People have been talking to each other for at least 100,000 years, as best we can tell. And in all that time, there has been only one thing in the world that could learn a human language to perfect fluency: a human child.
Now there are two.
Four short years after the release of ChatGPT, many of us now take it for granted that we can converse naturally with our phones or computers. LLMs like Claude, DeepSeek, and OpenAI’s GPT models are fluent and flexible enough to masquerade convincingly as humans. But peek behind the computational curtain, and there’s a catch: Teaching a computer to use human language still requires an inhuman amount of data. An LLM can easily churn through a hundred thousand times more words than a person will experience in the process of mastering their mother tongue—and way more than children might hear by their first birthday, when they typically start to grab hold of language.
“The progress recently has been amazing,” Michael C. Frank, a cognitive scientist at Stanford University, says of LLMs. “But we still have to burn down a forest and scrape the entire sum of all human knowledge to re-create this milestone that happens in our living rooms over the course of a year.”
This yawning divide between children and machines is called the data efficiency gap. And it raises a tantalizing question for cognitive scientists and a challenge for the architects of AI models: How is it that kids can still outperform the most linguistically sophisticated machines ever built?
Finding answers has stakes for both AI research and cognitive science. For the past decade, language models have mostly gotten better by getting bigger. Meta’s open-weight LLM Llama 3.1, released two years ago, chewed through 15 trillion tokens (word-like chunks of language) in pretraining—the main step of training a model that happens before it is fine-tuned for a specific task, like being a chatbot. Frontier models could be pretraining on 10 times more data, says Ethan Gotlieb Wilcox, a cognitive scientist and linguist at Georgetown University. But there’s only so much internet to train on, and eventually—perhaps as early as the 2030s—the well of easily available data could run dry.
Kids show that it could be possible to learn more with less. Far less. A preteen raised in a linguistically rich home may have heard something in the vicinity of 100 million words. Add literacy to the mix and you can boost that word count to maybe 300 million words by age 20.
The difference in scale is something that can only really be gestured at in analogy. “Claude has seen the amount of language that an entire city will experience in one generation,” says Wilcox. If you were to print out on paper all the words used to train a modern LLM, you could make a stack that would reach past the International Space Station. The human preteen’s 100 million words, meanwhile, would stack up just 20 meters. And we can make do with far less than that.
By reverse-engineering the way kids learn, scientists hope to be able to create more data-efficient AI models, which could be useful for everything from training AI effectively on video to creating chatbots that serve minority language communities. Testing hypotheses about human learning in machine models could also settle enduring questions about language and children’s developing minds. Are we born with a language instinct, or would it be possible, even in principle, for a child to learn language purely from experience? Is the way we process language a quirk of our biology, or might at least some of it reflect universal constraints on how languages can be used and learned?
The essential elements
Most of us realize language is hard only when we try to learn a new one after childhood. The past perfect tense, rolled rs and nasal vowels, the genitive case, phrasal verbs, grammatically masculine tables and feminine spoons—many are the instruments of linguistic torment for the adult language learner. It’s typically effortless to learn our mother tongues, however. Toddlers usually start producing grammatically correct sentences after hearing something like 10 million words, or 30 million on the high end.
“It’s just totally miraculous,” says Frank. “If you train GPT-2 on 30 million words, you get a nonsense generator; you don’t get a kid.”
Exactly how babies pull this off is a mystery. Researchers know a lot about what kids learn and how they use language at different stages in development, but there’s still a lot we don’t know. Perhaps the most enduring question is why babies can learn language at all. The syntax of human language—the rules for combining words into sentences—includes recursive, nested structures that allow us to express virtually infinite ideas with a finite lexicon of words and pieces of words. This seems like something that should be a problem for babies. They only splash about in the shallows of a fathomless ocean of language. And yet, somehow, that’s enough. From a drop, they infer the depths.
One solution, put forward in the 1950s by the MIT linguist Noam Chomsky, is that babies are born with hardwired knowledge of grammar. Chomsky was reacting to a rival view, championed by the psychologist B.F. Skinner, that language acquisition is entirely environmental. Skinner thought language was learned through conditioning and reinforcement, the way a dog figures out how to sit or shake for treats. Chomsky countered by citing the “poverty of the stimulus”—the idea that language, especially syntax, is too complex and children’s exposure to it too “impoverished” for them to learn entirely from experience. “His signature argument was, essentially, that language cannot be learned on the basis purely of statistics,” says Richard Futrell, a linguist and cognitive scientist at the University of California, Irvine. Instead, Chomsky posited that language is based on a set of logical rules and argued that children needed innate knowledge of those rules to deduce the grammar of their language from scraps of speech.
