Purpose: Children with specific language impairment (SLI) often have vocabulary impairments. This study evaluates longitudinal growth in a latent trait of receptive vocabulary in affected and unaffected children ages 2;6 (years;months) to 21 years and evaluates as possible predictors maternal education, child gender, and nonverbal IQ.
Method: A sample of 519 participants (240 with SLI; 279 unaffected) received an average of 7 annual assessments for a total of 3,012 latent trait Peabody Picture Vocabulary Test (PPVT) observations. Unconditional and conditional multilevel growth models were estimated to evaluate growth trajectories and predictor relationships over time.
Results: Children with SLI had lower levels of receptive vocabulary throughout the age range assessed. They did not close the gap with age peers. Children with higher nonverbal IQs had better PPVT performance, as did children of mothers with higher education. Child gender showed an advantage for young girls that leveled out with age and then became an advantage for boys from ages 10 to 21 years. All children's rate of vocabulary acquisition slowed around 12 years of age.
Conclusions: The outcomes of the study have implications for hypothesized causal pathways for individual differences; predictions differ for children under 5 years, 6-10 years, and later ages.