The author summarizes research results of the past two decades and puts a theoretical model up for discussion. Educational expansion proceeds via different levels representing a structural unity. Once the room for learning is exhausted on a development level, the leap onto a new level is provoked through the self-expansion of the educational system. In this, the number of students in teacher training as a proportion of the entire student body from 1820 up to the present is considered the best indicator. The driving force in educational expansion both in the individual career and in the collective historical process is a deficit provoking new expectations and efforts.