This article explores the influence of psychological language and discourses on the contemporary view of nationalism, an issue that has only begun to be studied in recent years (García-García, 2013; Sluga, 2006). On this occasion, the author focuses on two currents or schools that contributed decisively to the new view of nationalism after the Great War: first, degenerationist medicine and psychiatry, highly accepted in the European social and political debate since the late 19th century; second, and no less penetrating, the crowd or mass psychology of Taine, Tarde, Sighele, and, above all, Gustave Le Bon. After the Great War, as we shall see, nationalism was often represented as a form of degeneration, or a barbarous and cruel regression to a prior stage of development, embodied by the masses. This discourse and rhetoric was to condition the area of study for generations. In fact, the voices of medicine, psychiatry and mass psychology have not disappeared from the debate and continue to directly and indirectly influence the academic and popular comprehension of nationalism.