Language Acquisition: How Children Build a Language

A two-year-old says "I runned to the door." She has never heard an adult say "runned". Nobody told her the rule for the past tense. She worked it out, from the speech around her, and then applied it a little too widely. Within two years she will say things like "I wonder whether the dog that I saw is still outside", a sentence with a question inside a clause inside a clause, which she has also never been taught.

Every typical child, in every country, in every language and in sign, does this. No other animal does, and no adult learner finds it easy to match. The question of how is one of the oldest quarrels in the cognitive sciences, and it is more than a quarrel about children: it is about what a mind starts with and what it takes from the world. This piece goes past the basic timeline in How Children Learn Language (26/06) to the experiments and the arguments.

Not clinical advice. This describes research on typical development. Children vary a great deal. If you are concerned about a child's hearing or speech, a paediatrician or speech-language pathologist is the right person to ask.

The first year: statistics before words

Speech does not come with spaces. In a continuous stream of sound, a baby has to find where one word ends and the next begins, in a language she does not yet know. In 1996 Jenny Saffran, Richard Aslin and Elissa Newport tested a clever possibility. They played 8-month-old babies about two minutes of a made-up language, a monotone stream built from four nonsense words (tupiro, golabu, bidaku, padoti) in random order, with no pauses and no stress. Within a word, one syllable always follows another (tu is always followed by pi); across a word boundary, the next syllable is one of three possibilities. Afterwards the babies listened longer to new orders that mixed up syllables from different words than to the "words" themselves. They had tracked transitional probabilities: how likely one syllable is to follow another.

This finding has held up well, and it also works for non-speech tones and in other species (Hauser, Newport and Aslin found it in cotton-top tamarins in 2001). It does not mean this is the only cue: real infants also use stress patterns and pauses. Try the learner.

Without JavaScript: inside each word the transitional probability is 100%; at a word boundary it is much lower (in the simulation, roughly a third to two fifths). With enough listening, a rule "break where it dips" finds almost every word.

Babies also narrow their ears (the 9-month Mandarin experiment is in 26/06). Sound, rhythm and statistics are the first year's job. The first word usually arrives at around 12 months; by then the child has been learning her language for a year without producing any of it.

First words and the rules children invent

After the first words (typically around 12 months), vocabulary grows slowly, then faster. By 18 to 24 months many children combine two words ("more milk", "daddy go"). Normal range is wide: a child with 50 words and a child with 300 at two years can both be developing normally, and late talkers often catch up, though not always. Languages differ in what is easy. Turkish words are built by stacking suffixes, as in ev-ler-im-de, and children learning Turkish tend to use these suffixes correctly remarkably early, an observation of Dan Slobin's cross-linguistic work.

The error that captured everyone's attention is overregularization. For a while a child says went, then later goed, then went again. The child did not hear "goed"; she extracted the -ed rule (Jean Berko's 1958 "wug test" shows children apply it to new words) and applied it to a verb that breaks it. But the picture is easy to exaggerate. In their 1992 monograph, Gary Marcus and colleagues analysed 11,521 irregular past-tense uses from children's recorded speech and found overregularization in a median of just 2.5% of them. Later analyses give a higher average (4.2% in one dense corpus, with one child at 24%). Maratsos (2000) argued that averages hide the fact that rare verbs are far more error-prone. Children, in short, get irregular verbs right most of the time, and errors cluster on the less common ones.

A simple "memory plus rule" model reproduces this pattern. For each irregular verb, the child stores the past form once heard; the more often she hears it, the stronger the memory. If retrieval fails, the rule fills in. Steven Pinker and Alan Prince's "words and rules" account (1988) is built on this idea; connectionist models (Rumelhart and McClelland, 1986) argue that one network can do both jobs. The tool below is a toy of the first kind, with invented frequencies.

Without JavaScript: in the toy, errors begin at the age the rule is learned, fall steadily afterwards, and are highest for the rarest verbs; the overall average is a few per cent, as in the real data.

Poverty of the stimulus versus learning from use

How can a child learn grammar from input that is messy, incomplete and never labelled? Noam Chomsky's answer (from the late 1950s) is that she cannot do it from experience alone: the data underdetermine the grammar, so children must start with some built-in structure, which he called universal grammar. This is the poverty-of-the-stimulus argument. The standard example is question formation. A child hears "The man is happy" and "Is the man happy?" and could conclude: move the first "is" to the front. That rule fits every simple sentence perfectly and is wrong for "The man who is tall is happy".

Without JavaScript: for "The man who is tall is happy", the linear rule gives "Is the man who tall is happy?", which no English speaker says; the structural rule gives "Is the man who is tall happy?". The two rules agree on every simple sentence.

