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Raising a Child in Two Languages: What the Research Actually Says

By Leon MoriguchiPublished May 13, 2026Updated May 18, 20266 min read日本語版あり
Audience
Parents in international marriages; parents practicing or considering bilingual parenting

TL;DR

  • ·Bilingual children know fewer words in each individual language than monolingual peers, but their conceptual vocabulary — the set of distinct meanings they can express across both languages — is statistically equivalent; measuring only one language captures only part of what they know
  • ·Development in each language tracks the input it receives: reduced vocabulary in the minority language usually means reduced exposure, not a cognitive ceiling, and that is something parents can act on
  • ·The "bilingual advantage" in executive function is a hypothesis under active debate, not a settled finding; parents choosing bilingual parenting for cultural or practical reasons are on solid ground, but those choosing it primarily for cognitive enhancement should know the evidence is weaker than widely reported

Contents

  1. Lead
  2. Vocabulary in One Language vs. Vocabulary Across Two
  3. Executive Function: The "Bilingual Advantage" and Its Complications
  4. Family Language Strategy: OPOL and ML@H
  5. Records as a Bridge Between Languages
  6. Summary
  7. References

Lead

"If we use two languages at home, won't the child end up half-competent in both?" This question confronts nearly every parent who is raising a child bilingually — whether by deliberate choice or because an international marriage simply made it the situation. The concern is not baseless. But the research suggests that what looks like a limitation, when examined carefully, is something else.

This article reviews the key studies on bilingual child development — vocabulary acquisition, executive function, and family language strategy — and attempts an honest account of what the evidence shows and what it does not.

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