Brown and Bellugi (1964) write of "three processes in the child's acquisition of syntax." These are further discussed by Cazden (1972). Two of the processes are child-adult interactions involving language. The first is imitation with reduction-the child imitates the adult's utterances in an abbreviated form while maintaining word order. The complementary process is imitation with expansion-the adult imitates the child's utterance, but expands it to include those elements left out by the child. They point out that while these processes occur, the child is very rarely given direct feedback as to whether or not his utterances are grammatically correct. Generally, young children are given negative feedback only when the utterance is semantically incorrect within its context, or when it is not understandable. Brown (1973) also points out that the frequency with which a child has heard a linguistic structure used has less of an effect on when he will acquire it than does its semantic and grammatical complexity. The third process discussed by Brown and Bellugi is the least understood, as well as being the key to syntactic development:

The processes of imitation and expansion cannot teach more than the sum total of sentences that speakers of English have either modeled for a child to imitate or built up from a child's reductions. However, . . . all children are able to understand and construct sentences they have never heard but which are nevertheless well-formed, well-formed in terms of general rules that are implicit in the sentences the child has heard. Somehow, then, every child processes the speech to which he is exposed so as to induce from it a latent structure (1964, p. 143).

Brown and Bellugi view this induction of the latent structure to proceed by the progressive differentiation of syntactic classes, working simultaneously with an integrative process which enables the child to learn to correctly combine elements of the different classes.

Slobin (1971, 1973) has attempted to derive from his cross-cultural research "operating principles" that the child may be using in beginning to process his language. These operating principles are said to "guide the child in developing strategies for the production and interpretation of speech and for the construction of linguistic rule systems." They, therefore, would seem to provide some sort of link between listening and auding processes. However, Slobin's operating principles seem more like descriptions of what children across various languages appear to do, rather than explanations of how they acquire language. For example, Operating Principle F: "Avoid exceptions," reflects the common observation of children over-generalizing linguistic rules and thereby producing such forms as "sheeps" and "goed." Other operating principles include: "Pay attention to the ends of words," "Pay attention to the order of words and morphemes," "Avoid interruption or rearrangement of linguistic units," and others.