The tool shows the argument: with only simple questions the two rules are indistinguishable. The puzzle, said Chomsky, is that children never make the linear error. In 1987 Stephen Crain and Mineharu Nakayama tested children aged three to five by asking them to form questions from sentences like this and found no linear errors. Critics reply in three ways. First, the premise: Geoffrey Pullum and Barbara Scholz (2002) argued that such evidence is more common than claimed, and that "learning from the data" is not hopeless. Second, statistical learners: Florencia Reali and Morten Christiansen (2005) showed simple models trained on child-directed speech get most of these questions right with no built-in syntax. Third, the whole picture: Michael Tomasello and the usage-based school argue that children begin with specific phrases ("Where's the ___?", "I wanna ___") and gradually generalise them into abstract patterns, using general social and pattern-finding skills (reading others' intentions, categorising). The nativists reply that those models learn from far more or differently structured data than a child gets, and that cross-linguistic universals need explaining.

The debate is live and has moved. Chomsky's own proposals have shrunk the built-in part to little more than a combining operation (Merge), and Tomasello's side now accepts a rich set of innate learning skills. Language models, trained on enormously more text than any child hears, are now a new kind of evidence; what they show about children is argued by both camps (see Computational Linguistics, 36/30). Daniel Everett's claim that Pirahã in Brazil lacks recursion (2005) was meant to challenge the universal-grammar picture; Andrew Nevins and colleagues (2009) disputed his data. A fair summary: nobody now thinks the child is a blank slate, and nobody agrees about how much of grammar is built in.

Critical periods: Genie, Nicaraguan Sign Language and the age curve

Is there a window for language learning? The biologist Eric Lenneberg argued in 1967 that language is easily acquired in childhood and, after about puberty, no longer in the same way. Animal studies of imprinting and song learning in birds gave the idea a model. Humans can obviously not be experimented on, so we use accidents of history.

The best known is Genie. She was born in 1957 in Los Angeles and from about the age of 20 months was kept confined by her father, mostly alone, with almost no speech around her. She was discovered in November 1970, aged 13 years and 7 months, and linguists including Susan Curtiss studied her for years. Genie learned many words and some structure, but her grammar stayed limited and she never fully acquired a first language. It is tempting to treat this as proof of a critical period. It is not clean evidence: she had suffered severe abuse, malnutrition and trauma, and it is impossible to separate a missing window from everything else; Curtiss argued against a general intellectual impairment, others were less sure. The research itself has been criticised on ethical grounds, since the people studying her were also her carers. The testing ended in 1978, and her later years were hard. Genie is a case to tell with care and without sensationalism.

Better evidence comes from people who come to a language late, but without being deprived. Jacqueline Johnson and Elissa Newport (1989) tested Chinese and Korean immigrants to the United States on English grammar: those who arrived before about seven years old matched native speakers, performance fell as the age of arrival rose through adolescence, and after puberty it was scattered. In 2018 Joshua Hartshorne, Joshua Tenenbaum and Steven Pinker analysed an online English grammar quiz taken by roughly two-thirds of a million people. Their results fit a critical period that ends much later than expected: the ability to learn grammar stays high until about 17 or 18, then declines. They suggested cultural and social changes at that age might be partly responsible, in addition to brain maturation. Rachel Mayberry and colleagues found that deaf people who met their first language late (a sign language, for deaf children of hearing parents with limited access) showed lasting effects on grammar, even compared with those who learned a second language later. The effects are about first language exposure.

Without JavaScript: a sharp cut-off predicts high scores until the cut-off age, then low; a steady decline predicts a score falling gradually from birth; the 2018 results resemble a plateau into the late teens followed by a decline.

The most striking evidence is not a deprivation but a creation. Before 1977 deaf Nicaraguan children mostly lived at home, each with their own family's gestures. When special schools opened in Managua, starting in 1977 with about 50 deaf children and growing to more than 400 pupils in two schools by 1983, the children were together for the first time. They did not receive a sign language from teachers (who used Spanish and fingerspelling). They made one. Judy Kegl, invited by the ministry of education in 1986, and later Ann Senghas and Marie Coppola, documented how each cohort of younger children made the language more systematic. Senghas, Kita and Özyürek (2004) found that younger signers split a rolling-down-a-hill motion into separate signs for manner and path, while the older cohort's signing tended to express both at once, as hearing people's gestures do. A similar story is told for Al-Sayyid Bedouin Sign Language in the Negev, which arose within about three generations (Sandler and colleagues, 2005). What it shows is debated: nativists see children imposing structure; others stress that a community, its needs and its transmission across cohorts matter.

Bilingual children: myths and evidence

Perhaps half the world's people use more than one language in their daily lives (a common estimate by the psycholinguist François Grosjean). Bilingualism is the normal case, not a puzzle, and the research on bilingual children has gone through a cycle of myth, over-enthusiasm and correction.

