“Hearing two languages will confuse my baby and delay their speech.”
The confusion myth is one of the clearest cases in this library, and the developmental science field is unusually united here. Babies raised hearing two languages start babbling at about the same age as other babies, and their total vocabulary across both languages is on par with children learning one. In any single language they may know fewer words or use simpler sentences for a while, which is expected and not a delay. That is why how you count matters: their vocabulary lives across both languages, so measuring one language against monolingual norms undercounts them. Code-mixing, using both languages in one sentence, is not confusion. Bilingual adults mix in systematic, grammatical ways, and bilingual children's mixing follows rules much like theirs. Parents mixing languages in their own speech also appears, on current evidence, to be mostly unrelated to children's vocabulary, apart from a small, uncertain link with understanding words in the community language. Clinical reviews describe two practical harms of this myth. Some families are advised to drop a home language to help speech, but that advice is not supported by the science, may do less for the child's other language than hoped, and gives up a language that research links to stronger family relationships. Clinicians can also misread a bilingual child in either direction: sometimes mistaking normal bilingual development for a delay, and sometimes missing a real delay because it gets put down to bilingualism. The red lines for a language conversation with your pediatrician are the same for bilingual children as for anyone, counted across all languages.
Sources
- Byers-Heinlein K, Lew-Williams C. Bilingualism in the early years: what the science says. LEARNing Landscapes. 2013;7(1):95-112. (Research review: no confusion; code-mixing follows rules; total vocabulary)
- Hoff E, Core C. What clinicians need to know about bilingual development. Seminars in Speech and Language. 2015;36(2):89-99. (Clinical guidance: count total vocabulary; do not discourage the home language)
- Oller DK, Eilers RE, Urbano R, Cobo-Lewis AB. Development of precursors to speech in infants exposed to two languages. Journal of Child Language. 1997;24(2):407-425. (Longitudinal study of 73 infants: babbling begins at similar ages in bilingual and monolingual infants)
- Hoff E, Core C, Place S, et al. Dual language exposure and early bilingual development. Journal of Child Language. 2012;39(1):1-27. (Behind in single-language comparisons, comparable in total vocabulary)
- Verhoeven E, Oudgenoeg-Paz O, van Witteloostuijn M, et al. The relation between parental language mixing and children's vocabulary outcomes: an evidence synthesis. Child Development. 2026. doi:10.1093/chidev/aacag102. (Evidence synthesis: parental mixing mostly unrelated to vocabulary)
How claims get graded
Every claim is graded on two independent axes. Evidence strength asks: how good is the research behind the claim as parents actually state it? Randomized trials and meta-analyses outrank single observational studies, which outrank expert opinion. Consensus asks a different question: where does the field actually sit? A claim can rest on limited evidence while researchers still broadly agree, and both facts belong on the label.
Every empirical statement in a summary is tied to the sources listed on its page, and an automated check enforces the honesty rules: claims below strong evidence cannot use proof-language, and claims with limited or insufficient evidence must say so in plain words. The check runs on every update to this site; a summary that overclaims cannot ship.
The grades are educational summaries of published research, not medical advice, and they can change; each page shows when it was last reviewed.
This page summarizes published research for educational purposes. It is not medical advice, and it is not a recommendation for or against any practice for your specific child; those calls belong with you and your child's clinician.