Language acquisition is the process by which children acquire their native language with remarkable speed and accuracy despite impoverished input. By age 5–6, children achieve near-complete mastery of complex grammatical structures with little explicit instruction — a pattern Chomsky cited as the 'poverty of the stimulus' argument for an innate Language Acquisition Device. Acquisition proceeds through predictable stages: babbling, one-word (holophrastic), two-word, and telegraphic stages, followed by rapid grammatical development. Second language acquisition differs significantly from first language acquisition, particularly regarding the role of the critical period and explicit instruction.
Study transcripts of child speech (e.g., from the CHILDES corpus) to observe acquisition stages directly. Notice systematic errors (e.g., 'goed', 'mouses') as evidence that children are not simply imitating but generalizing rules.
Think about what you now know about language: morphemes combine to build words, syntactic structure governs how words arrange into phrases and sentences, phonological systems organize the sounds of a language, and phrases are built from hierarchically nested constituents. A competent adult speaker has mastered all of this — thousands of morphemes, complex phrase-structure rules, subtle phonological contrasts, and nuanced pragmatic conventions. The puzzle of language acquisition is that children achieve most of this mastery by age five or six, with no formal instruction, often from caregivers who correct meaning but almost never correct grammar, and from input that underdetermines the abstract structures they ultimately acquire. This is the problem the field of language acquisition exists to explain.
Children move through recognizable stages. Babbling (around 6–8 months) produces increasingly language-like sounds but no semantic content. The holophrastic stage (around 12 months) uses single words to communicate whole propositions — "milk" can mean "I want milk," "that's milk," or "I spilled the milk" depending on context. The two-word stage (around 18–24 months) shows the beginning of structure: "more milk," "daddy go." The telegraphic stage produces multi-word utterances omitting grammatical function words: "Daddy go store." Then, rapidly, morphological and syntactic complexity explodes. The clearest evidence that children are not merely imitating adult speech comes from overgeneralization errors: "goed," "mouses," "braked." Children produce forms they have never heard — forms that prove they are actively inducing and applying grammatical rules, not parroting.
Chomsky's influential argument is that this acquisition cannot be explained by the input alone. The linguistic structures children converge on are more abstract and constrained than what the data they hear can justify — the poverty of the stimulus. Children are never told which sentence types are ungrammatical; they simply don't produce them. This argues for some innate Language Acquisition Device — a species-specific biological endowment that constrains the hypothesis space for what a possible human grammar can look like. Critics of this view argue that statistical learning over rich input plus domain-general cognitive mechanisms can achieve the same result; the debate continues.
Second language acquisition differs in important ways. The critical period hypothesis holds that beyond a certain developmental window — roughly puberty for syntax and morphology, earlier for phonology — full native-like acquisition becomes unavailable. Adult L2 learners can achieve high proficiency but typically retain an accent and show residual difficulty with certain morphosyntactic features. Explicit instruction helps adult learners in ways it does not help children; children acquire rules implicitly and spontaneously, while adults benefit from conscious awareness of grammatical patterns. L1 transfer — the influence of the first language on the second — produces characteristic errors that predict learners' native languages and doesn't occur in first-language acquisition at all.