor

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Complicated-looking brain maps from today’s research article

I do brain outreach with children, meaning I go around to elementary and middle schools and teach about brain health, anatomy, and function. Without fail, someone always asks me about why some of the human brains we bring (donated to the university pre-mortem for the purposes of education) were smaller than others. Part of this is due to differences in how the brains were preserved but part of it is, I assume, due to how big the brain was to begin with. I would tell them this and would IMMEDIATELY remind them that just because a brain is bigger doesn’t mean it’s better because bigger people have bigger brains.

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Wrinkly bits seen here in this photo by Fred Hossler from the National Geographic website

Well that’s true and that isn’t true, apparently. According to the recent Science article, “Study Finds Some Significant Differences in Brains of Men and Women,” biological sex* might also play a factor. The gist of the article is that there are neuroanatomical differences between men and women in this sample of 5,216 citizens of the UK between the ages of 44 and 77. They looked at thickness of the cerebral cortex (cortex is the wrinkly stuff you think of when you think of a brain) and volume of 68 different “areas within the brain” (which I took to mean the areas of the brain underneath the wrinkly bits).

Look at all those areas that aren’t wrinkly bits! (Thanks University of British Columbia for this excellent image)

According to Science (the magazine, not the systematic study of the structure and behavior of the physical and natural world through observation and experiment), the research found that men have bigger subcortical regions (all 68 of them the way it sounds) but women have thicker cortex. But then they tell me that when a bunch of statistical controls were done (which I feel like probably should have been done in the first place) 14 subcortical areas were larger in men and 10 were larger in women (If you’re thinking, “wait, I thought you said they were all larger in men?” then you and me both, my friend).

As always, I wanted to go to the source. This one was less easy to find because Science didn’t directly cite it for me (the nerve of them, amiright?).I looked up “Ritchie UK Biobank” in Google Scholar and this research article by Ritchie, Cox, Shen, Lombardo, Reus, Alloza, Harris, Alderson, Hunter, Neilson, Liewald, Auyeung, Whalley, Lawrie, Gale, Bastin, McIntosh, and Deary (science means never publishing a paper alone, apparently) was the only one that fit the description.

I read this one on a plane flight complete with TWO tiny bags of pretzels

 Article Summary

What they did: The UK has this thing called a Biobank, where biological data on the population is stored (all volunteers!). This study used the data from 2,750 women and 2,466 men. This data was collected by putting people in a really big magnet and getting pretty pictures of their brains, essentially. Then they took all those images and compared them between the sexes to see if there were any differences. They specifically looked at the volume of 7 subcortical structures (hippocampus, nucleus accumbens, amygdala, caudate nucleus, dorsal pallidum, putamen, and thalamus, in case you were interested) as well as the volume, surface area, and thickness of 68 cortical regions. They also looked at white matter tract directionality/complexity as well as functional connectivity, but as that is not discussed in the original online article and I’m trying to keep this to ~1,500 words, I won’t be discussing that in this blog post (I’ll find another paper specifically on things like that for you lovely people, don’t worry!)

What they found: They found a lot of things. Overall, men had larger brains, even accounting for their larger bodies. So when they tell me that all seven subcortical structures were larger in men than in women I’m thinking, ‘of course they are, men

Here’s a nifty table I drew of their results in case you needed a visual representation of the chaos.

have larger brains in general.’ When they finally decided to control for overall brain volume, the results were more interesting (and, I’d like to think, more accurate.) They found that of those seven structures, men actually only had three larger than women, and women had one larger than men (the nucleus accumbens, if anyone is interested).

At this point I’m a little annoyed because they dragged me through all this suspense of men having larger volumes of all these things when they don’t actually, but I kept reading because I love science and also because I had already wrote the first half of this post.

Let’s get onto those cortical areas. Without adjusting for overall brain size, ALL 68 areas were larger in men than in women. But, and this should surprise no one, when overall brain volume was taken into consideration, men had 14 larger areas than women and women had 10 larger areas than men, and 44 of those areas there were no significant differences. Thanks, statistics (see the image at the top of the page and look at the difference between the first and middle panel for a visualization of this). Men also had larger surface areas than women. I could not tell if this was the result controlling for overall brain volume or not. Don’t worry ladies, even if men have larger cortices by volume and surface area, our slightly smaller cortices are thicker. This was found in 47 areas of the brain unadjusted for brain size and still 46 areas when adjusted. So, go us for being consistent I guess?

Inner Skeptic:

I wanted to take a quick moment to point out that the online news release from Science got the subcortical and cortical areas confused (saying there were 68 subcortical regions looked at but also talking about their thickness). I wanted to remind people that copywriters, regardless of the scientific background, might not always get everything right which is why we need to think critically about each article we come across (including, for example, mine!)

I had a few issues with this paper. I think the largest issue is that it wasn’t actually a peer reviewed publication. I know this because it told me at the top of each page (see below). This doesn’t mean it’s not science, nor does it mean it can’t be good, but it does mean that it hasn’t been critically looked at by other scientists yet, and the results should probably be questioned a bit more than your average paper. I will be interested to see if they publish in a peer reviewed journal and how similar that paper will look to this one.

Second, I didn’t like that I had to read through all that crap about brain area sizes being different when the overall brain volume wasn’t accounted for. I felt like they should have just automatically done that and used those results as their only results.

Third, their age range was 44-77. This does not take into account a very large part of the population. It is also at the time when women are going through menopause which does things to their brains and bodies that no one understands because no one controlled for it in this study. This means the results might not be able to be generalized to the rest of the population.

Final Thoughts:

My first thought is that I should have chosen an article with fewer list-like results. But sometimes that doesn’t happen so I will forgive myself.

More importantly, brain size doesn’t necessarily mean anything with regard to intelligence or daily activities. Remember that. It’s not the same as smartness or talent or anything like that. It is simply a physical attribute unique to you as a person. Overall I think it’s a really interesting thing to look at, because women and men have different outcomes in psychiatric situations so it would be amazing if we could find a good underlying reason for that. They weren’t able to do that in this study (which was never meant to be anything more than descriptive, according to the author), but if you want to go get a bunch of brain images and measure volume/surface area/thickness against different psychiatric disorders, I’m sure there’s a pretty sum of grant money in it for you.


*Because the study in question did not take into account gender versus biological sex, and did not discuss intersex, this is going to be about biologically male vs biologically female brains with complete disregard to whether a participant’s gender lines up with their sex.

Have a science news article you want a closer look at? A concept you want to learn more about? Leave it in a comment or email us at phteatime@gmail.com!
About the author
14595564_10155083719871840_4974523626401782470_nMari is a graduate student studying psychology and neuroscience at the University of Minnesota. Her research focuses on affective, behavioral, and molecular aspects of opiate addiction. Mari loves all science and wants everyone else to be as excited about it and as critical of it as she is.

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