According to Boudon, social background affects educational transitions as a result of differences in children's academic performance (primary effects) and differences in transition probabilities given children's level of academic performance (secondary effects). This study addresses historical changes in both primary and secondary effects on the educational transition from primary school to higher secondary education in the Netherlands. In addition, it considers changes over time in the relative importance of these effects. The study compares five cohorts of Dutch pupils, specifically those enrolling in secondary education in 1965, 1977, 1989, 1993 and 1999, and it employs counterfactual analyses. The main findings are that secondary effects have been stable and primary effects have fluctuated to some extent. As a result, the proportion of the total effect of social background accounted for by primary effects has increased somewhat, from 53 per cent to 58 per cent.