Acquisition of auxiliary and copula BE in young English-speaking children

This study tested the unique checking constraint hypothesis and the usage-based account concerning why young children produced tense and agreement morphemes variably via three experiments. Experiment 1 investigated whether subject types influenced the production accuracy of auxiliary 'is' in three-year-olds through an elicited production task. The rate of use of auxiliary 'is' increased as children's tense productivity increased, but the pattern was different for each subject type. The rate of use increased more rapidly with tense productivity for lexical NP subjects than it did for pronominal subjects.

Experiment 2 further examined the role of subject types, predicate types, and predicate word frequency on the use of copula 'is' in three-year-olds via an elicited production task. Overall, the production accuracy of copula 'is' was higher with nominal predicates than with permanent- or temporary-adjectival predicates, followed by locative predicates. Children also produced copula 'is' more accurately with low-frequency predicate words than with high-frequency predicate words. Moreover, the effect of subject types on the use of copula 'is' varied with children's tense productivity. For sentences with nominal, permanent-adjectival, or temporary-adjectival predicates, children with lower tense productivity used copula 'is' more accurately with lexical subjects than with pronominal subjects in. In contrast, children with higher tense productivity produced copula 'is' more accurately with pronominal subjects than with lexical subjects.

Experiment 3 extended Experiment 1 by exploring the degree of abstractness of representations of auxiliary BE via a structural priming task. The production accuracy of auxiliary 'is' in three-year-olds increased above the baseline when the prime-target pair shared the same structure and subject + auxiliary 'is' combinations, but not when the prime-target pair only shared the same structure. However, the production accuracy of auxiliary 'are' did not change with prime types.

These experiments suggest that young children have only lexically-specific representations of auxiliary BE. Frequency, rather than structural properties, of sentence elements influenced the production accuracy of auxiliary and copula 'is' in young children. These findings support the usage-based approach that young children use tense and agreement morphemes variably because they have not yet learned adult-like abstract representations and use highly frequent/ lexically-specific constructions for the production of these morphemes.