The vocabulary of bilingual children is determined by various linguistic factors that develop depending on the language input and individual factors of these children. To understand vocabulary development and to be able to support these children accordingly, the assessment instruments essentially need to be adapted to this individual process. The current study examined factors influencing productive vocabulary performance by means of explanatory item-response models on a sample of 126 German monolingual and Turkish—German bilingual preschool children aged between 3 and 6 years. We analyzed item features with respect to lexical category, length, frequency, age of acquisition, and complexity in dependence of the language background of the children. Apart from a general delay in the development and some language specific characteristics, the results indicate a parallel shifted acquisition of vocabulary in bilingual children, which is nonetheless shaped by the same factors as in monolinguals.
