Behaviourism (sometimes called learning-theory in a narrow sense of that phrase) suggests that language is an immensely complex chain of associations and that learning language is a process of selective reinforcement (Skinner, 1957). Behaviourism suggests that babbling is crucial to the development of language since the child is reinforced for producing speech sounds either directly by receiving attention from caretakers or indirectly by receiving pleasure at reproducing sounds he/she has heard before. Those sounds that are not part of the adult language are not reinforced and eventually disappear from the child's sound repertoire. This theory stresses the continuity of pre-linguistic and linguistic development, for all of language is a set of stimulus-response associations built up over years of trial and error, as in the child's learning to say apple elicits a response of a piece of fruit.
However, the nature of language itself argues against behaviourism as an adequate explanation. Babbling children's utterances are systematically different from the utterances they hear around them. Furthermore, children over-regularise consistently as they learn word endings, creating new construction that they have not heard before nor have been reinforced to produce. It is far more difficult for behaviourists to explain such creative learning.