Source Count: 14 | Weighted Score: 27 | Source Confidence: [3/5] | Primary Tier: 1 | Last Updated: March 12, 2026
Keywords: second language acquisition, SLA, interlanguage, Selinker, critical period, Lenneberg, Krashen, input hypothesis, monitor model, comprehensible input, i+1, universal grammar, L2, fossilization, error analysis, contrastive analysis, interaction hypothesis, Long, output hypothesis, Swain, sociocultural theory, Vygotsky, ZPD, task-based language teaching, TBLT, motivation, transfer
Category Tags: linguistics, applied linguistics, education, cognitive science, psychology
Cross-References: ZG_3_05 — Language and Thought · ZG_3_09 — Syntax · ZG_4_10 — Code Switching · ZG_5_08 — Neurolinguistics · T_3_03 — Learning and Memory

QUICK SUMMARY

Second Language Acquisition (SLA) — the study of how people learn languages beyond their first (L1) — is a multidisciplinary field drawing on linguistics, psychology, cognitive science, and education. Central questions include: How do learners develop knowledge of a new language? Why do some learners achieve near-native proficiency while others fossilize at intermediate levels? Is there a critical period after which native-like attainment becomes impossible? How do the first language and the second language interact in the learner's mind? Key theoretical frameworks include: Larry Selinker's concept of interlanguage (1972) — the idea that L2 learners construct a systematic, rule-governed linguistic system of their own that is intermediate between L1 and L2, developing dynamically through predictable stages and subject to fossilization (stabilization at a non-target level). Stephen Krashen's Monitor Model (1982) — five hypotheses: the Acquisition-Learning Distinction (unconscious acquisition vs. conscious learning), the Monitor Hypothesis (conscious learning acts only as a monitor/editor), the Natural Order Hypothesis (grammatical morphemes are acquired in a predictable sequence), the Input Hypothesis (acquisition occurs when learners are exposed to comprehensible input slightly beyond their current level — "i+1"), and the Affective Filter Hypothesis (anxiety and negative emotions block acquisition). Although Krashen's model has been enormously influential (especially in language teaching), it has been extensively criticized for vagueness, unfalsifiability, and overemphasis on input at the expense of output and interaction. Michael Long's Interaction Hypothesis (1981, 1996) emphasizes the role of conversational interaction — particularly negotiation of meaning (clarification requests, confirmation checks, comprehension checks) — in making input comprehensible and driving acquisition. Merrill Swain's Output Hypothesis (1985, 1995) argues that producing language (output) is also essential for acquisition — it pushes learners to process syntax more deeply than comprehension alone requires, tests hypotheses, and generates awareness of gaps. Sociocultural theory (Lantolf, drawing on Vygotsky) emphasizes that language learning is a socially mediated process — assisted performance in the Zone of Proximal Development (ZPD) and the role of scaffolding, mediation, and collaborative dialogue. The Critical Period Hypothesis (Lenneberg, 1967) posits that there is a biologically determined window (typically before puberty) during which language acquisition proceeds most naturally — after this period, native-like attainment becomes progressively more difficult, especially for phonology. Evidence (Johnson & Newport, 1989; Hartshorne et al., 2018) generally supports a decline in ultimate attainment with increasing age of onset, though the sharpness of the cutoff and whether a true "critical period" (vs. gradual decline) exists remain debated.

1. VERIFIED CLAIMS (Tier 1 — Peer-Reviewed / Experimentally Confirmed)

1.1 Interlanguage (Selinker, 1972)

• Interlanguage: the learner's developing linguistic system, which is:
• Systematic: governed by rules (not random errors) — interlanguage grammars show systematic patterns
• Dynamic: develops over time through stages as proficiency increases
• Influenced by L1: transfer (positive: L1 features that facilitate L2; negative/interference: L1 features that cause errors in L2)
• Subject to fossilization: some features may stabilize at a non-target level despite continued exposure and motivation — Selinker estimated ~95% of adult L2 learners fail to reach native-like competence
• Error analysis (Corder, 1967): errors are not random failures but evidence of the learner's current interlanguage system — systematic study of errors reveals acquisition processes
• Types of errors: interlingual (L1 transfer), intralingual (overgeneralization of L2 rules, e.g., goed for went), developmental (shared by L1 and L2 learners)

1.2 Critical Period Hypothesis

• Eric Lenneberg (1967, Biological Foundations of Language): proposed that there is a critical period for language acquisition, ending around puberty — after which full native-like attainment is no longer possible
• Evidence for age effects:
• Johnson & Newport (1989): Chinese and Korean immigrants to the US — age of arrival strongly predicted grammaticality judgment scores; those arriving before age ~7 scored like natives; scores declined linearly with age of arrival from 7 to ~17, then leveled off
• Hartshorne, Tenenbaum & Pinker (2018): massive online study (670,000+ participants) — grammatical ability continues to increase until ~17.4 years of age, then declines; critical period for immersion-based learning ends around 10–12 for grammar, with native-like attainment becoming unlikely after ~17
• Phonology is the domain most affected by late onset — most adult learners retain a non-native accent; morphosyntax shows moderate age effects; lexical learning shows the smallest age effects (adults can learn vocabulary efficiently throughout life)
• Debate: whether the decline is abrupt (true "critical period") or gradual (sensitive period), and whether non-biological factors (declining motivation, reduced input quality, L1 entrenchment) contribute

