One of the major imperatives behind the comprehensivisation of secondary education was the belief that postponing the age at which students are tracked in different educational routes would mitigate the effect of social background on educational outcomes. Comparative investigations of large-scale international student achievement tests in secondary education, such as PISA, have indeed suggested that individual test results depend less on social origin in countries that have postponed tracking age. However, a crucial pitfall in such cross-sectional studies is that many other factors influence the effect of social origin on achievement as well. In order to account for possible unobserved confounder bias, and to acknowledge the fact that part of the social origin effect already exists prior to the introduction of tracking, we apply a difference-in-differences analysis to data from PIRLS (primary education, 2006, N = 33, n = 171.486) and PISA (secondary education, 2012, N = 33, n = 235.378). Our results confirm that the introduction of tracking increases the effect of social origin on reading achievement between primary and secondary education. This lends further support to the argument that postponing the tracking age can foster social equity in educational achievement.