Men and women already differ in many ways. But whether the gender difference is also manifested in the brain has so far been a matter of debate. Now one of the most extensive comparative studies to date proves that there are neuroanatomical differences. Accordingly, women have more gray matter in the brain, for example, and the parietal lobe, while men have more volume in some posterior and lateral areas of the cortex, including the primary visual center. The researchers also found evidence that these differences are closely related to the activity of the sex chromosomes.
Typical man, typical woman? Regardless of gender stereotypes or role assignments, there are not only external differences between the sexes. Men and women are not always the same in many aspects of behavior, psychology and health. For example, autism or Parkinson’s disease is more common in males, but women are more likely to suffer from depression. In childhood, the brain maturation processes in boys and girls run at different speeds and there also seem to be differences in perception. However, whether there are clear morphological characteristics behind such gender differences in cognition and behavior is highly controversial. Some studies found local variations in the thickness of the gray matter in women and men. Others, however, came to the conclusion that the overlaps are so large that one cannot speak clearly of a female or male brain.
Gray brain matter is not the same everywhere
“A better understanding of whether and which gender differences exist in the human brain is very important for how we evaluate the established differences in cognition, behavior and psychiatry,” explains senior author Armin Raznahan from the National Institute of Mental Health in Bethesda. That is why he and his team have examined this question again. To do this, the researchers first evaluated the brain scans of 976 adult men and women whose brain morphology and activity had been investigated as part of the Human Connectome Project (HCP). Raznahan and his team specifically compared the volume of different areas of the gray matter in the cortex. Significant differences emerged that went beyond the general difference in size – the brain of men has around ten percent more volume on average. The scientists also found these characteristics and gender-specific deviations in a further data set of brain scans that had been collected by the British UK biobank.
“We find that the adult brain has a stereotypical pattern of regional gender differences in the gray matter,” the scientists report. Specifically, the volume of the 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 brains. In contrast, in men the cerebral cortex is thicker in the back of the brain, including the primary visual center. As a comparison with the functions of these areas of the brain showed, overall tendencies can be identified: “The regions in which the volume of gray matter in the brain is larger in men are mostly involved in object recognition and processing of faces,” say Raznahan and his colleagues. “In contrast, the more pronounced cortical regions in women are linked to the control of tasks, the impulse control and the processing of conflicts.”
Differences also in gene expression
In the next step of their study, the scientists wanted to know whether these morphological differences can be attributed to gene expression in the brain areas concerned. Studies in mice had previously shown that the sex chromosomes in the brain areas with pronounced gender differences are also read particularly actively. Raznahan and his team tested whether this is also the case in humans using maps of gene expression that show the locally active genes for 1317 brain tissue samples from six deceased donors. The analysis showed that there was also a clear pattern here: “The cortical regions with a relatively high expression of the sex chromosomes are in the areas that have a higher volume in men than in women,” report the researchers. The regions in which the gray matter of the brain was thicker in women, on the other hand, showed less activity of the X and Y chromosomes.
According to Raznahan and his colleagues, these findings indicate that there are deeper gender-specific differences in the brain, which can be seen in the morphology and gene expression. They also see the agreement between the patterns of volume differences and gene activity as an indication that these differences between the female and male brains are probably innate. “We do not 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 also how the differences now observed are linked to gender-specific differences in behavior, cognition or mental health have yet to be researched.
Source: Siyuan Liu (National Institute of Mental Health, Bethesda) et al., Proceedings of the National Academy of Sciences. doi: 10.1073 / pnase.1919091117