Some young bilingual children score lower than the monolingual based norms on standardized tests for vocabulary. This is why bilingual children are often over identified as having a DLD when tested in only one of their languages. The aim of this study is to assess the vocabulary development of young bilingual children in both of their languages and to provide more insight into the internal and external factors that can possibly account for the large variation between young bilingual children found in previous research. The outcomes of this study show that for two out of six children, their Dutch vocabulary is below the monolingual Dutch norms, but that this is certainly not the case when the vocabulary scores for both languages are taken into account. Therefore, this study shows how important it is to measure both of the languages of bilingual children. Furthermore, the pattern of vocabulary development that the bilingual children exhibit is similar to the pattern monolingual Dutch children follow. However, their developmental pattern can be influenced by the home language. The vocabulary in the home language is mostly influenced by the factor age. The Dutch vocabulary is mostly influenced by the quantity of the Dutch input the children receive. L1 typology only seems to have an influence on the pattern in which the children acquire the different grammatical word categories. The other factors that are predictors of the individual variation between the children are all related to the quality of the input the children are exposed to.
