Does your brain have a sex?

Examining the science behind the male and female brain

At the molecular level, it is quite easy to differentiate between men and women. Women have two X chromosomes while men have an X and Y. Those endowed with the Y chromosome develop the male anatomy via the presence of the SRY gene, the necessary component for male sex determination.

This biological definitions of the sexes, however, does not strictly apply to society with the concept of gender being very different from these molecular aspects and cast into a storm of sexism and inequality. Gender stereotyping has become subconscious. Yet, more people have begun to embrace the dismantling of gender and the emergence of non-binary definitions. Anatomically, men and women clearly differ, but do these differences apply to the brain? Do our brains have a sex?

Differences in the structure and function of the male and female brain are of scientific importance due to the apparent sex-differential susceptibility to psychiatric disorders. Alcoholism, anti-social personality and suicide predominate in males while anxiety, stress-related disorders and eating disorders are more prevalent in female populations. But why and how are these differences manifesting?

Brain scientists have long been aware that the male brain is larger than the female brain, even when height and weight have been corrected for. In a study by a group in the University of Edinburgh, they looked for sex differences in the structure and function of 68 different substructures in the human brain. Looking at brain-structural MRI data of approximately 2,500 males and 2,750 females aged between 44-77, they reported significant differences between the brains of the sexes.

Adjusting for age and size, it was found that on average women have significantly thicker cortices. Studies of the cortex indicate that general intelligence is positively associated with cortical thickness. It was found that men have greater volumes in all other substructures. When this observation was then adjusted for by the total brain volume of the individual, this significant finding became less relevant. This adjustment revealed that men had larger volumes in 14 regions versus 10 regions in the female brain.

What was notable was how men were much more variable in the volumes of their substructures and cortical thickness. This variability has also been discovered in studies looking at male IQ. These parallels give the findings increased significance.

It is hard to say why men show such high variability. It has been postulated that it is the female-protective mechanism involving effects on the X chromosome. Deleterious genetic variants found on one X chromosome in females may be buffered by the presence of the opposite allele. As mentioned, males only have one X chromosome so this effect cannot occur. The likelihood that the mutated allele will then be expressed increases and variation in phenotype is thus altered.

The paper states that overall, for every brain region that showed large sex differences, there was always overlap between males and females. This confirms that the human brain can not – at least for the measures observed in this study – be described as sexually dimorphic. This overlaps with the idea of the intersex-brain.

This hypothesis was put forward by Daphne Joel in a 2015 paper entitled: “Sex beyond the genitalia: The human brain mosaic.” The paper discusses how sex differences in the brain are not stringently male or female but rather a “mosaic” of features. Some forms more common in female brains and some more common in male brains. Some features were also a hallmark of both sexes. The paper sets out to debunk the theory that there are regions of the brain that are either completely male or completely female. i.e. they were looking for sexual dimorphism of brain substructures. Could it be that just as males and females blatantly differ in their genitalia, that they too have sex-specific structures in the brain?

In an analysis of the Joel study by Dr. Kevin Mitchell of the Smurfit Institute of Genetics here at Trinity, he cleverly analogises the structure of male and female faces to the human brain. For each feature of the face; the jaw, the nose, the mouth, the protrusion of the forehead etc., there is no definitive male or female form. There is, however, a distribution “which is shifted one way in males and one way in females”. Yes, one can easily tell a male face from a female face – however looking at the features individually produces a distribution. Classification of a single feature may place it anywhere on the distribution and subsequent amalgamation of all results leads to individuals falling in either the “more male” dense region of the distribution or the “more female” dense region of the data or even perhaps, right in the middle of the two.

If one looks at the brain as a distribution rather than strictly dimorphic; it appears that the high variability in the anatomically male participants of the study by the University of Edinburgh infers that they span more of this hypothesised “distribution”. The anatomically female subjects, however, inhabit a smaller proportion of this “distribution”, with both sexes capable of overlap. One can therefore not take a brain and definitively say whether it is male or female. You can take a brain and say that because it appears to have certain features it is more likely to be a female’s brain, but your answer cannot be conclusive. A distribution, of course, cannot produce discrete results.

It is also important to note that there were still regions assessed in the study where there was minimal overlap in the form. These must be termed sexually dimorphic according to the definition. Most of the results, however, should not be categorised according to gender. Sex has nothing to do with the variation. If there is a distribution and males and females may inhabit anywhere on that distribution it is wrong to say that these forms are either “male forms” or “female forms”.

So, do brains have a sex? Unfortunately, science cannot answer this question yet. There are structural differences in the brains of males and females. Males have larger brains and women have thicker cortices. Do these relate to intelligence or susceptibility to certain mental illnesses? Much more work must be done to answer these questions. What the science does consistently say is that male and female brains are more similar than they are different and that brains are not sexually dimorphic.