The old worry, that two languages confuse a child and delay speech, is not supported: bilingual children reach the main milestones in a similar window. What the evidence does show is that vocabulary depends on input. A bilingual toddler usually knows fewer words in each language than a monolingual of the same age, because she hears each for part of the day, but her total across both languages is comparable (Hoff and colleagues, 2012). Mixing languages in a sentence (code-switching) is not a sign of confusion: Shana Poplack's 1980 study of Spanish–English speakers in New York found it follows grammatical constraints and is commonest among the most fluent speakers. The more recent "cognitive advantage" claim, that bilinguals have a stronger executive control, went from headline to dispute: Minna Lehtonen and colleagues (2018) pooled 152 adult studies and found no advantage after correcting for publication bias. Similar disagreement attends the finding that bilinguals show Alzheimer's symptoms about four years later (Bialystok and colleagues, 2007), since later studies with better controls differ. Try your hand at sorting the claims.

Without JavaScript: bilingualism confusing children is not supported; smaller vocabulary per language but similar totals is supported; code-switching as confusion is not supported; an executive-function advantage and delayed dementia are mixed; most of the world using several languages is supported.

Practical points the research does support: amount and quality of input matter more than any one-parent-one-language rule; bilingualism does not cause a language disorder, and a bilingual child with a delay should be assessed in both languages by someone who knows the field. The evidence is about averages; any particular family has its own situation.

Sign languages are full languages

Until the 1960s many hearing people, including many linguists, treated sign as gesture or pantomime or a broken English on the hands. In 1960 William Stokoe, a professor of English at Gallaudet, published Sign Language Structure, showing that American Sign Language (ASL) signs are built from smaller meaningless units, as spoken words are built from sounds: where the hands are (location), their shape (handshape), the way they move (movement), and later also palm orientation. Change one, and you change the word. Robbin Battison's 1974 minimal pairs are standard examples. Try them.

Without JavaScript: APPLE and ONION differ in location; MOTHER and FATHER differ in location (chin or forehead); CAR and WHICH in handshape; PROPOSE and MAYBE in movement; TEMPORARY and TRAIN in palm orientation.

Everything else follows. Sign languages have grammar, new vocabulary, poetry, jokes and dialects; they use space and the face for grammar (raised eyebrows can mark a yes/no question) and can express two things at once, which speech cannot. Deaf children of Deaf parents learn them on roughly the spoken timetable, including "babbling" with their hands (Laura-Ann Petitto and Paula Marentette, 1991). Damage to the left hemisphere impairs signing as it impairs speech, and signers with strokes can lose grammar while keeping the ability to gesture (Ursula Bellugi, Howard Poizner and Edward Klima's work on sign aphasia). Sign languages are many and not universal: ASL descends from French Sign Language and is unrelated to British Sign Language, and the two are not mutually intelligible. Counts of sign languages vary widely (from well over a hundred to over three hundred) because it depends on what counts as separate. Most deaf children, about nine in ten by a commonly cited estimate, are born to hearing parents who have never met a sign language. That is why the age-of-exposure research is not abstract: it concerns real children who sometimes reach school with no full language. How best to provide early access to language, through sign, spoken language or both, is discussed in families and Deaf communities, and this piece does not advise on it.

What a graduate just sees

• Babies are statistical learners before they know any word, and "break the stream where predictability dips" is a real strategy.
• "Goed" is a sign that a rule has been learned; most irregular verbs are produced correctly most of the time.
• The poverty-of-the-stimulus argument is about what the data can and cannot tell a learner; both camps now accept some innate machinery, and disagree about how much and of what kind.
• Genie is a hard case with many confounds, not a clean test; the best evidence for a sensitive period comes from late-exposed learners.
• Deaf children made a language in Nicaragua; languages are made by communities of children.
• Bilingualism is normal, does not delay children, and its "cognitive benefits" are disputed.
• Sign languages are languages, with phonology, grammar and a brain.

Where it shows up next

• Psycholinguistics & the Brain (36/28): how an adult brain handles the language the child built.
• Computational Linguistics (36/30): what language models learn, and how it compares to what children learn from far less.
• Sociolinguistics (36/26): language shift, heritage speakers and the politics of which languages children are taught in.
• In the world: early-years teaching, speech-language therapy, deaf education, language policy in schools, and the design of reading programmes.

If you want to go deeper

• 📘 Textbook. Eve Clark, First Language Acquisition (Cambridge University Press, 3rd ed. 2016) is a standard course text; Barbara Lust, Child Language: Acquisition and Growth (Cambridge) is another.
• 📘 Gentler. Steven Pinker, The Language Instinct (1994) argues the nativist case in an engaging way; Michael Tomasello, Constructing a Language (2003) is the usage-based reply; Susan Curtiss's account of Genie and Russ Rymer's Genie: A Scientific Tragedy (1993) give the case in full.
• 🌐 Free text. The Noba Project and OpenStax psychology texts have free modules on language development; CHILDES (childes.talkbank.org) is a free archive of recorded child speech.
• 🌐 Free course. MIT OpenCourseWare 24.900 Introduction to Linguistics (Spring 2022) includes sessions on language acquisition and on language change.