Sex differences in corpus callosum morphology (when controlling for brain size) have been reported in the literature.  Females tend to have a larger relative corpus callosum cross-sectional area, with a more bulbous splenium.  This has lead to suggestions that these anatomical differences are the result of selection on different behavioral abilities during our evolutionary history (Holloway 1983, 1990).  A number of recent studies suggest that corpus callosum morphology is related to behavioral ability on some cognitive tasks.

The present study assessed the strength of corpus callosum / behavioral ability correlations for an expanded the range of behavioral domains, both between- and within-families.  36 pairs of sisters (72 individuals total) where given a diverse battery of cognitive tests (linguistic, mental rotation spatial ability, frontal-lobe specific, and general reasoning ability), a test of throwing ability, and were asked to provide information about their relative degree of sociality.  High resolution MRI brain scans (voxel size: ~1.3 mm3, with no gaps between slices) where obtained, from which several neuroanatomical features where quantified, including overall brain volume, cortical grey volume, total corpus callosum cross-sectional area, and cross-sectional areas of the genu, body and splenium of the corpus callosum.  Within- and between-family correlations were calculated (controlling for age and simple reaction time) among the behavioral and neuroanatomical variables.  Between-family correlations were either very small or negative.  Within-family correlations were similar, although with somewhat larger negative associations (some reaching significance).  Of interest is the fact that the highest positive correlations occurred between corpus callosum measures and sociality variables.  The possible evolutionary implications of these findings are discussed, as are questions regarding the difficulty of applying within-species associations to evolutionary questions.