Why building a love of the mother tongue matters most in the earliest years

There are two pieces on this site about adding a language to a six-year-old. This one asks the question that almost nobody asks first: what happens to the language the child already has?

It sounds like a non-question. A child born in Vietnam, living in Vietnam, schooled in Vietnam — surely Vietnamese takes care of itself.

But the ground is shifting fast. Many families now enrol children in schools where most of the day happens in English. Many parents speak English at home despite not being comfortable in it. And a growing number of city children reach ten without having finished a single Vietnamese book that was not a textbook.

The research on this is old, fairly consistent, and does not side with the common intuition.

The first language is not a rival to the second

The usual worry is that two languages compete for space: every hour of Vietnamese is an hour lost to English. If the brain were a bucket, that would be true.

It is not. Cummins (1979) proposed what he called the linguistic interdependence hypothesis: beneath the two languages sits a common underlying proficiency, and what a child builds in the first language — the ability to tell a story with a beginning and an end, to infer meaning from context, to grasp that writing encodes speech, to hold an argument in mind — transfers to the second without needing to be relearned.

A child who can read in Vietnamese does not have to relearn what reading is in English. They have to learn a script and a sound system. A child who cannot yet read in any language has to learn both at once, in the language they understand less well.

The largest evidence for this comes from Thomas and Collier (2002), who tracked the long-term academic outcomes of language-minority students in the United States over many years. Their conclusion cut against the policy of the day: the strongest predictor of academic achievement in English was not years of English instruction but years of instruction in the mother tongue. Long-term bilingual programmes outperformed programmes that transitioned quickly to English — even measured purely on English.

The limits should be stated. Those are studies of immigrant children acquiring a majority language, not Vietnamese children learning English in Vietnam. The setting differs. But the underlying mechanism — that language competence transfers, and that a firm base in one language beats a shaky base in two — does not depend on the setting.

What Lily Wong Fillmore found

In 1991, Lily Wong Fillmore published a study whose title says the finding: When learning a second language means losing the first. She surveyed more than a thousand families whose young children were learning English early and intensively.

What she found was not about test scores. It was about the breakdown inside the house.

In families where the child switched to English quickly and the parents did not, what was lost was not merely a language. What was lost was the ability to talk to each other about difficult things. Parents could still say eat your dinner, put your coat on, go to bed — those survive. What did not survive was advice, family history, the explanation of why we do it this way, the conversation you need when the child is fifteen and something is wrong.

Wong Fillmore describes families in which parents and children still loved one another but no longer shared enough language to say anything complicated. She called it the cost nobody warns you about.

Later work found the same shape. Tseng and Fuligni (2000) surveyed adolescents in East Asian, Filipino and Latin American immigrant families and found that those who spoke a different language from their parents reported weaker family cohesion and less discussion with them. Portes and Hao (2002) linked home-language loss to poorer family relationships and lower self-esteem. Oh and Fuligni (2010) found that higher heritage-language proficiency went with better parental relationships and firmer ethnic identity.

In Vietnam the risk takes a different shape but does not vanish. It is not that a child loses Vietnamese outright. It is that the child ends up with a thin Vietnamese — enough for daily talk, not enough to read a novel, not enough to follow a grandparent’s story, not enough to write down what they actually think.

Additive, or subtractive

Lambert (1975) coined a pair of terms that explains the whole business compactly: additive and subtractive bilingualism.

Additive is when the second language is added alongside the first and both grow. The child is richer.

Subtractive is when the second displaces the first, usually because the first is treated as worth less. The child is not richer; they have traded one thing for another, usually at a loss.

What decides which happens is not hours. It is attitude. Children read very early which language is valued at home. When a child works out that speaking English makes adults delighted and proud, while speaking Vietnamese is the unremarked default, that child has finished the lesson about worth — and that lesson will shape where they invest for the next ten years.

The first language is where the emotions live

This is the least-discussed part, and probably the most important.

Pavlenko (2005) and Caldwell-Harris (2015) synthesise a body of evidence showing that first and second languages do not carry the same emotional weight. Reprimands in a native language produce measurably stronger physiological responses — in skin conductance — than equivalent reprimands in a foreign one. Bilinguals routinely report that I love you is easier to say in the second language, precisely because it weighs less.

The most striking evidence comes from an unexpected direction. Costa and colleagues (2014), following Keysar and colleagues (2012), gave participants classic moral dilemmas — should you push one person to save five — in either their native or a foreign language. Same people, same dilemma, only the language of presentation differed.

The result: people make more utilitarian decisions when thinking in a foreign language. They are markedly more willing to sacrifice one to save five. The widely accepted explanation is that a foreign language creates emotional distance: it triggers fewer gut responses, so cold reasoning wins.

Read that backwards and you have the strongest thing that can be said about a mother tongue: it is the language a person’s emotions were installed in. Where the first lullaby sits. Where a word like thương carries a weight no translation can bear. Where a child learns to name what they feel — and naming a feeling is the first step in being able to regulate it.

A child with a strong second language and a thin first one is not a bilingual child. They are a child with one language to work in and none deep enough to live in.

What this language teaches that no other does

Every language forces its speakers to attend to things other languages let them ignore. Vietnamese has several such places, and they are not substitutable.

