Multivariate statistical genetic analyses provide an estimate of the degree to which the same genetic or environmental factors contribute to multiple neuroanatomic structures, such as distributed neural networks or ontologically related regions. In a large multivariate analysis of the CPB data, a single genetic factor accounted for 60% of variability in cortical thickness across the brain (Schmitt et al. 2007). Six factors account for 58% of the remaining variance, with five groups of structures strongly influenced by the same underlying genetic factors. These findings are consistent with the radial unit hypothesis of neocortical expansion proposed by Rakic (Rakic and Caviness 1995) and with hypotheses that global, genetically mediated differences in cell division were the driving force behind interspecies differences in total brain volume (Darlington et al. 1999; Finlay and Darlington 1995; Fishell 1997)
