L2 vocabulary acquisition of early sequentially bilingual children with TD and DLD affected differently by exposure and age of onset.

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Language exposure is known to be a key factor influencing bilingual vocabulary development in typically developing (TD) children. There is, however, a lack of knowledge in terms of exposure effects in children with developmental language disorder (DLD) and, especially, in interaction with age of onset (AoO) of second language acquisition. In the Helsinki longitudinal SLI study (HelSLI), we investigated the receptive and expressive second language (L2) vocabulary performance and cross-sectional vocabulary development of sequentially bilingual children with TD and DLD in order to resolve whether the groups could be differentiated based on their vocabulary performance. More importantly, we examined the effects of AoO and exposure in the vocabulary performance of these two bilingual groups. A total of 70 children with DLD from a hospital clinic and 82 with TD from kindergartens were recruited. Children were 3-7 years old with different AoOs for the L2 and varying degrees of language exposure. Multiple regression analysis was used to compare the groups in two receptive and three expressive vocabulary tests while considering the effects of AoO and language exposure. Children with TD outperformed children with DLD in both receptive and expressive vocabulary measures. Exposure predicted vocabulary but AoO did not. The effect of exposure was different in TD and DLD groups in receptive but not in expressive vocabulary. Additionally, the interaction of exposure and AoO was found for receptive vocabulary, but similarly in both groups. With increasing exposure, a difference in performance between the groups became more notable in receptive vocabulary compared with the expressive vocabulary. Bilingual children with TD and DLD can be differentiated by using L2 vocabulary tests when exposure is taken into consideration. Non-significant AoO effects in 3-7 year olds suggest flexibility in terms of when to start L2 immersion. However, exposure is important, and especially children with DLD would need a substantial amount of it relative to their TD peers, so that they would not fall even further behind over time. Differences in benefiting from exposure in receptive mode might offer clinicians and kindergarten personnel an insight for evaluating challenges in bilingual development. Severe challenges in vocabulary development in the DLD group also call for both individually targeted small-group activities for learning words as well as strategies for strengthening vocabulary in various environments and everyday life situations. What this paper adds What is already known on the subject Language exposure is often found to explain more of the variation compared with AoO in early L2 vocabulary. On the other hand, it has been suggested by some researchers, but not all, that AoO effects would be found. Exposure and AoO might also interact differently depending on the task and whether the development is typical or disordered. Contrary to the clinical observation, diagnostic value of receptive vocabulary has often been questioned. What this paper adds to existing knowledge This study indicates that receptive vocabulary might be useful in differentiating bilingual TD and DLD. Language exposure effects differ between TD and DLD groups, but depending on task. Lower performance, but also slower cross-sectional development, is found in receptive vocabulary for children with DLD. Interaction between language exposure and AoO stresses the importance of taking both into consideration when studying bilingual development. What are the potential or actual clinical implications of this work? L2 vocabulary tests can be used in differentiating bilingual children with TD and DLD when exposure is taken into consideration. Difficulties using exposure and, hence, slower development in children with DLD suggest that especially receptive vocabulary might be diagnostic by the accumulating exposure. Investing in ways of supporting vocabulary development through small-group activities and in everyday situations of bilingual children at risk of DLD is highly recommended. This is important to prevent them from incrementally falling further behind their TD peers over time. A limited AoO effect suggests that families have more flexibility in terms of when to place their child in L2 immersion in their early years.