How do infants learn to produce the sounds of their language? The vocal abilities of infants change dramatically over the first year of life. Beginning with the earliest, immature vocalizations, infants make rapid progress, typically producing their first words by 12 months of age. Along the way, they begin to produce speech-like syllables and to structure sequences of syllables in accordance with the phonological patterns of their language environment. While the vocal achievements of the first year are well-described, not nearly as much is known about the mechanisms of change that drive vocal development. However, many studies of song learning in birds, as well as the investigators' preliminary studies, indicate that the social environment is a source of rich structure and feedback that can guide early vocal learning and development. Can prelinguistic infants use social feedback to create new, more developmentally advanced vocalizations? The investigators hypothesize that two processes enable social influences on vocal learning. During the first 6 months, infants learn that their babbling reliably affects adults' behavior. From this early learning, infants become attuned to social responses to their vocalizing. Over the next 6 months, infants use the contingent reactions of caregivers to their babbling to guide their vocalizations towards the phonological patterns of their language. Acquiring knowledge of underlying patterns that govern the relations among multiple examples is a process known as statistical learning. To investigate these processes of vocal learning, the form and timing of caregivers' reactions to their infants' sounds will be manipulated. First, the process of learning associations between babbling and caregivers' reactions will be investigated in 2- to 5-month-old infants. Infants' information-processing capacities and caregivers' natural responsiveness to babbling will be used to predict individual differences in learning. Second, the statistical properties and contingency of speech to 9-month-old infants will be manipulated to assess their effects on vocal learning. The role of statistical learning in speech perception and production will be directly compared. Finally, the relation between the two learning mechanisms (associative learning at 2 to 5 months and statistical learning at 9 months) will be determined. Findings from these studies will allow the investigators to construct a new theoretical framework for vocal development, integrating data from social learning and speech perception. The experiments will benefit from new applications of digital technologies that allow caregiver behavior to be manipulated in real-time (via instructions given over wireless headphones) during naturalistic interaction. At the same time, changes in infants' vocalizations will be recorded and analyzed. The proposed research has broad and fundamental implications for educating parents in providing optimal learning environments for their infants. An understanding of the role of socially guided learning in speech and language could be used to help caregivers to be more sensitive to infants' behavior in ways that would facilitate development. Investigating social influences on phonological development will contribute to the study of speech-language pathology and of processes underlying both successful and disordered communicative development. This research will provide a framework for understanding the role of early social learning in communication and language deficits such as those seen in autism and Down's syndrome. By elucidating mechanisms by which infants learn from caregivers, these findings will be useful in designing behavioral interventions for infants with developmental disorders.