There is a heated and ongoing debate on the effects of early tracking on educational processes and outcomes. While there is a growing body of evidence suggesting that early tracking does not affect performance levels but increases educational inequality in achievement (e.g. Van de Werfhorst & Mijs, 2010), there is little evidence on the effects of tracking on educational processes. The social composition of schools is a key feature for educational environments and processes. From a system perspective, inequality in the social composition of schools is conceptualized as social segregation in the school system in a given country. Following the idea that the opportunity of choice can reinforce social inequality (e.g. Boudon, 1974), we hypothesize that the opportunity to choose between different school tracks increases the degree of social segregation in the school system. The main issue for the identification of the effect of tracking on social segregation are differences that existed already previously to the tracking (e.g. due to residential segregation). To address this issue, the present study made use of the accumulated and combined data of three large-scale international assessments: all the cycles in the Progress in International Reading Literacy Study (PIRLS), the Trends in International Mathematics and Science Study (TIMSS, grade 4 and 8), and the Programme for International Student Assessment (PISA). The primary school studies PIRLS and TIMSS (grade 4) provide data on segregation prior to the tracking and the secondary school studies PISA and TIMSS (eighth 8) on segregation after the tracking took place. The constructed a pseudo-panel database contains N=1,173 country-by-study-by-year-observations from 75 countries. To measure social segregation, we computed the index of dissimilarity for each country-by-study-by-year-observations using the number of books at home as an indicator for the socio-economic status. Our analytical approach is a simple difference-in-difference model where we regress the degree of social segregation in secondary school on the degree of segregation in primary school and an indicator for the tracking status. In line with our hypothesis, the results of preliminary analyses suggest that early tracking increases social segregation. The observed effect corresponds to an increase of 0.3 standards deviations (p < 0.001) in the degree of social segregation in secondary school. We conducted several robustness checks to explore the sensitivity of our analysis for different model specifications, measures of segregation, and subsamples. The findings remain qualitatively the same and suggest that early tracking increased social segregation.
