Previous research has documented that persons who obtain eligibility for higher education in the vocational school sector or through other non-standard routes show particularly low and decreasing transition rates into higher education. The project seeks to disentangle the social processes that are responsible for this phenomenon and aims at providing empirical evidence on the underlying mechanisms. Two different but potentially complementary processes have been suggested as explanations in the literature: First, selection effects arising from increasing upper secondary attainment rates among students who never intended to enter higher education. Second, idiosyncratic influences of alternative pathways to upper secondary education that have a negative impact on students’ competencies and bias educational and occupational aspirations towards non-academic preferences. The project aims at specifying these explanations further and will conduct the first empirical tests of the suggested mechanisms. The empirical data analyses will be based on the starting cohorts 3 (fifth-graders) and 4 (ninth-graders) of the National Educational Panel Study (NEPS). These data allow for a longitudinal assessment of the development of competences and educational and occupational aspirations through lower and upper secondary education. In addition, the data allow for linking this development both to the choice and to the influences of different learning environments provided by the various pathways to upper secondary education. By targeting at the causal processes behind the differences in higher education transition rates that are associated to different routes to higher education eligibility, the project will make an innovative contribution to the debate on inclusion versus diversion in the context of diversification in upper secondary education.