We first studied brain structures including volumes for lobar gray and white matter, caudate and lateral ventricle, and area of the corpus callosum in 90 MZ twin pairs, 38 same-gender DZ twin pairs, and 158 singletons (average age = 11.5, SD = 3.5; Wallace et al., 2006). All brain regions with the exception of the lateral ventricles and cerebellum were significantly heritable, with genetic factors responsible for between 70 and 90% of variance, similar to previous reports in adults. When we then looked at the interaction of age with heritability, we found that variance due to genetic effects increased with age in both gray and white matter. However, in white matter the environmental variance decreased, whereas in gray matter it increased, leading to a decrease in the heritability ratio. These findings suggested that heritability indeed changes during the age span within our study, and that effects are specific for different types of brain tissue.
