The research brought forward examines the evolution of the selective social processes that have accompanied educational expansion over the long term, referring to the case of France. It is based on an original index which addresses the issue of assessing inequality within the selection process for access to various educational levels. The results obtained support an understanding of contradictory prior findings attached to the thesis of uniform democratization and the thesis of "qualitative" or "intrinsic" democratization, respectively. They substantiate a new, alternative thesis, that of "contradictory democratization", according to which the effects of socioeconomic changes, and the correlative integration of the educational system, have tended to counterbalance the negative effects of educational policies on the relative achievement of children from disadvantaged backgrounds.