Despite decades of educational reform and expansion, social inequalities in educational attainment have been stubbornly persistent. Individuals from more advantaged backgrounds not only outperform those from less advantaged backgrounds in examinations (‘primary effects’), they also make more ambitious educational choices, even at comparable levels of prior performance (‘secondary effects’). From a policy perspective, it is essential to understand the relative importance of primary and secondary effects in order to determine when and how to intervene to reduce them. This is an issue which both sociologists and economists have addressed, but existing findings are inconclusive. In addition, certain considerations have thus far been neglected. First, existing studies have focused almost exclusively on the influence of family economic resources. This is likely to underestimate the size of both primary and secondary effects and can only tell a partial story about their sources and about over-time change. Second, little effort has been made to identify the actual mechanisms that generate these effects. We seek to address these neglected issues. We will do so by examining the British case in comparative perspective. In this way we should be able to shed light on the possible influence of different educational institutions and policies.