This dovetails nicely with my previous post on encephalization. Brain size not linked to Microcephalin and ASMP?. Normal Variants of Microcephalin and ASPM Do Not Account for Brain Size Variability:
Normal human brain volume is heritable. The genes responsible for variation in brain volume are not known. Microcephalin (MCPH1) and ASPM (abnormal spindle-like microcephaly associated) have been proposed as candidate genes since mutations in both genes are associated with microcephaly and common variants of each gene are apparently under strong positive selective pressure. In 120 normal subjects, we genotyped these variants and measured brain volumes using magnetic resonance imaging. We found no evidence that the selected alleles were associated with increases or decreases in brain volume. This result suggests that the selective pressure on these genes may be related to subtle neurobiological effects or to their expression outside the brain.
Related: Earlier ASMP & MCPH1 articles.