Children’s first words may bear little resemblance to adult forms (/ga/ for squirrel), and they may at first produce multiple versions of words. Mastering adult pronunciations takes time and practice. Babbling allows practice with some sounds, but those sounds may only appear in words years later. Early vocalizations, at times paired with gestures, often precede recognizable words. And children take time before they can produce versions that match adult productions. Some 70% of children’s words up to age four are inaccurate and variable in form. Children may select some word forms over others in production because they are easier targets. They simplify words by substituting sounds they can manage; they assimilate some sounds to neighboring ones; they omit some sounds, especially in clusters. And they may set up templates for multisyllabic words, focussing on stressed syllables over unstressed ones. As they get older, they make spontaneous repairs to words that don’t match their stored representations, and also repair when asked by others. Finally, they practice words, to themselves, and in their daily language use. Single words are easier than word combinations, so longer utterances take still more practice.

Chapter 10 discusses various characteristics of the overall developmental progression of language acquisition. We first discuss some general properties of this process and then show how it can be studied both with respect to language production and language perception. We discuss the stages and milestones that children go through for different aspects of grammar and ask whether the properties and timing of stages lend support to the Innateness Hypothesis for language. We then formulate the argument from stages. Here the idea is that a complex system like language “unfolds” in the human mind, step by step, each step occurring at more or less regular points in time, as determined by a biological clock. This process of unfolding is called maturation. Just as our body gradually changes into a mature system, so does our mind. This process of unfolding is biologically determined and largely outside the control of the organism, although external factors (“nurture“) play a role. We critically evaluate the argument from stages, asking how precisely it might support the Innateness Hypothesis.

During the first year of life, human infants undergo an extraordinary process of vocal learning, unmatched by other primates. This lays a key foundation for meaningful speech production. The first sections of this chapter describe major milestones and other features of the development of prelinguistic and early speech sounds, including the acquisition of new sound types and of conversational turn-taking skills. The chapter then discusses what we know about the roles of exploratory play, social input, and neural systems in human vocal learning. A section on computational modeling reviews theoretical work that informs our understanding of how these mechanisms interact. Effects of sociocultural and clinical differences on infant vocal development are then discussed. The final section of the chapter discusses policy perspectives on research and interventions in this domain.