Let’s start with the word – “differ”. We currently know of no brain measure on which all women differ from all men. When we talk about differences between the brains of men and women, we talk about a difference between averages – differences that you find when women and men are compared as two groups. But when you compare a specific woman to a specific man, you may or may not see the difference that you see at the group level.
Probably the largest group-level difference between the brains of men and women is in the size of the entire brain. It is larger, on average, in men by 7-10%. Note, though, that this difference disappears if you look at the size of the brain relative to the size of the body.
There are also many group-level differences in the size of different brain regions. Most of them reflect the general rule that in a larger brain, everything is larger. We know this from studies that compared the brains of women and men while taking into account the difference in total brain size, as well as from studies that compared women and men with a similar total brain size. Both approaches reveal much fewer differences between men and women than when women and men are compared without taking brain size into account. Moreover, in some regions the differences reverse themselves, that is, regions that in terms of absolute size are on average smaller in women compared to men, turn out to be larger in women compared to men, when their size relative to total brain size is considered. In any event, when brain size is taken into account, the differences between women and men are typically small.
(see https://bsd.biomedcentral.com/articles/10.1186/s13293-019-0245-7for a recent review of this topic).
Group-level differences also exist in connectivity patterns (some of which again reflect the difference in total brain size between the sexes), and in several chemical messengers called neurotransmitters. These differences are also mostly small, with lots of overlap between the scores of women and men.
Only a few regions in the human hypothalamus and brain stem show relatively large sex/gender differences even after total brain size is taken into account. These may be considered as “true” sex/gender differences, with “true” meaning that they are not simply a byproduct of the difference in total brain volume.
Up until now I was discussing differences in single brain features. But what happens if we look at the entire brain?
Team discovered that sex/gender differences rarely add up in a single brain. That is, very few people have brains in which all the group-level sex/gender differences show up consistently in the form that is typical of one sex. In other words, it’s extremely rare for allof these features to be present exclusively in the form more common in women than in men, or exclusively in the form more common in men than in women. In contrast, many brains have some features in the form more common in women than in men and other features in the form more common in men than in women. That’s what I call “mosaic brains”. Interestingly, we found a mosaic of female-typical and male-typical measures in some brains even when we analyzed the size of hypothalamic measures showing “true” sex/gender differences I’ve mentioned above.
(See https://journals.sagepub.com/doi/full/10.1177/1073858419867298for a recent review of the mosaic hypothesis).
Last point – If human brains are typically composed of a mosaic of female-typical and male-typical features, this means that knowing that someone is female or male does not help you much in predicting which features in her or his brain would be similar to yours, and which would be different. Indeed, we have recently found that although there are group-level differences between the brains of women and men, the brain ‘types’ typical of women are also typical of men, and vice versa. In fact, the chances that a woman and a man would have the same brain ‘type’ are very similar to the chances that two men or two women would have the same brain ‘type’.
(The paper describing the analyses that led to these conclusions can be found here (warning, it is quite technical) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6204758/)
0Comments