Why are human men taller than women?

The brains of men and women are different

Stereotypical than expected: The brains of women and men are more different than expected, as a study has now revealed. According to this, women have more gray matter in the frontal lobe and parietal lobe, while men have more volume in some posterior and lateral areas of the cortex, including the primary visual center. In line with this pattern, there are also differences in the gene expression of the sex chromosomes in the brain areas.

How far are the biological differences between men and women? It seems clear that apart from social influences there seem to be differences in behavior, cognition and health. Even newborns behave slightly differently, the female brain is on average more active and processes social information differently than the male brain. In addition, for example, autism or Parkinson's are more common in men, whereas women suffer more often from depression.

Is there such a thing as “the” male or female brain?

However, it is disputed whether there are clear morphological characteristics behind these gender-specific differences: Does “the” male or female brain even exist? While some studies have found indications of such differences - including in networking - others see the female and male brain as just a myth. The overlaps are simply too big.

To clarify this, researchers at the National Institute of Mental Health in Bethesda have once again undertaken a large-scale search for morphological gender differences in our thinking organ. To do this, the researchers evaluated the brain scans of 976 adult men and women whose brain morphology and activity had been examined as part of the Human Connectome Project (HCP). In particular, they compared the volume of different areas of the gray matter in the cortex.

Local differences in the gray matter of the brain

They found what they were looking for: "We find that the adult brain has a stereotypical pattern of regional gender differences in the gray matter of the brain," the scientists said. Specifically, the volume of gray matter in women is higher in parts of the prefrontal cortex, in the overlying orbitofrontal cortex and in parts of the parietal and temporal brain. In men, on the other hand, the cerebral cortex is thicker in the back of the brain, including the primary visual center.

Overarching functional patterns can be identified: "The regions in which the volume of the gray matter in men is larger are mostly involved in object recognition and the processing of faces," report Raznahan and his colleagues. "The more pronounced cortical regions in women, on the other hand, are linked to the control of tasks, the impulse control and the processing of conflicts."

Differences also in gene expression

But what are these volume differences based on? Studies in mice have already shown that these local differences are also related to gender-specific gene expression. Raznahan and his team checked whether this is also the case in humans by comparing them with maps of gene expression for 1317 brain tissue samples from six deceased donors.

The result: There was also a clear pattern in the activity of the genes in the brain cells. "The cortical regions with relatively high expression of the sex chromosomes are in the areas that have a higher volume in men than in women," the researchers report. In contrast, the regions in which the gray matter was thicker in women showed less activity on the X and Y chromosomes.

"Environmental factors not the main driving force"

Taken together, Raznahan and his team see these results as evidence that the brains of men and women are anatomically quite different - and that these regional differences are closely linked to the activity of the sex chromosomes. In her opinion, these gender-specific differences must also be at least partly innate.

“We don't believe that environmental factors are the main driving force behind these highly reproducible patterns in the volume of gray matter,” the researchers state. To what extent and in what way the differences observed now are linked to gender-specific differences in behavior, cognition or mental health, has yet to be researched. (Proceedings of the National Academy of Sciences, 2020; doi: 10.1073 / pnase.1919091117)

Source: NIH / National Institute of Mental Health

July 21, 2020

- Nadja Podbregar