The earliest sexual dimorphism observed in life is that of head circumference, which was assessed by prenatal ultrasound as early as the second trimester of pregnancy (Joffe et al. 2005). Although it is the least heritable, the cerebellum is the most sexually dimorphic brain structure. Boys have larger total cerebellar volume than girls and the magnitude of sex-differences varies significantly with age: 10% at 8 years and 13% at 20 years of age (Tiemeier et al. 2010). Likewise, the volumes of cortical gray and white matter are generally larger in boys than in girls (Figure 2A). Cortical gray matter volumes increase somewhat more rapidly with age in boys than in girls: 6–7.5% differences were observed in neonates and children, and ~10% differences during adolescence (Reiss et al. 1996; Caviness et al. 1996; Giedd et al. 1996; Giedd et al. 1999; Lenroot et al. 2007; Gilmore et al. 2007). Gender-specific differences in cortical white matter volume are greater in adults than in children and adolescents: 15–17% differences were observed from childhood to young adolescents (De Bellis et al. 2001; Perrin et
