Introduction

Human language is a unique natural communication system for two reasons. Firstly, the mapping from meanings to signals in language has structural properties that are not found in any other animal's communication systems. In particular, syntax gives us the ability to produce an infinite range of expressions through the dual tools of compositionality and recursion. Compositionality is defined here as the property whereby an expression's meaning is a function of the meanings of parts of that expression and the way they are put together. Recursion is a property of languages with finite lexica and rule-sets in which some constituent of an expression can contain a constituent of the same category. Together with recursion, compositionality is the reason that this infinite set of expressions can be used to express different meanings.

Secondly, at least some of the content of this mapping is learned by children through observation of others' use of language. This seems not to be true of most, maybe all, of animal communication (see review in Oliphant, this volume). In this chapter I formally investigate the interaction of these two unique properties of human language: the way it is learned and its syntactic structure.

Evolution without natural selection

Evolutionary linguistics is currently a growing field of research tackling the origins of human language (Bickerton, 1990; Pinker & Bloom, 1990; Newmeyer, 1991; Hurford et al., 1998). Of particular interest to many researchers is the origins of syntactic structure.