The human corpus callosum has been already demonstrated by a large number of studies to show a sexual dimorphism. This study investigated if the sex differences in brain size, larger in men than in women, or biological sex per se account for the apparent sex differences in callosal morphology. For this purpose, the authors compared the callosal thickness on MRI between 24 men and 24 women matched for overall brain size. In addition, they selected 24 extremely large male brains and 24 extremely small female brains to explore if observed sex effects might vary depending on the degree to which male and female groups differed in brain size.
The corpus callosum was always thicker in men than in women. However, the magnitude of this sex difference seems to be strongly determined by the cerebral sex difference overall. That is, the larger the discrepancy in brain size between men and women, the more pronounced the sex difference in callosal thickness. These findings suggest that individual differences in brain size account for apparent sex differences in the anatomy of the corpus callosum.