The relationship between WM development and testosterone was examined in a cross sectional sample of 408 healthy adolescents (204 males; age range 12-18)(Perrin et al., 2008). They measured serum testosterone levels and genotyped the androgen receptor, which contains a CAG repeat that affects testosterone activity. They further sought to separate out developmental changes in myelination by measuring changes in the magnetization transfer ratio (MTR), a measure sensitive to macromolecular structure and composition of tissue. In the absence of overt pathology such as edema, the predominant factor affecting MTR in WM is the amount of myelin, and thus developmental changes in MTR are thought to indicate changes in myelination. Similarly to previous studies, they found a much more rapid increase in WM volume in males than females. Although levels of bioavailable testosterone did not add significantly to chronologic age in explaining variation in WM volume in the overall group, it appeared to have a stronger effect in the subset of males with the AR genotype having fewer CAG repeats, a variation associated with higher transcriptional activity of the AR gene and
