Cross-morpheme facilitation: The systematic emergence of agreement in 2-year-olds

This article investigates the relationship between third person singular present tense agreement morphemes, copula is and verb-s, at 2;00 and 2;03. Language samples from 60 children at 2;00 were analyzed for the productivity of copula is as measured by the number of different morphemes preceding is in children's sentences. Samples at 2;03 were analyzed for the number of different verb stems affixed with -s or marked with verb past inflection. Concurrent control measures of vocabulary and MLU were obtained at 2;03. Two hierarchical multiple regressions were performed to determine whether there was a relationship between 2;00 copula is and 2;03 verb inflection. Each regression had a different 2;03 verb inflectional category, verb-s or verb past, as its dependent variable. The results of the verb-s regression showed that both MLU and copula is productivity accounted for significant portions of unique variance in 2;03 verb-s productivity. In contrast, the results of the verb past regression revealed that only MLU accounted for a significant amount of variance. These data indicate that the grammatical features connecting copula is and verb-s emerge soon after the second birthday.