Does bilingualism cause speech delay?

“He’s two and a half and barely talks. It’s definitely because you’re speaking to him in two languages. You’d better stop with the English and stick to Czech, otherwise he’ll never start talking.”

Have you ever heard advice like this? Perhaps it came from a well-meaning grandmother, a friend at the playground, or—worst of all—from a teacher, speech-language pathologist, or pediatrician. The fear that bilingualism causes speech delay is like a dark shadow hanging over many multilingual families.

When the monolingual boy next door is reciting nursery rhymes at age two, and your bilingual daughter at the same age operates with fifty words half in Czech and half in German, it’s easy to panic. Cue the parental guilt: “Is she going to be delayed because of us?”

Let’s look at what hard data and modern research have to say about this fear. Spoiler alert for tired parents: No, bilingualism does not cause speech delay.

The mathematics of bilingual vocabulary

Where did this myth actually come from? It originated from flawed scientific methodology. In the 1920s and 1930s, primarily in the United States, researchers began studying the language skills of bilingual children. These observations led to clear conclusions that children growing up in a multilingual environment were delayed not only in speech development but also in academic (school) performance.

The catch was that these studies tested children's language skills in only one specific language (in this case, English), and in many cases, involved children who had only relatively recently entered the new language environment. It is therefore logical that these children did not achieve the same results in English as their monolingual peers. Furthermore, many were children of immigrants who had minimal education themselves and whose socio-cultural background was significantly worse than that of the monolingual control groups. Simply put, researchers were comparing apples and pears and wondering why the pears weren't perfectly round and red.

Similar conclusions were drawn about toddlers based on vocabulary analysis. In the past, when scientists tested a bilingual child’s vocabulary, they often measured it in only one specific language (for example, Czech). And in that language, the bilingual child logically had fewer words than their monolingual peer.

But we cannot apply monolingual norms to bilingual children. Imagine a monolingual child has 100 Czech words in their vocabulary, while a bilingual child knows 60 Czech words and 50 English words. If you look only at the Czech vocabulary, they seem to be lagging behind. But if you add all the words together, you find that the bilingual child has a total vocabulary of 110 words. They are doing just as well, if not better!

If we were to measure the number of concepts (meanings—for example, cat and kočka counted as one concept, known as conceptual vocabulary), it is possible that the bilingual child might have fewer concepts than a monolingual child for a certain period. However, you should know that according to scientific research, a much more important factor for the size of school-aged children's conceptual vocabulary is their socio-cultural background. It is therefore not true that multilingual children have a lower conceptual vocabulary in each of their languages. They can even have a higher conceptual vocabulary than their monolingual peers. What matters most is the environment they grow up in—whether they spend more time talking about their experiences from a school trip or watching videos on TikTok.

All in all, bilingual children reach key milestones in speech development, such as first words or combining words, within the same age range as monolingual children. The only difference is that they divide their capacity between two languages.

When bilingualism is innocent

Now, let’s get to the most important point. What if a child really is significantly behind in speech? What if they are three years old, not forming sentences, their vocabulary is minimal in both languages, and communication is struggling?

At a moment like this, parents often make a fatal mistake: they tell themselves it’s “because of the bilingualism” and decide to wait for the child to grow out of it. Or they abandon one language to make things easier for them.

But bilingualism is innocent here. If a child has a specific language impairment (e.g., developmental speech disorder), speech delay, or another speech-language pathology issue, it would manifest even if they were growing up in a purely monolingual environment. Multilingualism itself does not cause speech disorders, nor does it make them worse.

If you have a genuine suspicion that your child's speech development is not age-appropriate (regardless of the number of languages), it is highly advisable to visit a clinical speech-language pathologist. A proper professional won't brush you off with the phrase “it’s just the bilingualism,” but will instead assess the child’s communication skills as a whole, across both of their linguistic worlds.

When to watch out

Instead of counting words, focus on these signs. Visit a specialist if the child:

• At 1 year of age: Does not respond to their name, does not understand simple instructions, or does not make eye contact.

• At 1 and a half years of age: Does not produce any intelligible words (in either language) and does not use gestures.

• At 2 years of age: Says only a few words and does not form word combinations.

• At 3 years of age: Does not combine words into simple sentences and cannot be understood even by those closest to them.

Remember that bilingualism is a gift and a wonderful workout for the brain. If speech development is hitting a roadblock, look for the cause where it actually lies—and don’t be afraid to ask a professional for advice. Dropping the second language won’t solve the problem; it will only deprive your child of a valuable part of their identity.

What's coming up next? The fear of speech delay often leads parents to hold off on multilingual upbringing. They tell themselves: “We’ll wait until they know Czech perfectly, and we’ll add English/Spanish later in kindergarten or school.” But is this the right strategy? When is actually the best time to start, and is there a moment when it’s “too late”? We’ll write about that in the next article.

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