Sexual dimorphism of the anterior commissure and massa intermedia of the human brain.

Neuroanatomical sex differences were observed in the midsagittal area of both the anterior commissure and the massa intermedia on analysis of postmortem tissue from 100 age-matched male and female individuals. The anterior commissure, a fiber tract whose axons in primates primarily connect the two temporal lobes, was an average of 12%, or 1.17 mm2 larger in females than in males. The massa intermedia, a structure that crosses the third ventricle between the two thalami, was present in 78% of the females and 68% of the males. Among subjects with a massa intermedia, the structure was an average of 53.3% or 17.5 mm2 larger in females than in males. Inclusive of subjects with and without a massa intermedia, this structure was a mean of 76% or 16.93 mm2 greater in females than in males. These sex differences were present despite the fact that the brains of males were larger than those of females. Since a majority of subjects were adults, it is unknown when sexual differentiation occurred. Anatomical sex differences in structures that connect the two cerebral hemispheres may, in part, underlie functional sex differences in cognitive function and cerebral lateralization.