We investigated when young children become sensitive to one evolutionary important signal of honest affiliative and cooperative intent: a genuine (Duchenne) smile. Altogether, we tested 168 children between 2 and 5 years of age in a series of studies aimed at mapping the development of children's ability to discriminate genuine from fake smiles, their preference for genuine smiles, and their understanding of how genuine smiles are linked with prosocial behavior. Studies 1-4 showed that children's ability to discriminate, and answer questions about, the different types of smiles gradually improves between the ages of 2 and 4 years: from implicitly discriminating the smiles in their gaze behavior (at age 3), to being able to identify genuine smiles explicitly in a verbal task (at age 4). Study 5 showed that 4- to 5-year-old children expect people displaying genuine smiles to be more prosocial than those displaying fake smiles. These results demonstrate that the origins of this evolutionarily important form of partner choice appear early in development. (C) 2016 Elsevier Inc. All rights reserved.