The next time you hear a babbling baby, awe might be a more fitting reaction than “aww.” The child’s brain is busy building a complex model of how language works from the ground up. That’s not just a matter of figuring out what words mean; an infant has to figure out what words are and pick them out from fast-running, fluid speech.

Children go from babbling, starting by about 6 months, to speaking their first words around the age of 1, to forming full sentences by their third year. This process, known as language acquisition, happens with hardly any structured adult guidance. Compare that with the slog of memorizing conjugation tables and word lists in Spanish class, or your half-finished attempts to learn French on Rosetta Stone or Duolingo. Our painstaking, deliberate attempts to learn language later in life never match the ease of those early childhood years.

That fascinates Evan Kidd, a psycholinguist at the Max Planck Institute for Psycholinguistics in the Netherlands. In a 2020 article in the Annual Review of Linguistics, Kidd and colleague Seamus Donnelly explore different theories for why some children acquire their first language faster than others. Knowable Magazine spoke with Kidd about how theories of language acquisition have evolved, what the field has learned and where it’s going next. This conversation has been edited for length and clarity.

How did you get into language acquisition and why are you passionate about it?

I studied psychology as an undergraduate and happened to come across psycholinguistics, and I found the problem of language acquisition really intriguing. Languages are these incredibly complex systems, yet children master them within their first five years. And even before that, as a 3.5- to 4-year-old, they are pretty good at engaging in conversations.

That became a problem for me to try to explain, though it’s not much of a problem if you’re a child — it’s probably a wonderful thing. But learning a language as an adult is incredibly effortful and error-prone, so it seems that children are pretty privileged language learners. And that’s the hook that made me interested in explaining the developmental processes related to language.

What are some of the different theories around language acquisition and how have they evolved?

Well, the influence of Noam Chomsky really is still all over the field. He came up with the idea that language is so complex, and children learn it so rapidly, that children must have innate knowledge about language. That’s an elegant solution to the problem, I suppose, but it’s probably not true.

And that’s because it’s hard to imagine how a child could have innate knowledge that would apply to all languages, right? That’s a point you make in the review — that any child from anywhere in the world can learn any language.

Exactly. Chomsky called this the Universal Grammar hypothesis: Children are born with what’s common to all languages across the world. But once you start looking, you start questioning whether there’s a lot that is common to all languages. And so the Universal Grammar hypothesis becomes hollow, because then you’re simply saying things like languages use little bits of sound to create syllables to create words. You’re not coming up with profound conclusions about what could be innate to all languages — such as children having innate knowledge of nouns, verbs and adjectives and how those combine to form sentences.

It sounds like you’re arguing that a child’s environment is a more important factor. What’s your view of the role of nature versus nurture in language acquisition?

I think there have to be innate processes that are involved, but I think processes is the key there. We’re born with what I like to think of as a cognitive tool kit. So very early on, as soon as you start hearing speech, you start to process it and then that starts creating knowledge.

We know, for instance, that children in the womb start to learn aspects of their language from the time the auditory cortex becomes mature. And in our own lab we found that, by around 9 months, kids can be very good at identifying words in running speech. Other people have shown this as well.

A timeline displaying various benchmarks in language development: birth to 3 month (pre-speech); 4 to 6 months (babbling stage); 7 months to 1 year (recognizing common words); 1 year is often when first words are spoken.

Babies reach a number of major language-learning milestones in their first year. During this time, children also make strides in picking out vowel sounds in their native tongue and recognizing what groups of sounds constitute words.

How do you know that infants can recognize words if they’re too young to speak?

We put electrodes on their head to record electrical activity from the scalp. And then we just play them sentences. These are really simple sentences, such as, “The eagle is in the nest.” And 300 milliseconds later, we play “eagle.” And then we compare the response from the electrodes to “eagle” to the response to another word that they haven’t heard. If we get a deviation in the brain activity in response to the new word compared to the replayed word, we infer that they recognize the replayed word.

Have you ever met a particular child whose language abilities gave you some new insight into language acquisition?

One of my colleagues and I wrote a paper years ago on the errors that her daughter was making. Scarlett was saying things like, “I’m are good girl” instead of “I am a good girl.” We analyzed it, and she made these errors only in specific contexts where she said “are” but meant to say “am.” This gave us insight into how she was analyzing the grammar. Scarlett gradually stopped making these errors, but at different rates in different sentences. This suggests that she linked the use of “am” to the specific sentence itself, rather than having a broad command of the use of the verb.

This is consistent with a learning-based approach to language acquisition, where children learn grammar in a piecemeal fashion, rather than learning the broad rules (for example, be, is, am, are) and quickly generalizing use across a range of contexts.

Maybe a more common mistake, one that you hear children make a lot, are these overgeneralization errors. Children will say things like, “He hitted me,” or “I sleepded,” and those errors indicate the kinds of analyses that children are making about how to form the past tense. Another one that’s a famous example is children saying, “Don’t giggle me” for “Don’t make me laugh.”

An image of an infant with a net of sensors on its head watching a television screen.

By using electrodes to measure baby brain activity, Evan Kidd’s lab has shown that children as young as 9 months old can pick out words from conversation.


How could studying language acquisition in children help adults or kids who lag behind their peers?

I’m not sure how much it would help an adult learning a new language, except to make them very jealous! I say that as an English speaker now living in the Netherlands. But if we can understand the mechanisms that underlie typical language development, then we might be able to understand how language acquisition is difficult for children with various types of language impairments, such as developmental language disorder, a common condition in which children struggle to use and understand language. This disorder can have knock-on effects for things like dyslexia. Understanding that typical process means that you can try and develop remediation strategies for children who are behind the normal curve.

There’s this oft-cited statistic that children in low-socioeconomic background households hear 30 million fewer words by age 4 than their more privileged peers. That finding has since been disputed, but how important do you think the number of words a child hears is?

Some children are going to do well with 5,000 words a day, some children will do better with 10,000 words a day. But my general guidance is that what is important is the quality of the interaction. A lot of our studies and those of other people have shown that meaningful social engagement with a child is the real basis of early language acquisition. Children’s tendency to engage in conversation with other people, the sort of to and fro of language, is highly predictive of their language development. That basically means sitting down and not talking at them, but with them.

What are some of the big unknowns in language acquisition, and what’s it going to take to answer them?

There are a few things that worry me. One is how many of our theories are based on a very small set of languages, yet our theories have to explain how a child could learn any language. We do a lot of research on English or other European languages, but languages can be crazily different in wonderful ways. We need to be able to understand how, say, a child in the Philippines can acquire the complex set of verb systems in Tagalog.

I also think the frontier of the field will be the integration of neuroscience because, ultimately, it’s our brains that are learning language. Understanding that process may well bring us closer to more psychologically plausible theories of how language is not only learned but represented in the brain.

Given how easily children pick up languages, are you ever surprised by how hard it is for researchers to understand this process?

It’s a crazy, silly paradox. We have all these people in linguistics departments building theories of grammar and rarely agreeing, but then a child in any culture can master that grammar very quickly. It’s exciting, humbling and also very annoying that they can do it so well and we can’t explain it!