This commentary makes the argument that the child-internal and child-external sources of individual differences in bilingual development are much the same as the sources of individual differences in monolingual development. It makes the further argument that the operation of the child-external influences results in differences between monolingual and bilingual development in the rate and sometimes in the outcome of language acquisition. An argument is made for the scientific and practical value of understanding the differences between monolingual and bilingual development, and future directions for research are suggested.

Bilingual children are a more heterogenous group than their monolingual counterparts with respect to the sources of variation in their language learning environments, as well as the wide individual variation in their language abilities. Such heterogeneity in both individual difference factors and language abilities argues for the importance of an individual differences approach in research on bilingual development. The main objective of this article is to provide a review and synthesis of research on the sources of individual differences in the second language (L2) and heritage language (HL) development of child bilinguals. Several child-internal and child-external individual difference factors are discussed with respect to their influence on children’s dual language abilities. In addition, the emergent research on individual differences in bilingual children with developmental language disorder is reviewed. Both the theoretical and applied relevance of individual difference approaches to bilingual development are discussed.

Previous work suggests that child-internal and -external factors influence dual language learning (DLL), although more work is needed – to clarify the role of phonological short-term memory (PSTM) and language exposure in particular. Accordingly, we investigated the role of language exposure and PSTM in phonological awareness and receptive and expressive vocabulary development in the majority language in DLL. PSTM was measured with both a language-specific and language-universal nonword repetition task (NWR). Sixty-five DLL preschoolers were assessed twice, five months apart. Results show that maternal majority-language proficiency as well as language-specific and universal NWR predicted all outcomes, whereas length of exposure to the majority language only predicted receptive and expressive vocabulary. Current exposure did not predict any outcome. The findings extend previous research on child-internal and child-external factors as predictors of DLL to phonological awareness and language-universal NWR.

Though previous research has shown that sentence repetition (SR) is an informative tool for identifying developmental language disorder (DLD) in bilinguals, little is understood about the skills that underlie children's performance on the task. With a population of 136 school-age Spanish–English bilinguals, the present study explores the contribution of verbal short-term memory, vocabulary, and language exposure on two SR tasks developed in English and Spanish. Results indicate that these skills may differentially underlie SR in typical versus disordered populations. Whereas the strongest predictors of English SR performance for typical bilingual children were expressive vocabulary and language exposure, bilingual children with DLD relied most heavily on verbal short-term memory. ROC curves to determine the classification accuracy of SR were conducted. Classification accuracy was excellent, with area under the curve reaching .92 for the English SR task and .87 for Spanish SR.

Research on the bilingual development of refugee children is limited, despite this group having distinct characteristics and migration experiences that could impact language development. This study examined the role of language environment factors, alongside age and cognitive factors, in shaping the Arabic as a first/heritage language and English as a second language of recently arrived Syrian refugee children in Canada (N = 133; mean age = 9 years old; mean family residency = 23 months). We found that Arabic was the primary home language with some English use among siblings. Children did not engage frequently in language-rich activities in either language, especially not literacy activities in Arabic. Parent education levels were low: most had primary school only. Hierarchical regression models revealed that stronger nonverbal reasoning skills, more exposure to English at school, more sibling interaction in English, more frequent engagement in language-rich activities in English, and higher maternal and paternal education were associated with larger English vocabularies and greater accuracy with verb morphology. Arabic vocabulary and morphological abilities were predicted by older age (i.e., more first/heritage language exposure), stronger nonverbal reasoning skills and maternal education. We conclude that proximal environment factors, like language use at home and richness, accounted for more variance in the second language than the first/heritage language, but parent factors accounted for variance in both languages.

Many children learn language, in part, from the speech of non-native speakers who vary in their language proficiency. To investigate the influence of speaker proficiency on the quality of child-directed speech, 29 mothers who were native English speakers and 31 mothers who were native speakers of Spanish and who reported speaking English to their children on a regular basis were recorded interacting with their two-year-old children in English. Of the non-native speakers, 21 described their English proficiency as ‘good’, and eight described their English proficiency as ‘limited’. ANCOVAs, controlling for differences in maternal education and child language level, revealed significant effects of group on lexical and grammatical properties of child-directed speech that the literature has identified as positive predictors of child language development. These results suggest that the child-directed speech of native speakers and non-native speakers with good proficiency provide a richer database for language acquisition than the child-directed speech of speakers with limited proficiency.

We examined how L2 exposure early in life modulates toddler word recognition by comparing German–English bilingual and German monolingual toddlers’ recognition of words that overlapped to differing degrees, measured by number of phonological features changed, between English and German (e.g., identical, 1-feature change, 2-feature change, 3-feature change, no overlap). Recognition in English was modulated by language background (bilinguals vs. monolinguals) and by the amount of phonological overlap that English words shared with their L1 German translations. L1 word recognition remained unchanged across conditions between monolingual and bilingual toddlers, showing no effect of learning an L2 on L1 word recognition in bilingual toddlers. Furthermore, bilingual toddlers who had a later age of L2 acquisition had better recognition of words in English than those toddlers who acquired English at an earlier age. The results suggest an important role for L1 phonological experience on L2 word recognition in early bilingual word recognition.

Carroll (Carroll) takes issue with the use of parent report to obtain quantity of language exposure measures in research on bilingual development. When discussing parent questionnaires, Carroll writes “Temporal units are crude measures of exposure and they tell us nothing about input”. While I agree that temporal units do not tell us much about the fine-grained details of the input within the temporal units, importantly, parent-report-based measures of input quantity have predicted variation in bilingual development of phonology, vocabulary and morphosyntax. These are robust and reliable findings across numerous studies, and yet, Carroll skates over them as if they did not matter, or dismisses them as trivial. Furthermore, Carroll seems to lead readers to believe that only coarse-grained, language-use temporal units have been obtained through this method; on the contrary, researchers have obtained fine-grained input quality details via parent report that also predict bilingual children's development. Finally, in some circumstances, parent report data is the only feasible method for obtaining language exposure information.