What would it mean for there to be a “male brain” or a “female brain”? Human genitals are mostly easy to categorise just by sight as either male or female. It makes sense to talk about there being different male and female types of genitals. What would it mean for the same to be true of brains? Daphna Joel and colleagues, in a 2015 paper Sex beyond the genitalia: The human brain mosaic have a proposal on what needs to hold for us to be able to say there are distinct male and female varieties of brains:
1. particular brain features must be highly dimorphic (i.e., little overlap between the forms of these features in males and females).
and
2. those features which are dimorphic must be consistent for each brain (i.e. a brain has only “male” or only “female” features).
They analyse MRI scans of 1400 human brains and find that these conditions don’t hold. There is extensive overlap, so that categorical brains, defined like this, just don’t exist. They write:
…analyses of internal consistency reveal that brains with features that are consistently at one end of the “maleness-femaleness” continuum are rare. Rather, most brains are comprised of unique “mosaics” of features, some more common in females compared with males, some more common in males compared with females, and some common in both females and males…Our study demonstrates that, although there are sex/gender differences in the brain, human brains do not belong to one of two distinct categories: male brain/female brain.
So the easy gender categorisation we can do on the genitals doesn’t translate to the (usually-unseen) anatomy of the brain. The ‘male/female brain’ doesn’t exist in the same way as the male/female sex organs.
Context for this is that there are differences between the average male and average female brain (for overall size, at least, these differences are large). Although there may not be categorical types, a follow up analysis showed that it is possible to classify the brains used in the Joel paper as belonging to a man or a women at somewhere between 69%-77% accuracy. A related study, on a different data set, claimed 93% classification accuracy.
Paper: Joel, D., Berman, Z., Tavor, I., Wexler, N., Gaber, O., Stein, Y., … & Liem, F. (2015). Sex beyond the genitalia: The human brain mosaic. Proceedings of the National Academy of Sciences, 112(50), 15468-15473.
Responses: Del Giudice, M., Lippa, R. A., Puts, D. A., Bailey, D. H., Bailey, J. M., & Schmitt, D. P. (2016). Joel et al.’s method systematically fails to detect large, consistent sex differences. Proceedings of the National Academy of Sciences, 113(14), E1965-E1965.
Chekroud, A. M., Ward, E. J., Rosenberg, M. D., & Holmes, A. J. (2016). Patterns in the human brain mosaic discriminate males from females. Proceedings of the National Academy of Sciences, 113(14), E1968-E1968.
The responses are linked to in Debra Soh’s LA Times article Are gender feminists and transgender activists undermining science?
Previously: gender brain blogging
Mind Hacks 
Might it primarily be that our current measuring tools aren’t sufficiently refined to discern dimorphic differences?
Well, we obviously can’t exclude the possibility that someone will invent a microscope of sufficient power that shows that all male brain cells have “bloke” etched in them at some hitherto invisible level of granuality (or less factitious example), but I think that isn’t the contribution of this paper. If we take “dimorphic” to mean literally having a different shape, our imaging technology is sufficiently powerful to reveal shape/size of different brain areas. And men and women don’t have brains with catgorically different shaped areas.
Also, here’s something I’m still trying to understand: how do you accurately measure something that operates akin to THIS (https://youtu.be/u28ijlP6L6M?list=PLWL8_SDV7ksrakh-CAI8rJKCj15ufIlTS
) times 11,000?
I’m kinda struggling to understand what this means, although it’s possibly just because of my lack of knowledge of statistics. Don’t suppose you could elaborate further? I’m thinking about lots of variables with different amounts of spread, and what would have to be the case in order for there to be a certain chance of correcting identifying the gender of a brain, and now I’m lost.
What it means is a bigger question!
The contribution of this paper, I think, is that it raises the issue of what people mean when they invoke “fe/male brains”. They set up conditions which need to be met for one definition (the categorical one), which they then test and refute.
There’s a philosophical issue about what defines a type, which may or may not be relevant, depending on what conceptual work you want the idea of “the fe/male brain” to do.
The follow up papers don’t — from my superficial reading — attempt to knock down the idea of what defines different categorical types, but merely show that the differences between brains belonging to men and women are significant enough that you can classify with a good level of accuracy a brain into which gender it is owned by from the structural features alone.
You could equally ask whether there are female and male stomachs, gall bladders, hearts, lungs, kidneys, livers etc. Or whether there are differences between tall brains and short brains, or white brains and black brains.
Some organs differ across people, some don’t. The brain and genitalia seem to be highly salient and overrated organs, doesn’t mean that meaningful connections exist between the two. The genitalia-brain connection is kinda like quantum consciousness, which just puts two mysterious phenomena together and claims the combo as insightful.
A better question to ask would be: what would be the possible insight if we do find some structural mappings (or not) between genitalia and the brain? That genitalia determines (or not) thinking?
It is a good point that although gender is a salient concept that doesn’t necessarily mean it is neurobiologically important one. You can’t infer its relevance to the brain from first principles, you need evidence.
However, the mention of the genitals by the authors is, I think, just a way to introduce the idea of gradual vs categorical differences. Are differences in male and female brains categorical? No. And that is the sense in which there is no “female/male brain”
Also, one thought – these are not newborn’s brains. So there is presumably no way to tell which of these brain features is due to innate biology and which due to differing socialisation (potentially including size, if your brain can grow bigger or smaller depending on how you are brought up? You tell me)
Yes, the real question is not “are there differences” but “why are there differences?”. We want to know if differences are immutable or contingent on life experience. For many biological differences connote innate differences, but we know it isn’t that simple. That said, the size of some differences (e.g. difference in brain volume between the sexes) are so profound environmental causes become unlikely. And there is direct evidence of early sexual differentiation in brain function (like this recent paper Sex-Specific Effects of Testosterone on the Sexually Dimorphic Transcriptome and Epigenome of Embryonic Neural Stem/Progenitor Cells).
Thanks for this series. I think the primary misunderstanding comes from this issue of: yes dimorphism could explain male / female differences in certain abilities. But do they? I hope you cc Larry Summers and Steven Pinker on your discoveries.
It looks like sex can be predicted with ~ 90% of accuracy so it’s hard to argue that there is no such thing as brain differences: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4833246/
I guess that is why nobody I’ve mentioned in this series of posts does argue that (although plenty of people falsely accuse other researchers of supporting the “gender identical brain hypothesis”)