The Impact of Bilingualism on Vocabulary Development and Sentence Comprehension in Preschool Children

In today’s globalized society, growing up with more than one language is increasingly the norm, particularly for children with a migration background. These bilingual children face the unique challenge of developing competencies in both languages for everyday and academic contexts. Vocabulary plays a key role in this process, as it is essential for reading comprehension, academic success, and social integration. This dissertation investigates the vocabulary development of bilingual children, focusing on the under-researched Turkish-German language combination. It explores influencing factors and the relationship between vocabulary and language comprehension, with the long-term goal of informing effective diagnostic and support strategies. Study 1 adopted a new validation approach for test items by applying a literature-based, multi-stepp 1-PL model approach from test theory (Hartig et al., 2012). Item features such as age of acquisition and frequency were analyzed in relation to monolingual and bilingual language backgrounds. Results show structural similarities in vocabulary development between the first and second language and indicate that bilingual children can draw on their L1 knowledge. Study 2 used a longitudinal design to investigate productive and receptive vocabulary growth and the role of language exposure in mono- and bilingual preschool children. Using parallel test versions enabled fine-grained comparisons across language groups. The findings show that, under favorable conditions, bilingual children can bridge vocabulary gaps in their second language and transfer concepts from their first language effectively. Study 3 examined the influence of vocabulary, fluid intelligence, and inhibitory control on sentence comprehension. It revealed strong links between vocabulary size and comprehension performance, as well as compensatory effects of cognitive abilities, especially in the heritage language. This dissertation makes a significant contribution to understanding bilingual language development. It highlights the importance of vocabulary and cognitive resources and supports the development of diagnostic tools and support strategies that integrate linguistic, cognitive, and sociocultural dimensions.