CLAIM: MALE AND FEMALE BRAINS ARE THE SAME
PLAIN ENGLISH: The brains of males and females do not differ, and if they do, it is the result of environmental factors rather than innate differences.
While it is true that male and female brains don’t differ fundamentally, it is misleading to state that male and female brains do not differ. Brains are complex organs that cannot be reduced down to a single factor. The differences between male and female brains, and also personality and behavior, are found in the correlational structure between many variables. Those claiming male and female brains do not differ are comparing them using improper univariate statistical methods when they should be instead take a multivariate approach. When multivariate analyses are used, male and female brains can be distinguished with high accuracy.
A DEEPER LOOK:
Few topics in science are more controversial than human neurological sex differences. The volatility surrounding this area of research results from a central fear: the fear of inequality. Many worry that the discovery of any innate neurological sex differences would embolden sexists and serve as a justification for discrimination. But if no such sex differences exist, it is thought, then discrimination would be entirely unfounded. Ignoring the problem of holding our fundamental human values hostage to future scientific discoveries, this fear has nevertheless resulted in some scientists and intellectuals downplaying sex differences or outright denying that such differences exist.
For instance, in 2019 Nature published a book review titled “Neurosexism: the myth that men and women have different brains.” The author, neuroscientist Dr. Lise Eliot, reviewed cognitive neuroscientist Dr. Gina Rippon’s 2019 book The Gendered Brain: The New Neuroscience That Shatters The Myth Of The Female Brain. In her review, Eliot writes that “...modern neuroscientists have identified no decisive, category-defining differences between the brains of males and females”.
Additionally, in a 2015 paper in PNAS by Joel et al. titled Sex beyond the genitalia: The human brain mosaic , the authors claim that sex differences were “mostly nonexistent or small” and that “human brains cannot be categorized into two distinct classes: male brain/female brain”.
Both of these articles received widespread media attention. According to Altmetric, these two articles combined appeared in 126 news outlets and were tweeted a total of 10,987 times. Vastly more attention than typical neuroscience papers.
But is it true that there is no such thing as a male and/or female brain?
To answer this question it’s important to first ask, “what evidence would be required to conclude the existence of male and female brains?” In the Nature and PNAS articles quoted above, I bolded relevant parts that indicate the authors’ requirements. In these quotes it is clear that in order for the authors to conclude that male and female brains exist as real natural categories, they must be reducible to “distinct” or “decisive, category-defining” differences. That is, male and female brains must boil down to some binary variable.
For example, the “sex difference” requirement for in the Joel et al. PNAS study required that traits be “dimorphic” (i.e. have very little overlap between males and females), and that “there should be a high degree of internal consistency in the form of the different elements of a single brain (e.g., all elements have the “male” form).” In other words, for the authors to conclude a sex difference exists, male brains must be typically masculine for every measured trait, and female brains must be typically feminine for every measured trait. With such a high threshold, it’s no wonder the study concluded male and female brains do not differ.
Univariate versus multivariate analyses
The issue with most studies concluding that male and female brains don’t exist is methodological in nature. In the above studies, the authors analyzed sex differences one trait at a time (i.e. using univariate analyses). But not all complex natural phenomena—such as male and female brains—can be accurately described by comparing single variables one at a time. Instead, some phenomena are best captured by the correlational structure between many variables (i.e. using multivariate analyses). Neurogeneticist Kevin Miller illustrated this concept well on his blog using male and female faces as an example:
Male and female faces differ on a wide range of parameters – size of the jaw, prominence of the ridge over the eyebrows, fullness of the lips, size of the bridge of the nose, and others. For each of these parameters, there is not a male form and a female form – there is a distribution, which is shifted one way in males and the other way in females. None of these markers by itself provides the means to accurately classify faces as either male or female. But taking all of them together certainly does. We are all very good at telling whether a face is male or female and computer programs can also very successfully perform this classification. So, “male” faces and “female” faces are clearly real things, even though the differences in specific parameters between them are not dimorphic.
Image from [clinicalgate.com]
The insistence that categories must be cleanly separable and reducible to a single essential factor in order for them to be considered real or “natural” categories is known as the univariate fallacy. Fortunately, this fallacy was quickly pointed out in several published responses in PNAS to the Joel et al. PNAS paper.
One of the responses by Jonathan Rosenblatt demonstrated that using only two variables, each exhibiting substantial overlap, that “a simple multivariate analysis using the same data suggests the opposite: Brains are indeed typically male or typically female” (see figure below).
In another response paper, Checkroud et al. , using different data but again only two variables, were able to distinguish between male and female brains with 93-95 percent accuracy and concluded that “multivariate analyses of whole-brain patterns in brain morphometry can reliably discriminate sex.” Several other papers come to similar conclusions: [nature.com] [onlinelibrary.wiley.com] [sciencedirect.com] [onlinelibrary.wiley.com] [sciencedirect.com]
Unfortunately, however, these responses did not receive nearly as much media attention as the original Nature and PNAS articles they debunked.
In conclusion, while male and female brain differences cannot be reduced down to a single binary factor, the claim “there is no such thing as a male and/or female brain” is misleading and the result of applying overly simplistic univariate statistics to a complex multivariate phenomenon.
What do you think?