All children appear to master the various grammatical complexities in about the same order, although at different rates. One child, at two years of age, may use longer, more complex sentences than another child utters at four years (see Figure). The rate of language acquisition undoubtedly depends on a number of factors, including intelligence and this amount of verbal exchange within the home.
The order in which the child acquires his linguistic knowledge, however, seems to follow a fixed sequence that is only minimally related to the frequency with which the child hears specific utterances or certain types of grammatical constructions. And the same development sequence found among English-speaking children seems to occur, with only minor deviations, among children learning any language.
How do we explain the universality in the stages of language acquisition? One possibility is that in human brain is innately programmed to process linguistic input; the child operates on the language le hears and extracts grammatical rules of increasing complexity as various biological mechanisms mature.
Some psychologists who hold to this view have suggested that there is a biologically- determined “critical period” for language development; if an individual is not exposed to language before age 14 or 15 (about the time the brain stops growing), he will be unable to acquire language (Lenneberg, 1967)
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Evidence bearing on this assumption is difficult to find. The few rare cases where attempts have been made to teach language to feral children (children who have been left as infants log row up in the wild) have failed; but these results are confounded by such factors as possible mental retardation.
Evidence does show that individuals who learn a second language in their teen’s platter do so with more difficulty and less “naturally” than a young child and usually retain something of an accent.
Another, more likely, possibility is that children acquire linguistic knowledge in an invariant order because the way they perceive and operate on their environment progresses in a similar sequence. Linguistic development may mirror cognitive development.
A child cannot use words correctly without, understanding the concepts they represent. For example, all children have some understanding of position in space before they understand the concept of time. In their early explorations children move themselves and their toys into various positions; and one of their first manipulations is to place a toy in something.
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In language acquisition the first prepositions a child uses consistently are in and on, both of which specify spatial location. The prepositions into (which specifies location plus direction) and over and under (location plus relation) tend to occur later.
The use of the preposition in to express temporal relations (in a minute, in a week) occurs still later (H. Clark, 1973). These, observations illustrate how the acquisition of specific word forms depends upon the child’s cognitive s development.
Leaving aside the question of the innateness of linguistic ability, we can summarize by saying that language acquisition depends on learning the meaning of words and on developing general rules that permit one to comprehend and generate an infinite variety of sentences. The child starts out with his own rule system and gradually modifies the rules until they approximate adult rules and hence adult linguistic performance.