We present a fluency framework of the development of affective preferences and beliefs, two domains often neglected in research on development. Fluency is the subjective ease with which a mental operation can be executed. The fluency framework of the development of preferences and beliefs starts from evidence that effects of fluency are present early in infancy and remain stable across the lifespan. The framework predicts that interindividual differences in preferences and beliefs will be few among newborns but increase with age, a process we call differentiation. Such differentiation goes along with what we call mental narrowing, which denotes the observation that preferences and beliefs become narrower in range and less flexible with increasing age. Therefore, with increasing age, fewer new preferences or beliefs develop, and it is more difficult to change them. We discuss alternative explanations for differentiation and mental narrowing and outline empirical tests for the predictions.