The growth of tense productivity.

PURPOSE This study tests empirical predictions of a maturational model for the growth of tense in children younger than 36 months using a type-based productivity measure. METHOD Caregiver-child language samples were collected from 20 typically developing children every 3 months from 21 to 33 months of age. Growth in the productivity of tense morphemes, centered at 21 months, was examined using hierarchical linear modeling. The empirical Bayes residuals from 21- to 30-month productivity growth trajectories predicted children's accuracy of tense marking at 33 months. RESULTS A random effects quadratic growth model with no intercept best characterized the growth of tense marking between 21 and 30 months. Average development was characterized by slow instantaneous linear growth of less than 1 morpheme per month at 21 months and acceleration overall. Significant variation around this trend was also evident. Children's linear and quadratic empirical Bayes residuals together predicted 33-month accuracy scores (r = .672, p = .008). CONCLUSIONS Acceleration and variation about this trend are consistent with maturational models of language acquisition. With an empirically sound characterization of early variation in morphosyntactic growth rates, future investigations can more rigorously test hypotheses regarding biological, environmental, and developmental contributions to the acquisition of morphosyntax.