Does a child who knows the word chien in French and dog in English know one word or two?

The answer seems obvious: two words.

But if we now ask, “Do they know one more concept than a child who only knows dog?”, the answer becomes far more interesting.

This is one of the questions researchers study when they look at bilingual children’s vocabulary. And it reveals something essential: measuring a bilingual child’s vocabulary in only one language gives a very incomplete picture of what they actually know.

Two languages, one brain

When a child learns French and English, they do not build two completely separate dictionaries. The two languages interact. A child may know:

chien → dog
maison → house
manger → eat
chaud → hot

In these cases, both words point to the same concept. Researchers call them translation equivalents.

That is why bilingual vocabulary is usually measured in two ways:

total vocabulary: add up the words known in both languages. Chien and dog count as two;
conceptual vocabulary: count concepts. Chien / dog = one concept, known in two languages.

This distinction is now standard in research on bilingual vocabulary development.

So is “dog” just one more word?

This is where things get subtler.

Saying that chien and dog share a concept does not mean the two pieces of knowledge are identical. The child has to learn a new sound, later a new written form, a new way of saying the word. Above all, they have to learn in which contexts that form is used.

Translation equivalents do not always overlap perfectly, either. Two words can broadly name the same thing while carrying slightly different uses or nuances.

A child knows the French word pain. When they learn bread, they do not just gain a new label. They also learn that this form is used by English speakers, in English sentences, with other word associations and other grammatical patterns.

Their knowledge of language becomes richer.

Bilingualism does increase the number of words known

On this point, research is fairly clear. When you add up the words a bilingual child knows in both languages, their total vocabulary can be considerably larger than what is measured in a single language.

This matters because a bilingual child may hold part of their vocabulary in one language and part in the other. For example, they might know:

in French: voiture, chaussure, jardin, fenêtre;
in English: train, playground, butterfly, cloud.

Test them only in French, and part of what they know vanishes from the result. Their vocabulary is not missing: it is spread across two languages. That is also why bilingual children sometimes mix their languages: they reach for the word that comes, in the language where they know it.

But more words do not mean more concepts

This is where a very appealing idea needs correcting:

“If my child knows 500 French words and 500 English words, they have a 1,000-word vocabulary, against 500 for a monolingual child.”

In number of words, yes. But that does not mean they know 1,000 concepts. If 300 of their English words are equivalents of French words they already know, they know 700 concepts, not 1,000.

That is exactly why researchers also use conceptual vocabulary, which counts a concept known in both languages only once.

So three things need to be kept apart: the number of words, the number of concepts, and how richly the child can use those words.

Why learn “dog” if the child already knows “chien”?

Because the child does not just gain a translation. They gain a second way into the same concept.

Picture a network. At the centre, the concept of a dog. One branch leads to the word chien, another to dog. The concept is largely shared, but the network grows, and the child can now reach it from two different language environments.

This becomes especially interesting when the child really uses both languages: at school, at home, with friends, in books or activities, each word comes alive in different contexts.

The two languages can help each other

So the two languages are not simply competing. Knowing a word in one language can make it easier to learn its equivalent in the other.

A study published in 2024 in the journal Developmental Science (Tan, Marchman and Frank, Stanford University) analysed four bilingual datasets, French-English and Spanish-English, covering 419 children in total. The finding: knowing a word increases the likelihood of knowing its equivalent in the other language, especially in younger children and when the two words sound alike.

In other words, vocabulary already built in one language can act as a springboard for the other. A rich bilingual experience is not about adding two word lists together: the two systems interact.

Two overlapping networks

Take a child who knows chat and cat. They have two words for one concept. But in English they may also meet cat → kitten → animal → pet, and in French chat → chaton → animal → animal domestique.

The two languages can organise some knowledge a little differently. So bilingual vocabulary does not look like two identical columns, but rather like two partly overlapping networks:

some concepts are known in both languages;
others mostly in one;
some words have a direct equivalent;
others much less so.

That is why researchers insist that a bilingual child’s two languages must be looked at together to assess their vocabulary properly. Studies of very young bilingual children also show that they draw on what they know in both languages to process the meaning of words.

Does every English word add a concept?

The wording matters.

Saying “every English word adds a new concept” would be wrong.

Saying “every new English word adds a new form and can enrich the child’s network of words, even when it matches a concept they already know in French” is far more accurate.

