Research in the sociology of education underscores the ambivalent role that schools can play in creating equity. On the one hand, schools are viewed critically, namely as institutions that create inequality by sorting students into unequal learning environments. On the other hand, from a positive perspective, schools can be seen as equalizing mechanisms, as they create standardized and more homogeneous learning opportunities compared to out-of-school environments, from which especially children from disadvantaged backgrounds should benefit.

To find out what role schools actually play in reproducing social inequality, Jan Skopek tested the "equalization hypothesis" with the relatively new Differential Exposure Approach (DEA). In DEA, the effect that school attendance has is determined by comparing children of the same age who (randomly) differ in the length of time they have been in school at the time of a skills test. Jan Skopek contrasted the DEA approach with the Seasonal Comparison Method (SCD), used primarily in the United States, which compares learning rates during and between school years ("summer vacation"), but which has yielded inconsistent results in recent studies.

To investigate the causal effect of schooling on social and ethnic differences in education in Germany using DEA, Jan Skopek worked mainly with data from the Starting Cohorts 1 and 2 of the National Education Panel Study, which is located at LIfBi and whose great potential he emphasized in his lecture. Furthermore, he used data from the USA. 

In the course of his presentation, Jan Skopek gave a differentiated account of how he has studied the role of (early) schooling in learning differences among students in Germany and the United States. While longer school attendance clearly boosts the school performance of all children, the data show little or very weak evidence for a social equalizing effect. But conversely, the gap in achievement between disadvantaged and better-off children does not widen during schooling either. His analyses also showed the potential of the differential exposure approach for cross-national research on the effects of schooling.