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Measures of verb lexicon size and diversity should be included as part of early language assessment to guide clinical decision making with young children at risk for language impairment.

Purpose The current study used an intervention design to test the hypothesis that parent input sentences with diverse lexical noun phrase (NP) subjects would accelerate growth in children’s sentence diversity. Method Child growth in third person sentence diversity was modeled from 21 to 30 months (n = 38) in conversational language samples obtained at 21, 24, 27, and 30 months. Treatment parents (n = 19) received instruction on strategies designed to increase lexical NP subjects (e.g., The baby is sleeping.). Instruction consisted of one group education session and two individual coaching sessions which took place when children were approximately 22 to 23 months of age. Results Treatment substantially increased parents’ lexical NP subject tokens and types (ηp2 ≥ .45) compared to controls. Children’s number of different words was a significant predictor of sentence diversity in the analyses of group treatment effects and individual input effects. Treatment condition was not a significant predictor of treatment effects on children’s sentence diversity, but parents’ lexical NP subject types was a significant predictor of children’s sentence diversity growth, even after controlling for children’s number of different words over time. Conclusions These findings establish a link between subject diversity in parent input and children’s early grammatical growth, and the feasibility of using relatively simple strategies to alter this specific grammatical property of parent language input.

The contribution of parent input to children's subsequent expressive verb diversity was explored in twenty typically developing toddlers with small verb lexicons. Child developmental factors and parent input measures (i.e. verb quantity, verb diversity, and verb-related structural cues) at age 1;9 were examined as potential predictors of children's verb production in spontaneous language samples at age 2;3. Parent verb input diversity, rather than input quantity, was the primary input factor contributing to children's subsequent verb diversity. Regression analysis showed that verb diversity in parent input at age 1;9 accounted for 30% of the variance in children's verb production six months later, with children's total vocabulary size at age 1;9 accounting for an additional 16% of the variance. These findings demonstrate the relative contributions of developmental and input factors to individual differences in toddlers' language development and establish the importance of input diversity to verb acquisition.

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