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 suggesting an interaction of testosterone level with genotype (see Figure 1 in the paper by Paus, this issue). Despite the increasing WM volume, the MTR ratio decreased with age, with age explaining a greater proportion of the variance in the MTR ratio in males (8%) than in females (1%). This pattern suggests that the rapid increase in WM volume in males may be related to other structural elements such as axonal volume rather than myelination.
