The present paper tries to study some of the basic rudiments that contributed to the change of paradigm about mental deficiency produced in the first decades of the XXth century. Taking as main sources the Binet and Simon's contributions from France, and the Sante de Sanctis' from Italy, it's analysed the construction of the category "abnormal childhood", relating it to the new and privileged space of observation that supposed the obligatory School. It concludes with a reflection about the fallacious use of the "intelligence" as "prejudice" in order to establish social taxonomies and meritocracies that they don't make but to justify the human inequality.