Allometry, the relationship between size and shape, is highly relevant to studies of sexual dimorphism. At least some changes in proportions of different brain regions occur solely as a factor of increasing brain volume (Finlay & Darlington, 1995). For example, it has been reported that the ratio of gray matter (GM) to white matter (WM) is larger in females (Allen et al., 2003; Gur, Gunning-Dixon, Bilker, & Gur, 2002). However, studies within and between species have found that WM volume increases more quickly than GM following a 4/3 power law (Zhang & Sejnowski, 2000). Studies directly comparing male-female differences to effects of overall brain size have found that sex differences in the GM/WM ratio are minimal once overall brain size differences have been accounted for, meaning that males and females with equal brain volumes will also have equal gray/white matter ratios (Leonard et al., 2008).
