However, researchers have challenged Chomsky's claim that language input to children is impoverished by showing that adults in fact simplify the way they use language to children. When speaking to children, we generally articulate more clearly, we talk about things which the child can see and feel, and we 'recast' (implicitly correct) and 'expand' on the child's utterance, as shown in the examples below.

It has been argued that these simplifications facilitate language acquistion. There is evidence to suggest that when adult and child focus on the same thing, and there is linguistic interaction involving the adult recasting and expanding the child's utterances, language acquisition is promoted. However, there is also research (eg Ochs, 1982) showing that not all cultures modify language to children in this way; in some cultures, adults appear to engage in far less linguistic interaction with young children than in the white middle-class anglophone environments which have been the main focus of language acquisition research - and yet, these children still acquire their first language. So while the typical form of child-directed speech (at least as far as Anglo-Western societies are concerned) may be helpful to language acquisition, it cannot be considered essential. Input there must be, but it is not clear of what kind. For a detailed review of this issue, see Pine (1994).

The debate about whether human beings are equipped with a pre-program for constructing language continues currently, although Chomsky's claim is largely assumed by British and North American linguists. There are some important challenges, eg Terrence Deacon's fascinating book (Deacon, 1997: chapter 4).

But even if we assume the Universal Grammar hypothesis for first language acquisition, can we assume that it is available for second language acquisition by adults? Here the consensus seems to be: no, except insofar as our first language provides us with a model of a language constrained by Universal Grammar.