The research will clarify the effect of words on categorization in the first months of life and trace the developmental trajectory of this effect over the first year of life. The starting point for these studies is a recent demonstration by the investigators that words promoted object categorization in infants as young as 3 months, and do so in a way that carefully matched tones do not. This result opens several new avenues for investigation, each of which will bear on fundamental questions concerning the relation between language and conceptual organization in the first year of life. The project will identify what it is about speech stimuli that promote object categorization over and above the effect of tone sequences in infants so young. The proposed experiments test the hypothesis that human speech engenders in very young infants a heightened attention to the surrounding objects, and that this very general attentional effect later becomes more specific as infants become attuned to the speech sounds of their own ambient language. Pursuing this hypothesis requires an examination of 3- to 12-month-old infants' treatment of a broad range of auditory stimuli. To discover whether the facilitative effect of words on categorization is specific to linguistic stimuli or evident for other complex stimuli as well, the proposed experiments use a preferential-looking task. In this task, the infant is presented with a series of individual pictures followed by a test trial in which the infant is presented with a novel and a familiar picture side-by-side, and the investigators measure how much time the infant spends looking at each picture. The project investigates the effects of auditory stimuli including naturalistic speech from a range of languages, filtered speech, backwards speech, mammal vocalizations, and bird calls. Another focus of this project is to investigate the developmental trajectory for infants growing up in bilingual homes. The project, focused on typically-developing infants, will have broad impact on theories of normative development in monolingual and bilingual infants and will have implications for interventions with atypically-developing infants and young children. The research focuses on two uniquely human capacities -- language and conceptual development -- and explores an emerging link between them. By mapping out the development of this link, the proposed studies will put practitioners in a better position to identify patterns that deviate from typical development. Moreover, by considering infants growing up bilingual, this work will address crucial questions about consequences of processing two languages in the first years of life, and will advance efforts to promote positive developmental outcomes for the ever-increasing number of bilingual infants and young children in the United States. Finally, this basic research can also serve as a springboard for developing targeted interventions for young children diagnosed with language delay and impairments, as well as those diagnosed with autistic spectrum disorders.