Male versus female brains: persistent myth or inconvenient truth?

Neurosexism, past and present

Science has been used as a tool of women’s oppression at least since Darwin formulated his theory of natural selection. In addition to On the Origin of Species, Darwin wrote a volume entitled The Descent of Man, and Selection in Relation to Sex, a book that attempted to explain observable differences between men and women in evolutionary terms. According to Darwin, “The chief distinction in the intellectual powers of the two sexes is shewn by man attaining to a higher eminence, in whatever he takes up, than woman can attain”—a difference that he attributed to the demands of competing for female mates.

Figure 1: The procedure that the authors used to decide which sizes were “male-typical,” which sizes were “female-typical,” and which were “intermediate” for a given brain region. Note that many women had male-typical sizes, and vice versa.

Such attempts to rationalize sexist perspectives have unfortunately not yet been consigned to the ash heap of history. On the fringes of society, pick-up artists and incels argue that fundamental differences between men and women legitimate rape and abuse. In the mainstream, autism researcher Simon Baron-Cohen holds that women are inherently “empathizers” and men are “systematizers.” Put more bluntly, he believes that inborn neurological differences make women unlikely to succeed in STEM fields, a conclusion that his many women colleagues at the University of Cambridge would surely dispute. Cordelia Fine, a historian of science, has coined the term “neurosexism” to describe the now-widespread practice of attempting to legitimize misogyny with neuroscience.

Daphna Joel et al.’s 2015 article, “Sex Beyond the Genitalia: The human brain mosaic” explicitly takes up arms against neurosexism. Studies that use modern neuroscientific techniques like magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) to seek  differences between male and female brains are extremely common, in part because it is trivial to test for those differences once a data set has been collected. Some physical differences between male and female brains have been observed so frequently and in so many people that they are indisputable. Joel and her coauthors do not deny this fact, though it is important to note that differences between men’s and women’s brains are much smaller than, say, differences between men’s and women’s heights. But she does take issue with the tendency that many scientists have to speak of a “male brain” and a “female brain” on account of these differences.

Do “male brains” and “female brains” really exist?

According to Joel, describing a brain as “male” or “female” is only defensible if most brains are consistently male-typical or female-typical. In other words, if we label an amygdala with a male-typical size blue and one with a female-typical size pink—and do the same for all of the other regions of the brain that show substantial differences in size between men and women—a female brain must be colored completely pink, and a male brain must be completely blue. But are brains consistent in this manner?

Using brain scans of 281 subjects, 169 women and 112 men, the authors identified 10 regions that show the largest differences in size between men and women. Importantly, for a given region, men’s sizes and women’s sizes exhibited considerable overlap, as is shown in Figure 1. Due to this overlap, the authors had to decide how to define “male-typical,” “female-typical,” and “intermediate” sizes for each region. For each region, they divided the range of possible sizes into thirds. A region was designated as “male-typical” if its size was in the most extreme 33% of sizes found in men, “female-typical” if its size fell among the most extreme 33% of women, and intermediate otherwise.

The authors then evaluated the extent to which their subjects’ brains were consistently male-typical or female-typical by counting the number of female-typical and male-typical regions in each brain. In very few individuals were all 10 regions male-typical or female-typical (6% of subjects, as shown in Figure 2). In contrast, more than a third of subjects had brains with both male-typical and female-typical regions. On this basis, Joel concludes that it is incorrect to speak of a male brain or a female brain, since brains tend not to be consistently female or male. Rather, she proposes, we should think of the brain as a “mosaic” of male-typical and female-typical anatomy.

Figure 2: A representation of the consistency (or lack thereof) in sex-typical brain region size for all of the participants in the study. Whereas only 6% of participants showed “internal consistency”—that is, all of their brain regions were either “male-typical” or “female-typical”—35% showed what the authors characterized as “substantial variability,” which meant that some of their brain regions were “male-typical” and others were “female-typical.” In this figure, blue indicates “male-typical” regions, pink indicates “female-typical” regions, and white indicates regions that are neither “male-typical” nor “female-typical.”

