The present study investigated the relationship between children's code-switching behaviour and their language competency in preschool settings. Five- to six-year-old English–Mandarin bilinguals were observed across five days in their childcare centers. The types and amount of code-switched utterances produced by children in their daily conversations, along with their NDWR per minute and MLU for English and Mandarin were measured and analyzed. In addition to the observation sessions, children were administered the English PPVT, and teachers were asked to rate the children's English and Mandarin language competency six months after the observation sessions.

Despite lower levels of proficiency in expressive Mandarin as compared to expressive English, results indicated that the number of code-switched utterances was positively related to Mandarin expressive language competency (NDWR per minute), over and above home language exposure. In other words, children who code-switched more tended to produce a larger variety of Mandarin words, even though this bilingual population is less dominant in Mandarin compared to English. In addition, English language competency (both expressive and receptive) was not significantly related to the amount of code-switched utterances. This is consistent with recent studies that showed that code-switching is not a result of language incompetency (e.g., Cantone, Reference Cantone2007). Most importantly, analyses conducted with teachers’ ratings of children's language competences for Mandarin and English showed that the amount of code-switched utterances (over and beyond the current levels of expressive English) positively predicted children's English and Mandarin competency six months later, but the latter relationship is mediated by their current levels of expressive Mandarin.

These findings illustrate that, contrary to popular belief, code-switching in bilingual children does not signal linguistic incompetency. Rather, code-switching is positively associated with language competency. These findings put forward the possibility that children may be using code-switching as a platform to aid the development of their languages, especially the weaker one. Young bilingual children may not be able to express themselves fully and accurately in both of their languages yet. Code-switching thus allows them to explore and use both languages (the weaker language with the stronger one) while keeping the intended meaning intact. This is in line with the Ivy Hypothesis, which argues that children code-switch to improve their weaker language by using the grammatical structure they have acquired in their stronger language (e.g., Bernardini & Schlyter, Reference Bernardini and Schlyter2004; Gawlitzek-Maiwald & Tracy, Reference Gawlitzek-Maiwald and Tracy1996). Bernardini and Schlyter (Reference Bernardini and Schlyter2004) found that the majority of the mixed utterances produced by Swedish–Italian and Swedish–French bilingual children consisted of single words or simple phrases in their weaker language combined with more complex phrases from their stronger language. Additional support for this hypothesis comes from a study illustrating syntactic transfers of wh in-situ interrogatives and prenominal relative clauses from Cantonese to English in a Cantonese–English bilingual child during the period when his Cantonese syntactic development was significantly ahead of his English syntactic development (Yip & Matthews, Reference Yip and Matthews2000). Thus, code-switching can be used as a scaffold for the weaker language, where more complex syntactic structures from the stronger language are used in combination with lexical items and simpler syntactic structures from the weaker language.

This argument that code-switching may be helpful to young bilingual learners is not dissimilar to that proposed by researchers on translanguaging. According to Garcia and Wei (2013), translanguaging refers to the idea that bilingual speakers have one linguistic repertoire that holds concepts socially constructed from both languages. Research on translanguaging, which focused mainly on the use of translanguaging within the classroom context, have found that translanguaging facilitates deeper thinking in bilingual students, and can be used by teachers to aid bilingual speakers in subjects taught in their weaker language (e.g., Creese & Blackledge, Reference Creese and Blackledge2010; Hornberger & Link, Reference Hornberger and Link2012). Teachers can leverage on the bilingual students’ stronger language to provide the students with a platform to participate, elaborate on their thought processes and raise questions. This is similar to our proposal that code-switching allows young bilingual children to leverage on their stronger language in daily communications and dual language learning contexts. It provides young bilingual children with an alternative tool to express their thoughts, feelings, and ideas. For these young learners of two languages, code-switching can be used as a form of communicative support and as a way to expand these emergent bilinguals’ understanding and linguistic competency.

