Why Your Bilingual Kid Needs Books in Both Languages
If you're raising kids in two languages and wondering whether it's worth keeping books in both — the short answer is yes, and the research is emphatic about it.
This matters because bilingual parents hear a lot of conflicting advice. "Focus on the dominant language or they'll fall behind." "Don't mix languages at home." "Heritage language reading isn't as important." The research says the opposite of all of that.
First-Language Literacy Supports Second-Language Literacy
In 1979, linguist James Cummins proposed what he called the developmental interdependence hypothesis — the idea that competence in a second language is partially dependent on competence already developed in the first.
In plain language: reading skills in your child's heritage language don't compete with the dominant language. They feed it. A child who reads well in Finnish, for example, builds cognitive and linguistic infrastructure that transfers directly to reading in English. The two languages aren't fighting for space. They're sharing foundations.
This has been confirmed repeatedly in the decades since Cummins published it, and it remains one of the most influential frameworks in bilingual education. If your kid reads in their heritage language, they're not falling behind in their school language. They're building the skills that make the school language easier.
The research: Cummins, J. (1979). Linguistic interdependence and the educational development of bilingual children. Review of Educational Research, 49(2), 222–251. doi.org/10.3102/00346543049002222
Reading Skills Transfer Across Languages
If Cummins provided the theory, Monica Melby-Lervåg and Arne Lervåg provided the data. Their 2011 meta-analysis looked at how foundational reading skills — things like phonological awareness and decoding — transfer between a child's first and second languages.
The transfer was moderate to large. Skills built in one language genuinely benefit reading in the other.
How reading skills transfer between languages
Skills built in Language A
What reading in one language develops
- →Phonological awareness
- →Decoding ability
- →Reading comprehension strategies
- →Vocabulary depth
Transfer to Language B
What crosses over — Melby-Lervåg & Lervåg, 2011
- →Moderate-to-large transfer of foundational skills
- →Decoding abilities cross over directly
- →Comprehension strategies generalize
- →Cognitive infrastructure is shared
This means a bilingual home library creates compounding returns. Books in Finnish help your child read better in English. Books in English reinforce what they've learned in Finnish. The two shelves aren't separate — they're one system.
The research: Melby-Lervåg, M., & Lervåg, A. (2011). Cross-linguistic transfer of oral language, decoding, phonological awareness and reading comprehension. Journal of Research in Reading, 34(1), 114–135. doi.org/10.1111/j.1467-9817.2010.01477.x
Bilingualism Is Cognitively Protective
Beyond literacy, bilingualism itself carries cognitive benefits. Ellen Bialystok and colleagues at York University in Toronto have spent decades studying what happens in bilingual brains, and the pattern is consistent: lifelong bilingualism enhances executive control — the ability to focus, switch between tasks, and manage conflicting information.
These effects are most pronounced in children and older adults. In older populations, bilingualism contributes to something called cognitive reserve — essentially, the brain's resilience against age-related decline. Some studies have linked lifelong bilingualism to a delayed onset of dementia symptoms.
Reading in two languages isn't just a literacy strategy. It's a cognitive investment.
The research: Bialystok, E., Craik, F.I.M., & Luk, G. (2012). Bilingualism: Consequences for mind and brain. Trends in Cognitive Sciences, 16(4), 240–250. doi.org/10.1016/j.tics.2012.03.001
The Library Is the Infrastructure
Here's the piece that ties it all together. Researcher Lucy Tse studied what predicts whether bilingual children in the U.S. successfully maintain their heritage language into adulthood.
Two factors stood out: access to heritage-language print materials, and seeing parents read for pleasure in the heritage language. A supportive peer group was the single most influential factor in sustaining heritage-language reading — but the home library came right behind it.
In other words, the books on your shelf send a message. Heritage-language books that are visible, accessible, and part of the household say: "This language matters here. Reading in this language is something we do."
The research: Tse, L. (2001). Resisting and reversing language shift: Heritage-language resilience among U.S. native biliterates. Harvard Educational Review, 71(4), 676–708. doi.org/10.17763/haer.71.4.ku752mj536413336
“A bilingual home library isn't a luxury — it's infrastructure.”
What This Means for Your Home Library
A bilingual home library isn't a luxury — it's infrastructure. Here's what the research actually asks of you:
Keep books in both (or all) languages. Let your kids see them. Read in your strongest language without guilt — that's not neglecting the other one, it's building the foundation for it.
And if you're raising kids in a country where the dominant language will inevitably take over at school, the home library is one of the most powerful tools you have for keeping the heritage language alive. It doesn't require a tutor or a special program. Just a shelf of books your kids can reach.
Plumerie supports 45+ languages — because multilingual libraries aren't edge cases. For millions of families, they're the whole point.