Reports regarding sex dimorphism and pubertal effects on brain development are beginning to appear from a large cohort of typically developing twins currently being followed longitudinally by researchers in the Netherlands (Peper et al., 2009; Peper et al., 2008). Lutenizing hormone (LH) levels were measured as an indication of onset of puberty in 57 male twins (age 9.20 +- 0.10) and 47 female twins (age 9.21 +-0.12), and voxel based morphometry was used to relate LH levels to regional gray and WM densities (Peper et al., 2008). LH levels were found to predict WM volumes when both sexes were looked at together, although not when they were looked at separately, potentially due to the loss in power from the smaller sample sizes. A second analysis in an overlapping cohort of 10-15 year olds, also using voxel based morphometry, compared brain gray and WM density and their relationships to estradiol and testosterone levels (Peper et al., 2009). Males (n=37; age 11.6 +- 1.0 yrs) had overall larger brain volumes than females (n=41, age=12.2 +-1.2yrs) after correction for age differences, although no