Differing definitions: what is a “male” or “female” brain?

At first glance, the study’s results might seem fairly straightforward; however, it sparked an unusual level of controversy, as the  four published responses to the piece demonstrate. These replies focused on the same question: what does the pattern of inconsistency observed by Joel actually mean? According to Jonathan Rosenblatt, an author of one of these critiques, this pattern fails to establish that we cannot, in theory, neatly divide brains into two separate groups. Even if, for example, many women have “male typical” amygdalae, that does not mean we cannot use amygdala size in conjunction with the sizes of other regions to divide the entire population into two groups with near-perfect accuracy. Figure 3 demonstrates how this might be possible. The smaller plots on the top and right of the figure illustrate hypothetical distributions of how men’s (say, yellow) and women’s (purple) values for a variable like amygdala size might overlap. When we plot each individual’s values for one variable against those for another variable (say, hippocampus size), we find that we can quite easily draw a line between men and women on the resulting graph in the center of the figure. We could then define brains that fall above that line (in the purple cloud) as female brains and those below the line as male brains. In other words, even though we can’t divide male brains from female brains based on hippocampus size and amygdala size taken separately, we might be able to make that categorization if we look at the two statistics simultaneously.

Figure 3: A demonstration of how multiple variables can be used together to separate groups, even when the groups show substantial overlap when the variables are taken individually.

Rosenblatt’s critique does not on its own establish that there are “female brains” and “male brains.” For starters, it is still a theory: researchers would need to make real comparisons of different brain region sizes to see if we can in fact separate male and female brains using multiple variables. But perhaps more importantly, the critique raises big questions about what it would mean to disempower neurosexists once and for all. On one hand, we can insist that the terms “male brain” and “female brain” imply a specific, easily observable type with which the brains of either sex must align perfectly; Joel’s analysis certainly demonstrates that such types do not exist. But, as Rosenblatt’s critique demonstrates, even apparently mosaic brains might be separable into male and female categories using more complicated statistical definitions. In other words, even if we cannot easily articulate what it means for a brain to be “male” or “female,” those definitions might still exist—and perhaps a computer could figure them out.

In fact, there are researchers who have attempted just this strategy. Interestingly, as both Joel and her critics admit, brains cannot be perfectly categorized, even by a computer. One can use anatomical information to predict someone’s sex based on their brain with 70% accuracy (versus 50% accuracy if the prediction were made entirely by chance). Some might focus on how it is possible to predict whether some brains belong to men and others to women; but perhaps our time is better spent interrogating why so many brains are impossible to categorize. Rather than focusing on the extreme examples, we could center the 30% of individuals who stump the machine.

Fight sexism, not data

Many studies tend to land on the conclusion that anatomical brain differences indicate a fundamental gap between men and women, but perhaps this finding should be viewed not as a conclusion, but instead as a starting point. What do differences in structure size actually mean in terms of perception, cognition, and behavior? After all, Darwin believed that the smaller skull size of women indicated their inferior intelligence; today, we know that this difference has absolutely nothing to do with intellectual capabilities. We need not necessarily prove that neurosexists present incorrect data to contest the misogynist purposes to which they put their results. Perhaps we should shift our focus from contesting the data, and instead apply ourselves to fighting scientifically empowered misogyny.

Edited by Isabel Low

Further reading:

Scientific American: the new science of sex and gender

Interview with and latest book by Dr. Gina Rippon

Delusions of Gender by Dr. Cordelia Fine

References:

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.

Rosenblatt, J. D. (2016). Multivariate revisit to “sex beyond the genitalia”. Proceedings of the National Academy of Sciences, 113(14), E1966-E1967.

Glezerman, M. (2016). Yes, there is a female and a male brain: Morphology versus functionality. Proceedings of the National Academy of Sciences, 113(14), E1971-E1971.

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.

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.