This follows that, despite mounting concerns about the potential negative impacts of parental code-switching on an infant's language development (Byers-Heinlein, Reference Byers-Heinlein2013), children's code-switching behaviour itself does not necessarily indicate linguistic incompetency, nor will it negatively affect children's language development. Research has shown that language input from parents and teachers are critical to children's language development, in terms of vocabulary size (Bowers & Vasilyeva, Reference Bowers and Vasilyeva2011; Hammer, Davison, Lawrence & Miccio, Reference Hammer, Davison, Lawrence and Miccio2009; Hoff, Reference Hoff2006; Hoff, Core, Place, Rumiche, Señor & Parra, Reference Hoff, Core, Place, Rumiche, Señor and Parra2012; Hurtado, Marchman & Fernald, Reference Hurtado, Marchman and Fernald2008), grammatical development (Blom, Reference Blom2010; Bohman, Bedore, Peña, Mendez-Perez & Gillam, Reference Bohman, Bedore, Peña, Mendez-Perez and Gillam2010) and comprehension skills (Dickinson & Porche, Reference Dickinson and Porche2011; Huttenlocher, Vasilyeva, Cymerman & Levine, Reference Huttenlocher, Vasilyeva, Cymerman and Levine2002). However, children's usage of the language(s) is also an important factor in language development. Studies, for example, found that increasing the use of a second language is associated with improved proficiency in that language (Freed, Segalowitz & Dewey, Reference Freed, Segalowitz and Dewey2004; Martinsen, Baker, Bown & Johnson, Reference Martinsen, Baker, Bown and Johnson2011). Thus, while it remains important that the language input children receive should accurately reflect the linguistic characteristics of the target languages, bilingual children's regular use of both languages should be highly encouraged too, even if it involves switching between the two languages. Our results elucidate that the act of code-switching by children may have provided them with a way to engage both their languages more frequently, particularly the weaker language. To put it simply, code-switching in a multilingual environment may present bilingual children with opportunities to use both their languages in ways that a pure language environment alone would not be able to provide them with. This, in turn, has a positive outcome on language development with improved proficiency.

Language and literacy development guidelines from the local curriculum framework were used in this study to create a teacher's rating scale to measure children's language competencies. This scale is relevant to the local context and it captures the major receptive and expressive language proficiency requirements as expected of a 6-year-old. Similar uses of teachers’ assessment of children's language skills are often employed in both research and educational settings (e.g., August, Shanahan & Escamilla, Reference August, Shanahan and Escamilla2009; Sundberg & Partington, Reference Sundberg and Partington1998). Nevertheless, this scale has some limitations. First, the rating scale is not a standardized measurement of language competency. Second, teachers’ ratings could be subjective and were based on retrospective reporting. Thus, measures of children's language competency based on teachers’ ratings may not reflect children's full extent of their language ability. However, finding an assessment tool that has been validated for use in multiple languages, in this case English and Mandarin, is a challenge. Standardized measures of language ability such as the Peabody Picture Vocabulary Test (Dunn & Dunn, Reference Dunn and Dunn2007), subtests of Woodcock-Johnson Tests of Achievement (Woodcock, McGrew & Mather, Reference Woodcock, McGrew and Mather2001), and Clinical Evaluation of Language Fundamental Preschool (CELF; Wiig, Secord & Semel, Reference Wiig, Secord and Semel2004) are available in English but not in Mandarin. Even if they are available in Mandarin, there remains a possibility that the materials of these language assessment tools may not be appropriate for the local children and thus may not accurately reflect their true language ability (Brebner, Rickard-Liow & McCormack, Reference Brebner, Rickard-Liow, McCormack and Lind2000; Carter, Lees, Murira, Gona, Neville & Newton, Reference Carter, Lees, Murira, Gona, Neville and Newton2005). Furthermore, researchers have not agreed on the assessment tools that are best for assessing the language ability of bilingual children (e.g., Bedore & Pena, Reference Bedore and Pena2008; Gutiérrez-Clellen, Reference Gutiérrez-Clellen2002; Saenz & Huer, Reference Saenz and Huer2003), and published self-report tools are not appropriate for use by children (e.g., LEAP-Q; Marian, Blumenfeld & Kaushanskaya, Reference Marian, Blumenfeld and Kaushanskaya2007). Taking these challenges into consideration, we believe that our teacher's rating scale of language competency is a relatively appropriate language measure in this context, albeit with limitations. Future studies should consider replicating the present study with a different population of children where standardized language assessment tools in both languages are available and well tested.

