Categories and induction in young children

One of the primary functions of natural kind terms (e.g., tiger, gold) is to support inductive inferences. People expect members of such categories to share important, unforeseen properties, such as internal organs and genetic structure. Moreover, inductions can be made without perceptual support: even when an object does not look much like other members of its category, and even when a property is unobservable. The present work addresses how expectations about natural kinds originate. Young children, with their usual reliance on perceptual appearances and only rudimentary scientific knowledge, might not induce new information within natural kind categories. To test this possibility, category membership was pitted against perceptual similarity in an induction task. For example, children had to decide whether a shark is more likely to breathe as a tropical fish does because both are fish, or as a dolphin does because they look alike. By at least age 4, children can use categories to support inductive inferences even when category membership conflicts with appearances. Moreover, these young children have partially separated out properties that support induction within a category (e.g., means of breathing) from those that are in fact determined by perceptual appearances (such as weight). Since we examined only natural kind categories, we do not know to what extent children have differentiated natural kinds from other sorts of categories. Children may start out assuming that categories named by language have the structure of natural kinds and with development refine these expectations.