And it is actually more interesting. The richness of language does not depend only on the number of different concepts: it also depends on how deeply those concepts are represented and how easily they can be reached.

How many words are “duplicates”?

There is no universal figure. You sometimes read that a large majority — say 80% — of words in the second language are duplicates, or on the contrary that they are all new concepts. The data support neither claim.

The share of translation equivalents varies a great deal from child to child, depending on age, languages and exposure to each. A study of young French-English children by a team at Concordia University in Montreal (Journal of Child Language, 2017) found that these equivalents are acquired gradually, and that there are more of them when exposure to the two languages is balanced.

None of this makes bilingualism less valuable

Quite the opposite. The trap is to reduce the value of bilingualism to a counter: 500 words, 1,000 words, 2,000 words. Language does not work like a collection of Pokémon cards.

What also matters is how those words are organised, understood, linked and used. A child who knows chien and dog can:

understand a story in French and another in English;
talk with a French speaker and an English speaker;
learn an idea in one language and find it again in the other.

The second language becomes a new environment in which all of their vocabulary can work.

Vocabulary needs to be rich in each language

It would be dangerous, though, to conclude: “The more languages you add, the bigger the vocabulary becomes.”

No. Exposure matters enormously. A bilingual child develops each language according to the quantity and quality of the conversations they have in it. A child who rarely hears a language will not build a rich vocabulary in it.

Bilingualism is not about splitting the same learning in two: each language needs enough experiences of its own. And at home, the language in which parents can talk most richly remains precious — we devote an article to it: Which language should you speak to your child at home?

Why regular immersion is not an English lesson

One hour of English a week and daily exposure to English are not the same experience. In immersion, a child meets the language:

during an activity;
in a story;
in an exchange with someone;
during a game;
in an instruction;
in a project;
in a conversation.

So they do not just meet words: they meet words in context. And context is what builds the associations behind a richer knowledge of a language.

The real advantage is not “twice as many words”

The phrase would look good in an advert. But scientifically, it is too simple.

The solid finding is this: a bilingual child builds a vocabulary spread across two languages. Looking at both together gives a far more complete picture of what they know. And when they know translation equivalents, they have not only two words for one concept, but also the ability to use that concept in two different languages.

It is a different kind of richness, not simply a larger quantity.

What this changes for school

For a bilingual school, the distinction matters. Developing bilingualism is not about making children memorise more vocabulary. Children need to live both languages: hear them, understand them, use them, meet words in different situations, build associations, reuse vocabulary.

And gradually move from an understanding that often leans heavily on context to more spontaneous use. That is the whole point of active learning: the child uses the language to do something that makes sense to them.

The aim is not to fill two dictionaries in a child’s head, but to build a rich network in which the two languages complete and strengthen each other.

What parents can look out for

Rather than only asking “How many English words does my child know?”, it is worth noticing:

what they understand without translation;
the words they use spontaneously;
the expressions they recognise;
whether they can follow a story;
whether they can follow an instruction;
the situations in which they use English;
how easily they switch from one language to the other;
how their vocabulary grows in both languages.

The number of words is still useful information. But the network of knowledge behind those words matters at least as much — it is also what tracking progress beyond marks looks at.

Bilingualism does not double vocabulary: it changes its structure

This is probably the best way to sum up what we know today.

A bilingual child does not necessarily have twice as many concepts as a monolingual child, and it would be wrong to claim that every word learnt in a second language is a new concept. They can, however, develop:

more words;
knowledge spread across two languages;
translation equivalents;
word associations in two language environments;
and sometimes, use what they know in one language to learn faster in the other.

That is far subtler than “two languages = twice as many words”. But it is also far more interesting.

At Da Vinci Private School

At Da Vinci Private School, we see bilingualism not as piling up vocabulary lists, but as gradually building genuine competence in two languages. The week is split equally between French and English, with one teacher for each language.

Children meet English in concrete situations, alongside French, so that words gradually settle into contexts, experiences and conversations.

The aim is not just for a child to know the word dog after learning chien. It is for them one day to hear dog, understand it instantly, use it naturally in a sentence, and draw on that knowledge when they listen, read, play or learn in English.

Discover our bilingual pre-school and our bilingual primary school.

So one more language is not just more words. It is a new way into language, knowledge and the world.

Do bilingual children really know more words?