A robust sex difference in the splenium of the corpus callosum, reflecting greater interhemispheric connectivity in women, was observed on magnetic resonance images from 114 individuals. In addition, bulbosity of the corpus callosum correlated with better cognitive performance in women but not in men, indicating that the degree of interhemispheric connectivity has different implications for men and women. These findings were based on a new image analysis technique which allows investigation of local variability in brain morphology.