https://www.sciencedirect.com/science/article/pii/S0149763421000804
https://theconversation.com/you-dont-have-a-male-or-female-brain-the-more-brains-scientists-study-the-weaker-the-evidence-for-sex-differences-158005
Since the dawn of MRI, neuroscientists have worked ceaselessly to find differences between men’s and women’s brains. This research attracts lots of attention because it’s just so easy to try to link any particular brain finding to some gender difference in behavior.
But as a neuroscientist long experienced in the field, I recently completed a painstaking analysis of 30 years of research on human brain sex differences. And what I found, with the help of excellent collaborators, is that virtually none of these claims has proven reliable.
Except for the simple difference in size, there are no meaningful differences between men’s and women’s brain structure or activity that hold up across diverse populations. Nor do any of the alleged brain differences actually explain the familiar but modest differences in personality and abilities between men and women.
My colleagues and I titled our study “Dump the Dimorphism” to debunk the idea that human brains are “sexually dimorphic.” That’s a very science-y term biologists use to describe a structure that comes in two distinct forms in males and females, such as antlers on deer or the genitalia of men and women.
When it comes to the brain, some animals do indeed exhibit sexual dimorphism, such as certain birds whose brains contain a song-control nucleus that is six times larger in males and is responsible for male-only courtship singing. But as we demonstrate in our exhaustive survey, nothing in human brains comes remotely close to this.
Yes, men’s overall brain size is about 11% bigger than women’s, but unlike some songbirds, no specific brain areas are disproportionately larger in men or women. Brain size is proportional to body size, and the brain difference between sexes is actually smaller than other internal organs, such as the heart, lungs and kidneys, which range from 17% to 25% larger in men.
When overall size is properly controlled, no individual brain region varies by more than about 1% between men and women, and even these tiny differences are not found consistently across geographically or ethnically diverse populations.
Other highly touted brain sex differences are also a product of size, not sex. These include the ratio of gray matter to white matter and the ratio of connections between, versus within, the two hemispheres of the brain. Both of these ratios are larger in people with smaller brains, whether male or female.
What’s more, recent research has utterly rejected the idea that the tiny difference in connectivity between left and right hemispheres actually explains any behavioral difference between men and women.
A zombie concept
Still, “sexual dimorphism” won’t die. It’s a zombie concept, with the latest revival using artificial intelligence to predict whether a given brain scan comes from a man or woman.
Computers can do this with 80% to 90% accuracy except, once again, this accuracy falls to 60% (or not much better than a coin flip) when you properly control for head size. More troublesome is that these algorithms don’t translate across populations, such as European versus Chinese. Such inconsistency shows there are no universal features that discriminate male and female brains in humans – unlike those deer antlers.