Often questioned, the phenomenon of the diffusion of the smile in photographic portraiture during the 20th century resists explanation. Recalling the conventional dimension of portraiture, in this article, Gunthert describes this evolution as the historical modification of a social norm: the passage from a culture of bashfulness to one of expressiveness between the 1930s and the 1950s. This transformation is based both on the reflexivity of media representations and the valorisation of an ethos of authenticity in self-presentation, promoted by amateur photography and the cinema. As a touchstone of this evolution, the full-toothed smile became an autonomous photographic sign, the modernist translation of the middle classes’ sociability and their attainment of a new historical status.