“It’s just totally miraculous … If you train GPT-2 on 30 million words, you get a nonsense generator; you don’t get a kid.”
Michael C. Frank, cognitive scientist, Stanford University
The Chomskyan view of language dominated linguistics in the US for decades under the moniker of generative grammar. And it was a major influence on computer science in the 1950s and ’60s, when AI was enjoying its first boom time and the lines between linguistics and natural-language processing dissolved in a flood of military funding; the Pentagon wanted computers that could understand English and translate Russian.
Despite early successes of simple neural networks, which learn to recognize and reproduce statistical patterns, AI researchers in the United States largely adopted a rule-based framework influenced by Chomsky’s theories. They tried to teach language to computers by explicitly coding the rules into programs—think less immersion experience, more grammar class. This approach, part of a broader trend called symbolic AI, prevailed for decades. It also largely failed to produce models actually capable of handling human language at scale. Interest in natural-language processing chilled in the “AI winter” that began in the 1970s.
In the aftermath, neural networks started to make a comeback. But it wasn’t until the 2010s, when computer hardware was getting cheap and capable and the internet was getting big, that their performance began turning heads. By 2018 and 2019, the models BERT and GPT-2, which were built on a new architecture—the transformer—and trained on billions of tokens, made it clear to insiders that learning from a massive glut of data could work for language. In 2022, with the breakout success of OpenAI’s chatbot ChatGPT, it was clear to everyone.
LLMs are not brains. What they are is powerful statistical learners—naïve pattern-learning machines without any of the evolved biological quirks folded into the human cortex. In other words, they are exactly the kind of thing a generative linguist two decades ago would have thought could not learn language. And yet here they were, writing believable sonnets and passing grammar tests.
“No matter how skeptical you are about AI, the thing that everyone has been really impressed with is: These things learn syntax,” says Alison Gopnik, a developmental psychologist at the University of California, Berkeley. “I didn’t think that was going to turn out to be true. And I think most people didn’t think that you could just look at the statistics of a large sample of language and figure out grammar.”
But what about learning from a small sample of language—a child-size one, say? Is it possible to build a baby-scale model that’s anything more than a nonsense generator?
Baby talk
Alex Warstadt, a linguist and data scientist at the University of California, San Diego, remembers the years around the release of BERT and GPT-2 as a heady time. Back in 2019, he was still a PhD student in linguistics at New York University, watching his field change before his eyes. The mere fact that language models could learn English by churning through text was a challenge to prevailing Chomskyan ideas. But many linguists remained skeptical that LLMs could tell us anything about how humans acquire language.
“I always got pushback on one issue in particular. And that was the size of the data sets of the model,” says Warstadt. “There was never a time when people were training language models at human scale where we were impressed by them.”
But Warstadt saw promise in LLMs: A scientific model doesn’t have to be perfect to be informative, and LLMs were clearly powerful simulations of human language use. By building hypotheses about how children learn into models and measuring their performance—how close they came to closing the data gap—might scientists be able to put their ideas to the test? In August 2022, Warstadt posted a Twitter thread laying out an argument that neural networks could be useful models of language acquisition. After some back-and-forth in the comments with AI researcher Leshem Choshen, Warstadt floated the idea for what would become BabyLM, an annual competition organized by Warstadt, Choshen, and several other researchers to train models on small data sets.
That was four years ago. Since then, BabyLM has added workshops and inspired spin-offs including a competition for baby models trained on Chinese. The main event challenges researchers to train language models on a “developmentally plausible” corpus of just 100 million words (for the toddler-scale track, 10 million) drawn from storybooks, dialogue, movie subtitles, Simple English Wikipedia, normal Wikipedia, and actual transcripts of speech directed at children. The models are evaluated on the kinds of grammar benchmarks that psycholinguists use with humans, says Georgetown’s Wilcox, one of the organizers.
One kind of task involves presenting test subjects—human or machine—with sentences and looking for indications of confusion or surprise at ungrammatical features. For instance, a test might compare the sentences The keys to the cabinet are on the table and The keys to the cabinet is on the table. “When humans see ‘is,’ they’re like: What? That’s not supposed to be ‘is,’ ” says Wilcox. For a human, that surprise might be measured by tracking eye movements. For language models, researchers use a measure called surprisal, which assesses how unlikely the model predicts a sentence or part of a sentence to be.
