It is said that men are from Mars and women are from Venus, but in fact it seems both are very much from Earth. New research finds that while there are sex/gender differences in the brain, human brains do not belong to one of two distinct categories: male brain or female brain.

While some ‘male’ and ‘female’ features do exist, the vast majority of people have a mix, so a general binary categorisation does not emerge.

Research by Joel et al studied the physical features of the brains of 1400 people aged between 13 and 85.

The team looked for variations in the size of brain regions as well as the connections between them. In total, the group identified 29 brain regions that generally seem to be different sizes in self-identified males and females. These include the hippocampus, which is involved in memory, and the inferior frontal gyrus, which is thought to play a role in risk aversion.

When the group looked at each individual brain scan, however, they found that very few people had all of the brain features they might be expected to have, based on their sex. Across the sample, between 0 and 8 per cent of people had “all-male” or “all-female” brains, depending on the definition. “Most people are in the middle,” says Joel.

This means that, averaged across many people, sex differences in brain structure do exist, but an individual brain is likely to be just that: individual, with a mix of features. “There are not two types of brain,” says Joel.

Since the research is paywalled I haven’t read the paper, so I can’t say why the features designated “male” and “female” were so designated. I gather they were not completely different.

The bottom line is that the terms ‘male’ or ‘female’ brain make no sense when referring to the physical nature of the brain. However, that still leaves open the question of whether there are gender differences in what the brain does.

Gina Rippon in commenting about the article refers to other gender research, including work by Harry Reis and Bobbi Carothers. They say:

Many people assume that sex differences in social behavior are categorical—that these differences represent fundamental distinctions between two distinct categories (taxa) of humans. Contrasted with this view is the idea that sex differences are dimensional—that differences between men and women indicate nothing more than relative positions along overlapping continuous dimensions.

To illustrate what a taxon is they use human measurements and strength. If you plot height and hair length, for example, you come up with something like this:

While there is overlap, the proportion of men who are taller than all women is quite substantial. In these circumstances it makes sense to think in terms of categories or taxa.

When they turn to social, psychological and personality characteristics, the categorical or taxonic analysis makes no sense. While there are statistical differences they lie along a dimension. What stands out is the overlap, the similarities.

Their analyses:

were run on a series of 22 discrete data sets in the published literature, which comprised 122 indicators and 13,301 individuals from 13 unique studies.

They point out that their samples were limited to modern North America, possibly a culture that may have become “detaxonified.” There may be cultural differences elsewhere in the world. They conclude:

At least with regard to the properties that we studied, rather than representing distinct categories of humans, women and men are cut from the same psychological cloth.

To follow their title, it’s a case of shades of grey rather than black and white.

I’ve turned on the “moderate comments” button on the discussion thread, because I’d like to stay on topic rather than to stray into gender relations and feminism.