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).
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