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.

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.

Paradis’ (2023) keynote article is a timely documentation of the ongoing shift in focus within childhood bilingualism research from investigating the factors that modulate majority or second language (ML/L2) attainment (Chondrogianni & Marinis, 2011) to understanding the sources of variation that lead to minority heritage language (HL) maintenance. This shift of focus into individual differences (IDs) in bilingual performance across both languages reflects converging research carried out in the past twenty years reporting that, when learning barriers are not at stake, bilingual children can reach ML outcomes. It also aligns with research findings suggesting that ML educational or linguistic outcomes are directly related to the successful attainment of HL educational and linguistic milestones, which, in turn, reinforce the need for the HL to be supported.

Large individual differences in language skills are well documented in monolingual children (e.g., Kidd, Donnelly & Christiansen, 2018). In bilinguals, the broad variation is even more pronounced. Interestingly, some bilingual children might be weak in their Heritage Language (HL, also labeled as Minority Language, Home Language, Community Language), to which they have naturalistic exposure from birth. Others might be weak in their Societal Language (SL), the language of the surrounding and educational environment. Large individual differences are observed in neurotypical bilingually exposed children as well as in their bilingually raised peers with developmental language disorder, autism spectrum disorder, and hearing impairment (see also Armon-Lotem & Meir, 2016; Meir & Novogrodsky, 2020). Figure 1 visualizes individual differences in morphosyntactic skills of monolingual and bilingual children with typical language development aged 5;5–6;8 as indexed by the LITMUS Sentence Repetition tasks (the data are drawn from Armon-Lotem & Meir, 2016; Meir, 2018). While monolingual preschool children (MonoRU and MonoHE) show little variation, bilinguals with different levels of dominance (balanced bilinguals: BB; HL dominant: HL-D; SL dominant: SL-D) as determined by standardized tests exhibit large individual differences within each language and across their two languages.

Paradis’ keynote article provides a comprehensive overview of factors influencing bilingual children’s dual language abilities. It includes the ‘usual suspects’, such as input quantity, and also highlights areas requiring further investigation, such as cognitive abilities. As such, it will no doubt serve as a valuable basis for the field as we move forward. Paradis quite rightly points out that whilst some of these factors may be unidirectionally related to language abilities, suggesting causality, for many others such relations are bi- or multidirectional and as such, caution is required in interpreting them. In order to pinpoint the nature and direction of these relations (currently absent from Figure 1 in the keynote), more complex analytic techniques are needed, as Paradis herself notes: “The relations among attitudes/identity, input and interaction, and perhaps social adjustment and wellbeing, are likely to be complex; therefore, more complex analytic techniques are needed to understand the path(s) between family attitudes about the HL on one hand, and children’s HL outcomes on the other.” (Paradis, 2023: 19). In this commentary, we provide an illustration of how the complex relationships between the variables discussed in Paradis’s keynote article could be conceptualised within a causal inference approach. We offer a modest starting point by summarising key features of causal inference modelling and by illustrating how it might help us better understand what causes what.

Children growing up bilingually are often treated as a monolithic group; however, heritage language (HL) bilinguals constitute an extremely heterogenous group that vary due to a wide variety of factors. In her keynote, Paradis provides a thought-provoking analysis of the research literature and identifies key internal and external factors that lead to individual differences. Specifically, she identifies age of second language (L2) acquisition, cognitive abilities, and social-emotional wellbeing as important internal factors. She also discusses both proximal and distal external factors. Proximal factors include children’s cumulative exposure to L2 and HL, L2 and HL usage at home, and the richness of the L2 and HL environment. Distal factors involve education in HL, parent language proficiency, socioeconomic status (SES), and family attitudes and identities. In my commentary, I expand on Paradis’ keynote to include the role of culture, which I consider to be both an internal and external factor, and respond to her discussion of two external factors, the impact of SES and the role of the classroom environment.

We investigated online early comprehension in Italian children aged 12 and 20 months, focusing on the role of morphosyntactic features (i.e., gender) carried by determiners in facilitating comprehension and anticipating upcoming words. A naturalistic eye-tracking procedure was employed, recording looking behaviours during a classical Looking-While-Listening task. Children were presented with sentences and pictures of two objects representing nouns characterised by either the same gender (determiner was uninformative) or a different gender (determiner was informative). As expected, 20-month-old children recognised the target picture when this was named, and they were faster in the different-gender condition. Interestingly, 12-month-old infants identified the target picture only when presented with an informative determiner (different-gender condition). These results suggest that, as early as 12 months of age and with an improvement seen at 20 months of age, toddlers can extract and use determiner gender features to enhance comprehension and make predictions about upcoming words.

