The acquisition of language by children is an intriguing phenomenon and a fascinating topic in developmental psychology. It is a multifaceted process that involves the employment, coordination, construction, and mapping of many human faculties (perceiving, thinking, hearing, speaking, acting, etc.) as well as environmental resources (objects, relations, times, places, events, etc.). Language acquisition also constitutes an enormous task in terms of magnitude. To get an idea, adult speakers of English use between twenty thousand and fifty thousand word forms when speaking, and a considerably larger vocabulary when comprehending. In the case of young children, according to some estimates, they should master around ten new words a day on average in order to arrive at a vocabulary of fourteen thousand words by the age of six. The growth of vocabulary from then on to about age of seventeen averages at least three thousand new words a year (Clark 1994). Starting at age ten, children encounter about ten thousand new words a year, and at least one hundred thousand distinct word-meanings in their textbooks alone (ibid.).

The complexity and enormity of this phenomenon poses serious questions for linguists, psychologists, and AI practitioners who have worked hard to understand and explain it but who are, by far, remote from a comprehensive and concurred-upon theory of language acquisition. As a companion to our study of connectionist models of language learning, this appendix provides a brief introduction to some of the key issues of language learning.