ABSTRACT
Imagine a newborn infant, lying on her back or being held in someone’s arms. A face is looking at her, and that face is making noises. What is the infant to make of this? What is she thinking about these noises, and, even more mysteriously, what is she doing with these noises that will allow her—within the space of a few years—to figure out the system that generates these noises? We know that by the time this newborn is 4 years old, she will be able to make those noises, too, and she will be able use that repertoire of noises to do all the things that humans do with language. How is that possible? It is a fact that all normal children reared in anything remotely like a normal environment learn to talk. A central aim of research on language development for the past 50 years has been to explain what makes language acquisition a universal achievement. It is also a fact, however, that language development is highly variable. At every point in development, children differ in the size of the vocabularies they command, the complexity of the structures they produce, and the skill with which they communicate (Fernald & Marchman, 2011). Another aim of research on children’s language development is to account for those individual differences.
