This study compares lexical development in a sample of 25 simultaneous bilingual and 35 monolingual children for whom semilongitudinal data were collected between the ages of 8 and 30 months. A standardized parent report form, the MacArthur Communicative Development Inventory (1989), was used to assess the children's receptive and productive vocabulary in English and/or Spanish. A methodology was devised to assess the degree of overlap between the bilingual children's lexical knowledge in one language and their knowledge in the other. Using the measures presented here, there was no statistical basis for concluding that the bilingual children were slower to develop early vocabulary than was the monolingual group. The wide range of vocabulary sizes observed at these ages in normally developing children (Fenson et al. 1991) was observed in these bilingual children as well. The close correspondence of the pattern of the bilinguals' growth in two languages at once to monolinguals' growth in one suggests that norms for lexical development in bilinguals should be made with reference to the children's performance in two languages together.
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