In some countries more than others, factors like parental socio-economic status (SES) can cause inequalities in educational achievement. Here we show how the mechanisms leading to such inequalities can be scrutinized by involving background variables which impact the relation between SES and achievement. We use the intercepts-and-slopes-as-outcomes paradigm which recognizes that the outcomes of schooling systems are not only characterized by average achievement (the intercept) but also by the achievement–SES regression slope. We show how certain background variables moderate the relationship between SES and achievement. As an illustration, we examine the relationship between reading achievement and SES, and how this is moderated by school funding and curriculum. This is done for several countries that participated in the PISA 2009 cycle.