This study investigated why object-gap relative clauses (RCs) are dominant in early child Mandarin. We discuss how restrictive-RCs differ from pseudo-RCs syntactically and pragmatically, and examine how these two types of RCs are distributed in the RC utterances of ten children and their caregivers. The results showed that (a) Mandarin-speaking children produce many more pseudo-RCs than restrictive-RCs, (b) restrictive-RCs exhibit a subject-gap advantage and are dominantly headed, and (c) pseudo-RCs exhibit an object-gap advantage and are dominantly headless. We propose that the development of restrictive-RCs is mainly influenced by the structural factor, and that the extensive use of pseudo-RCs is attributed to the communicative needs of young children. Our findings also suggest that young children’s pseudo-RCs tend to have a subject-focus reading, and the object-gap dominance observed in the pseudo-RCs of child Mandarin is related to the head-final RCs and the special structural features of the cleft construction in Mandarin.

Language in touchscreen apps could be useful as an additional source of children’s language input, alongside child directed speech (CDS) and books. Here we performed the first analysis of language in apps, as compared with books and CDS. We analysed language in 18 of the most popular educational apps targeting pre-schoolers and compared their language content to children’s books and CDS with respect to types of constructions and psycholinguistic features of words. We found that apps contained lower frequency words and had lower lexical diversity compared to CDS, and shorter utterances compared to books. Apps may thus provide an enriched supplementary form of input for young children, due to containing less frequent words. However, apps do not expose children to a high proportion of questions and complex sentences, both of which are crucial for supporting child’s development of structurally rich constructions.

This paper examines the acquisition of demonstratives (e.g., that, there) from a cross-linguistic perspective. Although demonstratives are often said to play a crucial role in L1 acquisition, there is little systematic research on this topic. Using extensive corpus data of spontaneous child speech, the paper investigates the emergence and development of demonstratives in three European (English, French, Spanish) and four non-European languages (Japanese, Chinese, Hebrew, Indonesian) between age 1;0 and 6;0. The data show that, across languages, demonstratives are among the earliest and most frequent child words, but their frequency decreases with age and MLU. As children grow older, they tend to use other types of referring terms (e.g., anaphoric pronouns) and other types of spatial expressions (e.g., adpositions). Considering these results, we hypothesize that children shift from using a body-oriented strategy of deictic communication to more abstract and disembodied strategies of encoding reference and space during the preschool years.

The current study sought to investigate whether word properties can facilitate the identification of developmental language disorder (DLD) in sequential bilinguals by analyzing properties in nouns and verbs in L2 spontaneous speech as potential DLD markers. Measures of semantic (imageability, concreteness), lexical (frequency, age of acquisition) and phonological (phonological neighbourhood, word length) properties were computed for nouns and verbs produced by 15 sequential bilinguals (5;7) with DLD and 15 age-matched controls with diverse L1 backgrounds. Linear mixed modelling revealed a significant interaction of group and word category on phonological neighbourhood values but no differences across imageability, concreteness, frequency, age of acquisition, and word length measures in spontaneous speech. Outcomes suggest that group-level differences may not be apparent at the word-level, due to the heterogeneous nature of DLD and potential similarities in production during early L2 acquisition.

In this preregistered, longitudinal study of early code-switching, 34 US-born, Spanish–English bilingual children were recorded with a bilingual family member at 2;6 and 3;6, in Spanish-designated and English-designated interactions. Children’s Spanish and English expressive vocabulary and their exposure to code-switching were measured through direct assessment and caregiver report. The children code-switched most frequently at speaker changes; within-turn and within-utterance codeswitching were rare. By 3;6, switches to English were significantly more frequent than switches to Spanish. At both ages, Spanish proficiency was a negative predictor of the frequency of switching to English, but children’s degree of English dominance uniquely explained additional variance. Thus, children appear to code-switch not merely to fill gaps in their weaker language but to maximize their expressive power. Neither individual differences in exposure to code-switching nor in the interlocutors’ language proficiency were consistently related to the children’s rate of code-switching.

The study investigates whether Italian verbal inflectional morphology is associated with inhibitory control skills after controlling for receptive vocabulary and verbal working memory. A sample of Italian preschoolers aged 4;0 to 6;0 was assessed using a standardized inhibitory control task tapping two different inhibitory skills (response inhibition and interference suppression), and a morphological task requiring simple and complex inflections of verbs. The hierarchical multiple regression analyses showed that working memory and the interference suppression scores were significantly associated with complex inflections but not with simple inflections of the verbs.

The aim of the present study was to investigate the acquisition of ditransitive structures beyond production. We conducted an elicitation task (production) and a picture-sentence matching task measuring accuracy and response times (comprehension). We examined German five-to seven-year-old typically developing children and an adult control group. Our data showed quasi-perfect performance in comprehension in adults and in those children who had already mastered ditransitives productively. However, children who had not yet mastered the production of ditransitives showed comprehension abilities preceding production abilities. Unlike adults, in the comprehension task children did not react explicitly before the end of the auditory stimulus.