It is worth noting that a bilingual child's exposure to his or her two languages may vary a lot during six months due to external factors, such as changed input at home or at school, or internal factors, such as when the child identifies himself or herself more with one language's culture or when the child refuses to speak in one language, Such factors may affect a child's code-switching behavior as well. While we agree that bilingualism could vary substantially in an individual within six months, we believe that the bilingual status of the children in our study is relatively stable throughout the study. The children in our study had been in the same preschool for the past 6 months with no known significant changes to their family or parental background, or to their preschool routines and teachers. Thus, it is unlikely that these children experienced a significant change in their language environments. Nevertheless, future studies should consider collecting information about possible external and internal factors affecting children's language balance during the period of study, such as through additional parental and teachers’ surveys.

We have earlier raised the possibility that the use of MLU to assess children's language complexity in this study may be limited due to a ceiling in age and morpheme count. Compounding this issue is the challenge of computing MLU as an indicator of language competence across two typologically different languages like English and Mandarin. The difficulty in using MLU to assess children's English language complexity is particularly noted in the case of Singapore Colloquial English (SCE). Two common features of SCE are the absence of subject, for example, (e.g., “(That car) very expensive, you know”) and the deletion of the copula ‘be’ (e.g., “that boat ø very short one”) (Leimgruber, Reference Leimgruber2011). Thus, the use of English MLU to assess children's English language complexity in this context may be limited. Future investigations examining whether code-switching affects children's syntactic development of their two languages could employ more specific measures other than MLU, e.g., an elicited imitation task to assess sentence formation (Lust, Chien & Flynn, Reference Lust, Chien, Flynn and Lust1987; Lust, Flynn & Foley, Reference Lust, Flynn, Foley, McDaniel, McKee and Cairns1996), the truth-value judgment task (Lust & Blume, Reference Lust and Blume2016), or an adaptation of adult syntactic complexity tasks that use not just length measures but also ratio measures such as sub-clauses/sentence (e.g., Bulté & Housen, Reference Bulté and Housen2012; Norris & Ortega, Reference Norris and Ortega2009).

Children code-switch for various reasons. Future studies can tap on multiple age groups or conduct a longitudinal study to further investigate the developmental shifts in the use of code-switched utterances. For example, studies that have found that children engaged in code-switching behavior in order to fill their lexical gaps considered younger children typically around or before the age of 3 years (Cantone, Reference Cantone2007). Studies that have found that children make use of code-switches for sociocultural or pragmatic purposes have focused mainly on older children (Chung, Reference Chung2006; Reyes, Reference Reyes2004; Vu et al., Reference Vu, Bailey and Howes2010). It is logical that very young bilingual children first start off with a limited lexicon, and would, therefore, code-switch when they do not have the translation equivalent of a particular concept (De Houwer, Reference De Houwer, Kroll and de Groot2005; Cantone, Reference Cantone2007). After acquiring a sizeable lexicon in both languages and learning their sociolinguistic rules and cultural practices, older bilingual children would begin to code-switch depending on the social demands of the conversation (Nicoladis & Genesee, Reference Nicoladis and Genesee1997). The children in our study were 5 to 6 years of age and were likely aware, to some degree, of the sociolinguistic rules used in their community. Including multiple age groups in future studies would shed light on the developmental trends in bilingual children's code-switching behavior.

In conclusion, the goal of the present study was to further elucidate the relationship between bilingual children's code-switching behavior in their larger language environment and their linguistic competency. Debate about whether bilingual children's code-switching behavior reflects their linguistic incompetency is ongoing. The present study is the first attempt to investigate this relationship using a quantitative approach. Findings from the present study provide counter-evidence against the linguistic incompetency hypothesis – there was no indication that bilingual children's code-switching behavior was a result of their linguistic incompetency. Instead, bilingual children's code-switching behavior suggests greater language competency. The findings from the present study provide an alternative perspective on the linguistic incompetency hypothesis – that, far from being debilitating, code-switching plays an important and positive role in language development of bilingual children, especially in the context of the larger language environment.