The regions most frequently reported by imaging studies as showing morphological sex differences include the basal ganglia and limbic structures. The caudate has been reported as proportionately larger in females by several studies across different ages and using different methodologies (Filipek, Richelme, Kennedy, & Caviness, 1994; Giedd et al., 1997; Sowell et al., 2002), which is intriguing given the involvement of the basal ganglia in disorders with pronounced sex differences in incidence such as Attention Deficit/Hyperactivity Disorder and Tourette's syndrome. The other areas most frequently reported as being different even after accounting for overall differences in brain size are the hippocampus and amygdala (Giedd et al., 1997; Goldstein et al., 2001; Suzuki et al., 2005; Wilke et al., 2007), with larger size or more rapid growth of the hippocampus typically reported in females, and of the amygdala in males. These findings appear consistent with observations of greater densities of androgen receptors in the amygdala versus higher levels of estrogen receptors in the hippocampus, as well as with the preliminary data combining steroid levels and brain volumes, which seem to indicate