The kinship pronoun system. This is the most distinctive of them. In English a child says I and you to everyone, for life. In Vietnamese a child cannot open their mouth without locating themselves in a relationship: con, cháu, em, anh, chị, cô, chú, bác, ông, bà. To utter one sentence, the child must first answer a question — who is this person to me?

That is a lesson in social position wired directly into the grammar, and the child learns it before knowing it is being taught. No other language in their life will teach it again.

Reduplication. Lấp lánh, lung linh, lộng lẫy, lấp ló, lững thững. Vietnamese has an unusually rich stock of reduplicative words carrying shades that plain words cannot. A child raised on nursery rhymes and folk verse has that stock; a child who hears only transactional Vietnamese does not.

Tone. An ear trained early on six tonal contrasts is an ear trained early. As the Chinese roadmap on this site argues, that advantage does not transfer automatically — but the underlying concept does.

Folk verse, proverbs and old stories. This is where a child first meets ideas they will need later: fairness, obligation, patience, consequence. They meet them as stories rather than as lectures, at an age when lectures do nothing.

Two honest notes

Two things commonly cited in articles like this will not appear here, because they do not hold up.

The thirty-million-word gap. Hart and Risley (1995) reported that by four, children in professional families had heard some thirty million more words than children in poor families. The figure has been quoted everywhere for twenty-five years. But Sperry, Sperry and Miller (2019), sampling more broadly and counting speech around the child rather than only speech directed at them, could not replicate the gap. The original rested on a very small sample and carried cultural assumptions about what counts as talking to a child. So: talk to your children, but do not count.

The bilingual cognitive advantage. For years the popular story was that bilingual children have superior executive function. Paap and Greenberg (2013) argued the evidence is incoherent, and de Bruin, Treccani and Della Sala (2015) showed clear signs of publication bias — studies finding an advantage were far likelier to be published than studies finding none.

The point worth making is that the argument here needs neither of them. The reason to keep the mother tongue is not that it makes a child cleverer. It is that it keeps the child inside their own family, and keeps a language deep enough to feel and think in.

What building the love actually looks like

Not more Vietnamese lessons. The child already has those at school, and school tends to get the grammar right and the affection wrong.

Read Vietnamese aloud to them, nightly, for as many years as they will tolerate. The meta-analysis by Bus, van IJzendoorn and Pellegrini (1995) found a reliable effect of shared book reading on later literacy. And keep reading after the child can read alone — the book you read to them is always harder than the one they read themselves, which is why it is where new language enters.

Give them Vietnamese books they choose. Comics included. Books you find dull included. What needs protecting is the habit of reading Vietnamese for pleasure, not the literary quality of any given volume.

Keep dinner conversational. Snow and Beals (2006) showed that mealtime talk — especially narrative stretches and explanations — predicts later vocabulary and comprehension. Dinner is the cheapest language class a family owns.

Tell family stories. Grandparents, the village, what your own childhood was like. This is where a child meets the Vietnamese no textbook contains, and simultaneously learns who they are.

Sing lullabies and nursery rhymes. They load structures and rhythms that prose cannot, and they arrive wrapped in affection — which is to say, wrapped in the signal that this language is lovable.

Let grandparents talk to the child, a lot. They carry a layer of Vietnamese that parents have often half lost: idiom, older turns of phrase, regional accent, and a storehouse of stories. In many families they are the only ones still holding it.

The commonest mistakes

Parents speaking English to their child despite not being comfortable in it. This is the most expensive mistake and the best-intentioned. The problem is input quality: a parent with moderate English gives the child a stream that is thin in structure and thinner in feeling, while forgoing the rich Vietnamese stream they already possess. The child loses the best on offer to gain the worst.

Treating Vietnamese as automatic. We live in Vietnam, of course they’ll know Vietnamese. True of spoken Vietnamese. Not true of reading and writing it, which must be fed exactly like any other language.

Correcting the child’s regional accent. A Nghệ, Huế or Mekong accent is not a pronunciation error. Correcting it teaches a child that the way their family speaks is something to hide — which is precisely the mechanism of subtractive bilingualism Lambert described.

Praising a child’s English in front of guests and never praising their Vietnamese. The child hears it, and ranks accordingly.

How to tell it is working

One: the child picks up a Vietnamese book unprompted. Not for homework.

Two: the child can tell a long story in Vietnamese. With a beginning, a middle and an end. Coherent narrative is exactly the underlying competence Cummins described — and it transfers to every other language.

Three: the child can say a difficult thing in Vietnamese. Not only I’m hungry but I felt left out. That is the sign the language runs deep enough to carry feeling.

Four: the child understands their grandparents. Including the idioms, the regional accent, and the stories about long ago. No test substitutes for this one.

What is actually lost when a language goes

The title has a short answer: because the earliest years are the only stretch in which that love forms without effort.

It can be learned later. People do recover a mother tongue at twenty, and many have. But by then it is a subject — with books, classes and effort. What cannot be recovered is the state in which the language was never a subject: when it was simply the sound of a mother, a grandmother, a song before sleep, a story told so often it was memorised.

And if you are weighing this while worrying that your child is behind in English, here is where the other two articles and this one meet: they do not conflict. A firm Vietnamese base makes English and Chinese easier, not harder — Cummins called it common underlying proficiency, and Thomas and Collier measured it across hundreds of thousands of students.

What you are protecting in these years is not a grade in Vietnamese class. It is the ability, twenty years from now, to sit down beside your child and say something difficult — in a language you both hear the full weight of.