The joint effects of bilingualism, DLD and item frequency on children's lexical-retrieval performance

Bilingual children and children diagnosed with developmental language disorder (DLD) are characterized by reduced lexical-retrieval abilities. Few studies examined their joint contribution and the mechanisms underlying these effects in the lexical domain.

To explore the joint effects of bilingualism and DLD by adopting a four-group comparison in which the difference between bi- and monolingual children with DLD is directly compared with that of bi- and monolingual children with typical language development (TLD). In addition, to examine the mechanisms underlying the effects of bilingualism and DLD on children's lexical-retrieval abilities, we tested how item's characteristics (frequency of use) modulate the effects of bilingualism and DLD.

Fifty-eight children (aged 9-14 years) participated in the current study. They were either Hebrew monolingual or Hebrew-English bilinguals and were either diagnosed with DLD or had TLD. Children completed a Hebrew picture-naming task and verbal short-term memory tests. The influence of participants' characteristics, including bilingualism, DLD and verbal short-term memory, as well as item's characteristics (frequency of use) were tested.

Accuracy analysis revealed that bilingual children scored lower than monolingual children and that children with DLD scored lower than children with TLD. Critically, the two factors interacted such that their joint presence resulted in less-than-additive effects. Specifically, although bilingual children with DLD performed worse than all other groups, they performed better than expected under an additive model. Interestingly, monolingual children with DLD performed similarly to bilingual children with TLD. Increased verbal short-term memory was associated with better performance across the four groups. Finally, bilingualism and DLD interacted with item frequency, such that being bilingual, having DLD, or both, resulted in increased sensitivity to item frequency manifested in exceptionally lower performance on low-frequency items.

The findings suggest that the strength of linguistic representations contribute to the effects of bilingualism and DLD. Further, the presence of bilingualism in the context of DLD does not exaggerate the impact of DLD. Clinically, this suggests that differences between bi- and monolingual children with DLD must be considered in reference to the gap in lexical-retrieval performance observed between bi- and monolingual children with TLD. Finally, because monolingual children with DLD and bilingual children with TLD performed similarly, sensitive diagnostic tools and intervention programmes should be adopted to allow correct identification and treatment of bilingual children with DLD.