This work presents a new theory of how children acquire language and discusses its implications for a wide range of topics. It explores the roles of innateness and experience in language acquisition, provides further evidence for the theory of Universal Grammar, and shows how linguistic development in children is a driving force behind language shifts and changes Charles Yang surveys a wide range of errors in children's language and identifies overlooked patterns. He combines these with work in biological evolution in order to develop a model of language acquisition by which to understand the interaction between children's internal linguistic knowledge and their external linguistic experience. He then presents evidence from his own and others' research in the acquisition of syntax and morphology and data from historical language change to test its validity The model is makes quantitative and cross-linguistic predictions about child language. It may also be deployed as a predictive model of language change which, when the evidence is available, could explain why grammars change in a particular direction at a particular time.

It is a simple observation that children make mistakes when they learn a language. Yet, to the trained eye, these mistakes are far from random; in fact, they closely resemble perfectly grammatical utterances by adults - who speak other languages. This type of error analysis suggests a novel view of language learning: children are born with a fixed set of hypotheses about language - Chomsky's Universal Grammar - and these hypotheses compete to match the child's ambient language in a Darwinian fashion. The book presents evidence for this perspective from the study of children's words and grammar, and how language changes over time.