The Human Brain “Mosaic”

In 2015 a study titled “Sex Beyond The Genitalia: The Human Brain Mosaic” took the media by storm. Well, at least by the rather modest standards of neuroscience publicity. Its authors claimed to have proven that the distinction between male and female brains is no longer tenable. In a field that (undeservedly) lurks in the shadows of mainstream discussion the new findings quickly spread everywhere from TED talks to American Scientific. Not bad for a study on brain morphology.

Is there a distinctly “male” or “female” brain?

In an effort to be as transparent as possible, I’ll quote their main finding here:

“…regardless of the cause of observed sex/gender differences in brain and behavior (nature or nurture), human brains cannot be categorized into two distinct classes: male brain/female brain.”

Joel et al decided that in order for human brains to be classified in two distinct groups based on biological sex they would need to satisfy two criteria:

“…one, the form of the elements that show sex/gender differences should be dimorphic, that is, with little overlap between the forms of the elements in males and females. Two, 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 order to test this they analyzed the MRIs of 1,400 human brains and found the following:

“…extensive overlap between the distributions of females and males for all gray matter, white matter, and connections assessed. Moreover, analyses of internal consistency reveal that brains with features that are consistently at one end of the “maleness-femaleness” continuum are rare.”

In other words, because the data failed to satisfy either of their criteria they draw the conclusion that it’s impossible to categorize human brains as male or female. Unlike biological sex, which we classify based on chromosomal constitution and genitals, brain morphology is more nuanced. Instead, the authors claim that the morphology of the human brain is better described as a “male — female continuum” within which brains are comprised of a “mosaic” of morphological features.

The Media Gets It Wrong, Of Course

It’s worth noting that the central claim of this research, although still far from agreed upon within scientific circles, is rather narrow. It’s simply that brain morphology (or structure) can not be categorized as strictly “male” or “female”. It says nothing about the functional differences between male and female brains and even explicitly admits that “there are sex/gender differences in brain and behavior”.

There’s been a number of headlines which paint a much more exciting picture such as “Men are from Mars….and so are women! Scans reveal there is NO overall difference between the brains of the sexes” or “A welcome blow to the myth of distinct male and female brains” (notice the not-so-subtle indication that the news is “welcome”? More on that later).

But even the comparatively modest conclusions of the study have hardly produced a consensus within the scientific community. It’s worth noting that there’s already been a number of scholarly responses that have cast some doubt on the conclusion that human brains are “intersex”.

Published Criticisms

In 2015 Guidice et al. published a critique of the methodology of the study which claimed that:

“Joel et al. did not conduct analyses (e.g., discriminant analyses) designed to test how well various brain features predicted participants’ sex. Performing such analyses on the data of Joel et al, we found that brain features correctly predicted subjects’ sex about 69–77% of the time. Moreover, the multivariate overlap of female and male distributions based on the same variables was moderate(42% on average), and certainly not so large as to invalidate the idea of overall sex differences in brain structure.”

They also added that,

“the definition of “internal consistency” Joel et al. use is so extreme that, in realistic conditions, it can only generate results consistent with their hypothesis.”

Chekroud et al. conducted a multivariate analysis of structural MRI scans for 1,566 individuals and found that “multivariate analyses of whole-brain patterns in brain morphometry can reliably discriminate sex”. Furthermore,

These two results are not mutually inconsistent. We wholly agree that a strict dichotomy between male/female brains does not exist, but this does not diminish or negate the importance of considering statistical differences between the sexes (e.g., including sex as a covariate in morphometric analyses).”

This is highlighted on a more fundamental level by Rosenblatt:

How can one assess the multivariate overlap between groups? One simple way, certainly not unique, is by using a classification approach. If a binary classifier has good performance, then clearly the groups have restricted overlap. Put differently, a classifier can only achieve perfect classification if the data points are well separated (note the converse does not hold: the data may be well separated, even if a particular classifier is no better than random guessing).
By fitting a linear support vector machine to the voxel-based morphometry data reported in ref. 1 we achieve a cross-validated misclassification rate of about 80% (depending on the random splits). We thus conclude that, whereas the univariate brain attributes (voxel morphometry) are bad predictors of gender, the multivariate brain morphometry is a very good predictor of gender.”
“Univariate overlap with multivariate separation….Two groups are perfectly separable when considering both variables but inseparable when considering only one variable at a time.”

The Source of The Confusion

The above responses demonstrate that the link between biological sex and brain morphology is certainly dimorphic even when using the data provided by the study itself. More importantly, it’s worth noting that Joel et al. use an incredibly arbitrary and strict methodology when considering “internal consistency”. Some of the critical responses were understandably confused as to why “internal consistency” was defined so strictly. After reading Joel’s previous work the reasons become quite clear.

