<p>This article addresses the well-known question of the effects of cognitive differentiation on the emergence of social stratification in educational success for a hardly investigated special case: the so-called collateral stratification as the only correlative covariation of cognitive differentiation and social stratification via conditional, unnecessary, and avoidable side effects of achievement differentiation at the end of primary school, especially from the interaction of cognitive and social conditions. The study is based on data from the National Educational Panel Study (NEPS) for the German federal states. Three processes are analysed, collateral stratifications in educational attainment, in the structuring of school classes in the course of the transition from primary school to secondary school and in secondary school achievement. The result supports the more recent findings on the effects of stringently organised cognitive differentiation. In the case of attainment, there is neither a&#xa0;reinforcement of the origin effect nor collateral stratification; the cognitive and social composition of the school classes are decoupled; and in the case of achievement at the secondary level, there is even collateral “compensation” in favour of the less talented children from the lower social classes. However, there are also indications of stratifying effects of the interaction between cognitive abilities and social background, namely disadvantages especially for talented children from the lower social strata and scissor effects in favour of children from the upper social strata. These can be explained by shortcomings in practices and loopholes in the regulations governing the factual implementation of differentiation. These are also consequences of institutional gaps in stringency that could be closed.</p>