DESCRIPTION: How do children organize their knowledge about the world? Recently there has been a shift in the treatment of concepts, from traditional views assuming that concepts can be characterized by superficial features, to theory views treating concepts as embedded in common-sense explanatory frameworks. The finding that children's concepts are tied to theories is especially striking, as it runs counter to a previously widely accepted view of children's concepts as perceptually bound. In contrast, preschoolers expect category members to share nonobvious similarities, even in the face of salient perceptual dissimilarities, and judge non-visible internal parts to be especially crucial to the identity and functioning of an item. In a sense, very young children acts as if members of a category share an underlying essence. However, little is known regarding: the developmental course if essentialist beliefs below age 4; what role language plays in transmitting essentialism to children; what implications essentialism has for causal reasoning; or how to reconcile essentialism with traditional views of children as focused on salient appearances. The proposed research uses naturalistic language analyses and experimental studies with children 2 to 10 years of age to address these questions. Specifically, the proposal has three aims: 1) To examine how essentialist beliefs are expressed in natural language by children (age 2-4 years), and how natural language expressions of essentialism are interpreted by children. The studies of natural language expression will reveal the scope of essentialist beliefs early in development; the studies of natural language interpretation will reveal a mechanism whereby essentialist beliefs are transmitted; 2) To examine the developmental incorporation of non-obvious features (causes, ontologies, and internal properties) into children's categories; and 3) To reconcile essentialism with the traditional view of children as focused on perceptual aspects of the world, by clarifying the conditions that lead to different profiles of performance. This work will allow researchers to go beyond simple models that posit dichotomous shifts over development, to build more subtle and informed understandings of the conditions that lead to flexibility in performance -- and why. Altogether, these studies will provide converging and precise evidence regarding the links among concepts, language, and theory construction in early childhood.