In 2011 Joel published “Male or Female: Brains Are Intersex” in which she makes a number of reasonable biological observations followed by strange assertions:

“There are only a few brain characteristics for which the term sexually dimorphic, which literally means having two forms, is appropriate, that is, for which there is minimal or no overlap between the form of this characteristic in males and females (e.g., the size of the sexually dimorphic nucleus of the preoptic area, which is three to eight times larger in male rats compared to female rats, Swaab, 1995). For most documented sex differences in the brain, however, and in particular in regions involved in behavior, emotion, and cognition, there is a considerable overlap between the distributions of the two sexes. It follows, that individuals may have the “female” or the “male” form for any of these non-dimorphic characteristics. The question then becomes whether the brain of a given individual is homogenous or heterogeneous with respect to the “male/female” type of its different brain characteristics

You’ll notice that the last sentence above is the dichotomy she claims to have overthrown in her 2015 study. And later on she adds:

“We can thus conclude that sex interacts with other factors to determine the structure of the brain, and that these interactions are complex. The result is therefore a multi-morphic, rather than a dimorphic, brain, that is, different individuals will have different combinations of “male” and “female” brain characteristics. In this sense brains are neither “male” nor “female,” they are “intersex””

Actually, the text in bold doesn’t follow at all. Just because there are differences in group averages (for example, between males and females) does not mean that there only exists two categorically different brain morphologies (i.e. a “male” brain or a “female” brain). It simply means that certain aspects of brain morphology differ on average between the sexes. Rosenblatt and Chekroud et al have clearly demonstrated that, above.

Distinctions between primary sex characteristics (genitals, chromosomal constitution, etc) are indeed categorical and the 1% of the population that doesn’t fit the male — female biological sex categorization is considered intersex. However, brain morphology is not a primary sex characteristic.

By referring to brain morphology as “intersex” Joel is applying the categories of primary sex characteristics to characteristics which are only influenced by biological sex, not necessarily entirely determined by them. Just because the interactions between biological sex and other factors are “complex” doesn’t mean that biological sex doesn’t influence morphological characteristics of the brain. And it certainly doesn’t imply that human brains are “intersex”.

That still leaves the very valid question of to what extent is brain morphology affected by biological sex. Certain aspects of brain morphology could be secondary sex characteristics. To be clear, the differences between male and female brain morphology have been demonstrated rather thoroughly. A meta study in 2014 has shown that:

“At a regional level, males on average have larger volumes and higher tissue densities in the left amygdala, hippocampus, insular cortex, putamen; higher densities in the right VI lobe of the cerebellum and in the left claustrum; and larger volumes in the bilateral anterior parahippocampal gyri, posterior cingulate gyri, precuneus, temporal poles, and cerebellum, areas in the left posterior and anterior cingulate gyri, and in right amygdala, hippocampus, and putamen. Females have on average higher density in the left frontal pole, and larger volumes in the right frontal pole, inferior and middle frontal gyri, pars triangularis, planum temporale/parietal operculum, anterior cingulate gyrus, insular cortex, and Heschl’s gyrus; bilateral thalami and precuneus; the left parahippocampal gyrus and lateral occipital cortex (superior division).”

Indeed, Joel seems to admit as much later on in the same 2011 publication:

We are discussing here the reverse problem, that is, whether we can predict the structure of one’s brain on the basis of one’s sex. The findings reviewed here lead to the conclusion that although we can predict that on average, females will have more brain characteristics with the “female” form than with the “male” form and vice versa for males, we cannot predict the particular array of “male/female” brain characteristics of an individual on the basis of her/his sex.

To further demonstrate this logic, pick any secondary sex characteristic at random. Say, human height. Can you predict an individual’s height based simply on their biological sex? How about their facial features? Body fat percentage? Lung capacity?

Of course not. You can’t predict specific secondary sexual characteristics of an individual based solely on their biological sex. Yet all of these characteristics demonstrate dimorphic differences. This is as true for brain morphology as it is for these other characteristics. There’s plenty of short men with masculine facial features and tall men with more feminine facial features. Joel’s insistence that demonstrating “internal consistency” requires that “a brain has only ‘male’ or only ‘female’ features” doesn’t make sense. When defined in such a way, the fact that her 2015 study found complete “internal consistency” to be rare is exactly what we should expect. By the same logic we could create a “human body size mosaic” as follows:

“Although we can predict that on average, females will have more body size characteristics with the “female” form than with the “male” form and vice versa for males, we cannot predict the particular array of “male/female” body size characteristics of an individual on the basis of her/his sex.”

Useful Explanations For Popular Misconceptions

It’s no secret that the existence of innate differences between men and women is controversial in many circles. The far left and the far right often fall victim to the Naturalistic Fallacy, which insists that what ought to be is based solely on what is. Sure, the evidence within neuroscience documents rather clearly that the brains of men and women differ in function as well as morphology, on average. But why not use a better understanding of these differences to influence public policy and help prevent things like sexism? Surely, all other things being equal, we would be more prepared to handle these issues with a better understanding of the biological differences between men and women.

The truth is that many people (including scholars) have a vested interest in there being no innate differences between the sexes. Some deny the existence of biological sex altogether! But of course, the real world is more complex than ideology can explain. And as Joel recognizes, the interplay between biological sex and other influences is indeed complex. But the only way we can better understand these complexities is by acknowledging what the data clearly demonstrate: innate biological sex differences exist. Only once we as a culture mature enough to handle these realities will real progress be made in our understanding of the human brain and whatever legal, political, and personal implications these realities entail. Sometimes science doesn’t tell us what we want to hear. But the path towards a more enlightened culture requires listening.