Sexual dimorphism in the human brain: evaluation of tissue volume, tissue composition and surface anatomy using magnetic resonance imaging.

Magnetic resonance imaging (MRI) was used to evaluate sex differences in brain morphology by comparing measures of brain tissue volume, brain tissue composition (proportions of gray and white matter), and measures of cortical surface anatomy. A large and well-matched sample of healthy women (n=42) and healthy men (n=42) were evaluated. There was a significant gender effect on intracranial volume, males being larger. However, this increase in size was limited to the cerebrum as there was no sex difference in the volume of the cerebellum. The gender difference in size of the cerebral volume was evenly distributed, with all four lobes equally larger in males compared to females. Gray and white matter tissue proportions were similar between the sexes globally. Regional tissue composition analysis showed sex differences within the parietal lobes with females having proportionately more gray matter on the right side. There were no differences between the sexes in cortical surface anatomy measures. Overall, against the background of similarity in morphology, there are differences between the sexes with regard to general and regional brain measures. The functional significance of these sex differences is unclear, but may represent the differential effects of gonadal hormones during brain growth and development.