What is language acquisition?

The Language Acquisition Device is Chomsky's proposed innate mechanism — a biologically endowed neural system specific to humans — that enables children to acquire the grammar of their native language from impoverished and inconsistent input. The LAD is hypothesised to contain Universal Grammar: abstract structural principles (constraints on phrase structure, movement, binding) shared across all human languages. The 'poverty of the stimulus' argument supports the LAD: children acquire grammatical rules they have never heard violated or been explicitly taught, far faster than general learning algorithms could account for. Critics (Tomasello, Elman) argue that general statistical learning, social cognition, and input frequency can explain language acquisition without positing innate grammatical knowledge.

Eric Lenneberg's critical period hypothesis (1967) proposes that language must be acquired within a biologically determined window — roughly from birth to puberty — during which the brain is maximally plastic for language learning. Evidence includes: (1) Sensitive period for first language: cases of extreme deprivation (e.g. Genie, isolated until age 13) show severely impaired grammar acquisition despite language exposure after rescue. (2) Second language acquisition: native-like phonological and grammatical competence is substantially harder to achieve for learners who begin after puberty. (3) Sign language: congenitally deaf children exposed to sign language from birth acquire it like a native language; those exposed late show permanent grammatical gaps. The term 'sensitive period' is now preferred — acquisition remains possible after puberty, but the rate and ceiling of attainment decline.

Language development follows a broadly universal sequence: (1) Pre-linguistic (0–6 months) — cooing, turn-taking in vocalisations, sensitivity to native phonemes begins narrowing; (2) Canonical babbling (6–12 months) — reduplicated syllables (ba-ba, da-da), gradually reflecting the native language's phonology; (3) One-word/holophrastic stage (12–18 months) — single words carry the meaning of whole sentences; overextension and underextension common; vocabulary explosion often follows; (4) Two-word/telegraphic stage (18–24 months) — two-word combinations, no function words ('more milk', 'daddy go'); (5) Multi-word speech (2–3 years) — rapid grammatical development; basic sentences with subject-verb-object structure; (6) Full grammatical competence is largely in place by age 4–5, though complex syntax continues developing into adolescence.

Nativist accounts (Chomsky, Pinker) argue that children are born with domain-specific innate knowledge of grammar — a Language Acquisition Device containing Universal Grammar — that explains the speed and universality of acquisition. Language is a biological endowment, not primarily learned from social experience. Social-interactionist accounts (Bruner, Vygotsky, Tomasello) argue that social input and interaction are essential, not merely triggering: caregivers provide a Language Acquisition Support System (LASS) of scaffolded joint attention, child-directed speech, and contingent responses that actively shape language development. Modern research increasingly integrates both: statistical learning in the first year shows impressive computational abilities to extract language patterns (Saffran et al., 1996), while the role of social referencing and joint attention is also well-established. The debate has shifted from 'nature vs nurture' to specifying which abilities are innate, which are emergent from general learning mechanisms, and how social experience interacts with each.