Purpose The goals of this study were to quantify longitudinal expectations for verb lexicon growth and to determine whether verb lexicon measures were better predictors of later grammatical outcomes than noun lexicon measures.
Method Longitudinal parent-report measures from the MacArthur-Bates Communicative Development Inventory (Fenson et al., 2007) from ages 21 to 30 months from an archival database were used to model growth in common noun and verb lexicon size for 45 typically developing toddlers. Communicative Development Inventory growth coefficients and 24-month measures of lexical diversity from spontaneous language samples were used to predict 30-month grammatical outcomes on the Index of Productive Syntax (Scarborough, 1990).
Results Average verb growth was characterized by 50.57 verbs at 24 months, with linear growth of 8.29 verbs per month and deceleration overall. Children with small verb lexicons or slow linear growth at 24 months accelerated during this developmental period. Verb lexicon measures were better predictors of grammatical outcomes than noun lexicon measures, accounting for 47.8% of the variance in Index of Productive Syntax scores. Lexical verb diversity in spontaneous speech emerged as the single best predictor.
Conclusion 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.
