Simultaneous Bilingual Development Is Additive Except in Early Expressive Vocabulary Growth: Longitudinal Evidence From 2.5 to 12 Years

A basic question about bilingual development is how the acquisition of one language affects the acquisition of the other. Previous findings are few and mixed. The present study addressed this question with longitudinal data on the dual-language vocabulary growth of 149 US-born children from Spanish-speaking immigrant families, who were followed from the age of 2.5 to 12 years. Expressive and receptive vocabulary skills in English and Spanish were assessed with standardized tests, and primary caregivers provided estimates of the relative quantity of the children's English and Spanish exposure at home. The relations between English and Spanish skill levels and growth rates were assessed with concurrent correlations at each age and longitudinal bivariate structural equation models. The relations of skill levels in each language to the relative quantity of home exposure were assessed with concurrent correlations at each age. Findings indicated a tradeoff between English and Spanish expressive vocabulary growth through age 4 only and no competitive relation between English and Spanish in receptive vocabulary development at any age. English vocabularies were related to relative exposure quantity at home only through age 6; Spanish vocabularies were related to home exposure through age 12. The findings thus suggest that bilingual development is subtractive only in the first years of development when both languages compete for the same limited resource-home exposure, and that bilingual receptive language development is additive from an earlier age than is expressive language development. SUMMARY: The relation between two simultaneously acquired languages is subtractive only during the first 4 years of development and only in expressive language skill. After age 4 years, simultaneously acquired languages grow independently of each other. Growth in the societal language is unrelated to home exposure after the preschool years, while minority language growth continues to depend on home exposure. The quantity of home language exposure is more of a limiting factor in expressive vocabulary development than in receptive vocabulary development.