The competition has already challenged some assumptions, such as the effectiveness of curriculum learning. Curriculum learning starts with simple training data and works up to more complex inputs—a bit like starting with baby talk and getting more sophisticated over time. And it was by far the most popular approach taken in the first round of BabyLM, says Warstadt. But it didn’t work as well as expected.
“The appeal is just kind of hard to resist, you know. [Curriculum learning] seems to really line up with ways that we believe humans are learning,” says Aaron Mueller, a computer scientist at Boston University and one of the BabyLM organizers. “But it seems like these transformers don’t really need to have their data ordered in such a way to learn effectively.”
Perhaps a touch ironically, the best BabyLM models aren’t inspired by babies at all. The 2024 champ, GPT-BERT, is a transformer trained partly to predict the next token in a sequence, like modern LLMs, and partly to act like BERT, a “masked language model” that fills in the blanks in sequences of tokens Mad Libs style. Impressively, when GPT-BERT was pretrained on about 100 million words, it was able to beat the performance of Meta’s Llama 2 70B—an LLM pretrained roughly 15,000 times that amount—on one of the BabyLM benchmarks.
Still, BabyLM models are not on the same level as LLMs. Many can’t produce text at all, and even GPT-BERT would seem clunky next to a modern commercial model. Ultimately, while they are “baby”-size, the way these models learn isn’t very baby-like. Kids are not disembodied computer programs whose only “experience” of the world comes through written text. They take in the world via their senses—especially vision and hearing. To close the data gap, some researchers think, machines will need to start learning through the eyes and ears of children.
Taking it all in
When Michael Frank started his lab at Stanford about 15 years ago, scientists didn’t really know how babies experience the world. Developmental psychologists were just beginning to glimpse babies’ lives through headcams.
“The insights that came out from that early research were that kids’ experience looks really radically different than we thought,” says Frank. “It’s much more focused: They’ve got these little short arms, so the objects are, like, right in front of them. And they live in a forest of knees.”
Frank was excited to use headcam footage to train machine-learning models to test hypotheses about how kids learn language, but he needed more data. So he and four colleagues recruited three babies—all the children of psychologist mothers who knew what they were getting themselves into—to don headcams for science. The project, called SAYCam, recorded two hours a week of each child’s life between six months and two and a half years of age.
“[The families] were willing to release that video, and that’s critical,” says Frank. “So we released it, and people started training models on it.”
One of those people was Brenden Lake, a cognitive scientist and AI researcher at Princeton. In 2024, when he was working out of New York University, he and his colleagues presented a model trained on 61 hours of raw SAYCam data that learned to identify objects and associate them with words. Many theories in developmental psychology propose that children need some biases to help them pick out particular parts of their raw sensory experience and associate them with bits of language. For instance, it’s thought babies assume that a new word like “shoe” refers to a whole object rather than a part of it (like a shoelace), says Lake. But the model Lake’s team built was able to learn to identify objects in the video footage and associate them with words without any such biases. “It turns out you can get a real start on language learning using a lot less than what a number of theories suggested,” says Lake. Still, he adds, “we don’t get a two-year-old out of [training] when we’re done.”
But perhaps it’s not surprising that such models can’t replicate childlike capabilities by working with a few dozen hours of footage cobbled together from short snapshots over several years of a child’s life. It could be that the shortfalls just indicate a lack of realistic data. After all, babies can’t wear a headcam 24-7; efforts like SAYCam and its successor, BabyView, record at best a few hours a week. So researchers have the choice between working with a tiny slice of the life of a single child or with larger data sets of footage pooled from many kids. Either way, a model’s training data is still a far cry from the lived experience of a child.
That could be changing. Uri Hasson, a neuroscientist and psychologist at Princeton, spent the last five years on a project to record the first 1,000 days of 17 children’s lives. The participating families wired every living area in their homes (except bedrooms and bathrooms) with cameras and microphones and recorded 12 hours a day, almost every day. The resulting data set, described for the first time in a recent preprint, is of a scale that would have simply been impossible to work with absent new AI tools for transcription and video analysis, says Hasson. “For the first time, we have the input,” he says. “It’s really only the beginning.”
Missing ingredients
So far, training models on video has proved difficult. While text-based models emerge fully fluent (after ingesting huge training data sets), multimodal models trained on video from kids are far from that. Lake’s model, for instance, learned simple words, like “ball” and “cat.” Attempts to supplement text with visual data haven’t worked for BabyLM participants, says Warstadt. Gopnik thinks the issue could be that kids do not simply sit and watch the world go by. “Children are actively exploring, which means that they’re actively choosing their own data,” she says. “Kids are constantly experimenting.” Maybe that’s the missing ingredient.
