Growing up in a family where more than one language is spoken is an increasingly common experience in the UK and around the world. Yet many parents of bilingual children receive anxiety-inducing advice suggesting that exposure to two languages might confuse their child or cause speech delay. The evidence does not support this concern. Bilingual development is different from monolingual development in some ways, but it is not deficient. Understanding how language develops across two languages makes it easier to assess whether a bilingual child is developing as expected.
Healthbooq (healthbooq.com/apps/healthbooq-kids) covers language development in bilingual and multilingual families.
What the Research Shows
The idea that bilingualism confuses children or causes speech delay has been extensively tested and repeatedly found to be wrong. Ellen Bialystok at York University in Toronto, whose research on bilingualism and cognitive development is among the most cited in the field, has documented that bilingual children acquire language on a similar timeline to monolingual children when total vocabulary (across both languages) is considered. The apparent gap in any single language's vocabulary at a given age is a consequence of the fact that a bilingual child's word learning is distributed across two languages rather than concentrated in one.
Virginia Yip at University College London, who has studied simultaneous bilingual acquisition extensively, documents that children exposed to two languages from birth or very early in life acquire both, and that the structure of each language develops appropriately for its own grammatical rules – bilingual children do not blend grammatical structures confusingly. They develop two separate linguistic systems simultaneously.
The Total Language Concept
When assessing a bilingual child's vocabulary, the relevant measure is conceptual vocabulary – the total number of distinct concepts the child has a word for in either language. A child who knows "dog" in English and "perro" in Spanish has one conceptual category, not two separate words. Research-based vocabulary assessments for bilingual children attempt to measure total conceptual vocabulary precisely because single-language assessments (like the MCDI – MacArthur Communicative Development Inventory) will underestimate a bilingual child's language development if only one language is assessed.
If a bilingual child is being assessed for potential language delay, the assessment should include information about both languages. A child who has limited vocabulary in Language A but normal vocabulary in Language B may simply be less exposed to Language A; a child with limited vocabulary in both languages may genuinely have language delay.
Code-Switching Is Normal
Code-switching – mixing the two languages within a conversation or even within a sentence – is entirely normal in bilingual children and adults. It reflects competence in both languages, not confusion. Research by François Grosjean at the University of Neuchâtel, one of the foundational researchers in bilingualism, established that code-switching is a skilled communicative strategy, not a sign of inadequacy. Young bilingual children code-switch especially when they don't yet have a word in the language they are currently using, borrowing from the other – an effective and intelligent strategy.
Parents sometimes worry that code-switching means their child is "mixing up" the languages. It doesn't; children who code-switch have solid foundations in both languages and are using all their linguistic resources.
Supporting Language Development in Bilingual Families
Input volume matters. The critical factor for good bilingual development is sufficient input in both languages. A language that is heard very rarely will not develop well. One common concern is the "minority language" – the language spoken at home by parents but not used at school or in the community. This language is at risk of being underdeveloped unless deliberate effort is made to maintain rich input (books, conversation, family, media in the minority language).
Consistent language strategies. Many bilingual families use "one parent, one language" (OPOL) – each parent speaks their language consistently. This is a widely recommended approach and has good evidence of supporting both languages, but it is not the only effective approach. The most important factor is that each language receives enough input.
Don't abandon either language. Some parents, worried about language delay or school readiness, are advised by well-meaning but uninformed sources to "just speak English" at home. Abandoning the home language does not accelerate English development; it reduces total language input and removes a resource that supports cognitive and cultural development.
Key Takeaways
Children raised in bilingual or multilingual homes develop language on a similar overall timeline to monolingual children, but the distribution of words between languages means the size of each language's vocabulary may appear smaller at any given point. This is normal and not a sign of delay. Bilingual children often mix languages (code-switching), which is a normal feature of bilingual language development and not a sign of confusion. The most important factors for good bilingual language development are rich language input in both languages and responsive, language-rich interaction. Bilingualism does not cause speech delay.
