The contrast between findings of increased heritability of cortical thickness with age seen here and decreased heritability of lobar gray matter volumes reported in a subset of this population (Wallace et al., 2006) appears to be driven chiefly by differences in the interaction of environmental variance with age, which increased for gray matter volume but decreases for cortical thickness. Cortical gray matter volume is affected both by the thickness of the cortex and its surface area, which are determined by different types of cell division during the original formation of the cortex (Rakic, 1988). The surface area is also dependent on factors such as gyrification, which have been found to increase during this age range (Sowell et al., 2002). Although developmental changes in both cortical thickness and surface topology during childhood and adolescence have been reported (Gogtay et al., 2004; Sowell et al., 2002), little is known about how longitudinal changes in these measurements may relate to each other.