Research by Gopnik’s group—including studies of grade schoolers exploring a Minecraft-inspired game—shows that what looks like child’s play is in fact an effective way to learn cause and effect. Kids seek out experiences and take actions that maximize their “empowerment,” or the ability to make a predictable impact on the world.
Unlike models, children are aware of what they don’t know and have a drive to fill their knowledge gaps, says Elizabeth Bonawitz, a developmental cognitive scientist at Harvard. And children’s social lives also help them learn, she says. Her research has shown that children interpret information differently when they know an adult is trying to teach them something. “Children are not only reasoning about the evidence they’re being told,” says Bonawitz. “They’re reasoning about the teacher, about the teacher’s knowledge, and about why the teacher is telling [them] this particular information.”
That’s very different from how models learn: passively and in isolation. Perhaps if models were built to seek out information to fill in their own blind spots, experiment with language and observe how other language users react to their babbling, and reason about some kind of simulated social world, they’d learn better. Last year’s BabyLM actually opened the competition to models that could learn by interacting with other models. But the social models didn’t outperform standard ones.
Of the leading industry labs, Meta seems the most interested in taking inspiration from kids—specifically for training models from video. Two Meta researchers were involved in BabyLM’s multimodal branch, and Meta scientists—together with academic researchers, including Frank—recently announced a benchmark and challenge for training models on baby headcam footage. Frank also says a stealth-mode AI startup called Flapping Airplanes has taken interest in his research. Neither Meta, Google DeepMind, OpenAI, nor Flapping Airplanes agreed to an interview.
For now, frontier labs aren’t exactly racing to borrow tricks from children, says Gopnik. She thinks it’ll be the next generation of AI—whatever replaces the transformer—that will take lessons from developmental psychology.
Perhaps the most enticing reason to close the data gap is that it could help us understand ourselves.
In general, the machine-learning community is less interested in mimicking the brain than in just building something that works, says Mueller. But he thinks awareness of—and interest in—the data efficiency gap is growing. An example is the NanoGPT Slowrun benchmark, launched by Q Labs in March 2026. “They have very similar goals to BabyLM,” says Mueller. “But they’ve dropped the motivation from human language learning and really just focused on the data efficiency angle.”
One reason Warstadt wants to close the data gap is to democratize AI so that universities and others without the resources to hyperscale can train good models and stay relevant in AI research. David Samuel, a machine-learning researcher at the University of Oslo and one of GPT-BERT’s architects, has a more personal reason to work on this problem. He’s Czech and works in Norway, and there’s a lot less data in Czech and Norwegian available for training LLMs than there is in English. Minority languages like Sami might have just tens of millions of tokens available, says Samuel—about the scale of a toddler’s exposure. “The question was,” he says, “how can we develop language models that are just as capable as the English ones for small languages?”
But perhaps the most enticing reason to close the data gap is that it could help us understand ourselves.
Bonawitz says she was initially skeptical that large language models could reveal anything about cognition. LLMs and brains are, after all, very different. Brains are embodied. Our neurons are not tidy lines of code but living cells. And our brains grow and change as we learn and age—LLMs pretrain once and never again. But as different as the two systems are, says Bonawitz, “I’m sort of revising my beliefs.” She’s been won over by the idea of studying models the way comparative psychologists might study animal minds to illuminate our own.
Researchers like Warstadt, Frank, Wilcox, Lake, and Hasson are already using language models as a kind of linguistic lab rat, an imperfect but informative stand-in for a real human language user—especially for questions that are more about learning and language and information processing than anything specific to our brains or biology. When models can do things with language we thought were impossible, it challenges old assumptions. And researchers can build hypotheses about language learning into models—say, by simulating different degrees of bilingualism or depriving models of exposure to certain grammatical forms—and test those hypotheses in a way that would be impossible to do with real children. Futrell compares the situation to teaching language to an alien and then opening up its brain to see what happened.
While other animals communicate, only humans converse. Now there’s something neither animal nor human that can talk, too. LLMs open up the possibility for comparative studies, even if models and minds are vastly different. “For the last 100,000 years or however long human language has existed, humans have been the only entities in the universe that use language. Now there’s this other linguistic entity,” says Warstadt. “Finally we have a model; not in the sense of a language model, but in the sense of a model organism.”
Elise Cutts is a science writer based in Austria.
