Individual differences in the onset of tense marking: a growth-curve analysis.

The purpose of this study was to explore individual differences in children's tense onset growth trajectories and to determine whether any within- or between-child predictors could account for these differences. Twenty-two children with expressive vocabulary abilities in the low-average to below-average range participated. Sixteen children were at risk for specific language impairment (SLI), and 6 children had low-average language abilities. Spontaneous language samples, obtained at 3-month intervals between 2;0 and 3;0, were analyzed to examine change in a cumulative productivity score for 5 tense morphemes: third person singular present, past tense, copula BE, auxiliary BE, and auxiliary DO. Hierarchical linear modeling was used to model intercept and linear growth at 30 months and quadratic growth overall. A growth model that included mean length of utterance (MLU) and MLU growth better explained within-child productivity score growth trajectories than a parallel model with vocabulary and vocabulary growth. Significant linear growth in productivity scores remained even after a control for MLU was in place. When between-child predictors were added in the final conditional model, only positive family history approached statistical significance, improving the overall estimation of the model's growth parameters. The findings support theoretical models of language acquisition that claim relative independence of tense marking from other more general aspects of vocabulary development and sentence length. The trends for family history are also consistent with proposals implicating faulty genetic mechanisms underlying developmental language disorders. Systematic use of familial risk data is recommended in future investigations examining the relationship between late-talking children and children at risk for SLI.