The concept of differentiation holds immense significance in education, touching upon aspects like access, inclusion, justice, and equality. However, it is also a complex and elusive notion, which acquires different meanings across historical and cultural contexts. This article explores the shifting reasoning about differentiation in the Swedish educational context. Inspired by Foucault’s account of disciplinary power, it conceptualizes differentiation as a technique for marking and addressing gaps between individuals. Drawing on an analysis of governmental and scholarly reports from 1842 to the late 1960s, the article identifies three shifts in the reasoning on differentiation: 1) from differentiation by socioeconomic class as a given factor to the search for scientific rationales for differentiation based on measurement of intellectual ability, 2) from viewing differences in intelligence as biologically conditioned and stable to viewing them as amenable to training and correction through education, and 3) from a focus on inputs to a focus on outputs. Overall, the article argues that even if the term ‘differentiation’ itself has been discursively replaced by others, the ideas underlying it—the search for gaps—continue to shape education in Sweden and beyond.