The Oaxaca-Blinder decomposition tells us that unobserved components are important but do not account for the bulk of wage differences. Nevertheless, the decomposition does not reveal whether what is at issue is classic discrimination by employers or unobserved heterogeneity in productivity associated with the performance of mothers. First, we select one mother from the sample without replacement. Second, we select all non-mothers who share the same characteristics as the mother selected in the first step.