Size of the human corpus callosum is genetically determined: an MRI study in mono and dizygotic twins.

The factors determining the large variation seen in human corpus callosum (CC) morphology are as yet unknown. In this study heritability of CC size was assessed by comparing the concordance of CC midsagittal area in 14 monozygotic and 12 dizygotic twin pairs with a mean age of 27 years, using magnetic resonance imaging and various methods of calculating trait heritability. Heritability was high regardless of method of assessment. The application of a structural equation model resulted in the estimate that 94% of the variance in CC midsagittal size is attributable to the genome. This indicates that under normal conditions and before the effects of normal aging, there is very modest influence of the environment on CC morphology. The results suggest that correlates of CC size, such as the pattern of cerebral lateralization, cognitive abilities and neuropsychiatric dysfunction may be associated with the genetic determinants of CC morphology.