The recent surge in inequality has been linked to growing disparities both within and between economic strata. Existing approaches to analyzing changes in inequality, however, often ignore within-group inequality by solely analyzing mean differences between groups. Approaches that do allow examining both changes in within- and between-group inequality in turn are limited in addressing causal questions about why inequality is changing. This paper introduces a novel approach to analyzing how a treatment variable affects both changes in within- and between-group inequality and decomposing these changes into compositional and behavioral effects. The procedure combines a classic variance decomposition with the Kitagawa-Blinder-Oaxaca (KBO) decomposition approach. Compared to KBO, however, the method allows analyzing treatment effects not only on the mean but on the whole conditional distribution. I demonstrate the utility of the method with an application analyzing the changing impact of parenthood on earnings and its consequences on earnings inequality.