1.3 Krashen's Monitor Model

• Five hypotheses (Krashen, 1982, Principles and Practice in Second Language Acquisition):

1. Acquisition-Learning Distinction: "acquisition" (unconscious, implicit — the way children learn their L1) is distinct from "learning" (conscious, explicit knowledge of rules) — and only acquisition leads to fluent spontaneous production
2. Monitor Hypothesis: learned (conscious) knowledge functions only as a "monitor" — editing output before or after production — and is available only when there is sufficient time, focus on form, and knowledge of the rule
3. Natural Order Hypothesis: grammatical morphemes are acquired in a predictable order (Brown, 1973, for L1; Dulay & Burt, 1974, for L2 — e.g., -ing and plural -s before third-person -s and possessive -'s)
4. Input Hypothesis: acquisition occurs when learners understand input containing structures slightly beyond their current level — "i+1" — the idea that comprehensible input is the essential driver
5. Affective Filter Hypothesis: negative emotions (anxiety, low motivation, low self-confidence) raise an "affective filter" that blocks input from reaching the acquisition device

• Criticisms: acquisition-learning distinction is difficult to operationalize; i+1 is vaguely defined; the model undervalues output and interaction; some claims are unfalsifiable

2. CREDIBLE CLAIMS (Tier 2 — Supported by Multiple Scholars / Strong Circumstantial Evidence)

2.1 Interaction Hypothesis (Long, 1981, 1996)

• Conversational interaction — particularly negotiation of meaning — facilitates acquisition by:
• Making input more comprehensible through clarification requests, comprehension checks, confirmation checks, and recasts
• Drawing learner attention to form-meaning mappings
• Providing negative feedback (indication that a form is non-target) in the context of meaningful communication
• Recasts (reformulations of learner errors in correct form without explicit correction) are particularly effective in promoting noticing and acquisition (Lyster & Ranta, 1997; Long, 2007)

2.2 Output Hypothesis (Swain, 1985, 1995)

• Producing language (speaking and writing) promotes acquisition through three functions:

1. Noticing function: output pushes learners to notice gaps between what they want to say and what they can say — driving attention to needed forms
2. Hypothesis-testing function: output allows learners to try out new forms and receive feedback
3. Metalinguistic function: output (especially collaborative dialogue) promotes conscious reflection on language

• Swain argued that comprehensible input alone is insufficient — Canadian French immersion students received massive comprehensible input but remained weak in grammar and accuracy, suggesting that producing output is also necessary

2.3 Sociocultural Theory (Lantolf & Thorne)

• Lev Vygotsky's concepts applied to SLA:
• Zone of Proximal Development (ZPD): the gap between what a learner can do alone and what they can do with assistance — learning occurs through collaborative interaction in the ZPD
• Scaffolding: graduated support provided by a more proficient interlocutor (teacher, peer, or text) — withdrawn as the learner gains ability
• Mediation: language (including the learner's L1) mediates cognitive development — private speech, collaborative dialogue, and self-regulation
• Language learning is inherently social — not just a cognitive process happening inside individual minds

2.4 Individual Differences

• Aptitude: Carroll & Sapon's MLAT (Modern Language Aptitude Test, 1959) measures phonemic coding ability, grammatical sensitivity, rote learning ability, and inductive language learning ability — predicts L2 success in formal instruction settings
• Motivation: Gardner & Lambert (1972) distinguished integrative motivation (desire to identify with the L2 community) and instrumental motivation (desire for practical benefits — jobs, travel, academic requirements). Dörnyei's (2005) L2 Motivational Self System reconceptualized motivation in terms of ideal L2 self, ought-to L2 self, and L2 learning experience
• Anxiety: Horwitz, Horwitz & Cope (1986) identified foreign language anxiety as a distinct construct — negatively correlated with L2 achievement

3. SPECULATIVE CLAIMS (Tier 3 — Limited Evidence / Emerging Hypotheses)

3.1 Usage-Based SLA

• Drawing on Construction Grammar and usage-based linguistics (N. Ellis, 2002): L2 acquisition is the gradual abstraction of patterns from exemplars in the input — frequency, salience, and L1-L2 similarity determine what is learned first. Whether this approach can fully replace UG-based or cognitive processing accounts is debated

3.2 Multilingualism and Third Language Acquisition (L3)

• L3 (and Ln) acquisition is an emerging field — available evidence suggests that both L1 and L2 influence L3 learning, and that multilingual speakers develop metalinguistic awareness that facilitates further language learning (Cenoz, 2013)

4. DUBIOUS CLAIMS (Tier 4 — Fringe / Not Supported by Evidence)

4.1 "Adults Cannot Learn Languages"

• Adults can and do learn second languages — often more efficiently than children in the short term (faster initial vocabulary and grammar learning). What adults rarely achieve is native-like phonology and near-perfect grammaticality in long-term attainment. The claim that adult language learning is impossible is false

4.2 "Immersion Alone Is Sufficient"

• While immersion provides rich input, comprehensive reviews (Norris & Ortega, 2000; Spada & Tomita, 2010) show that some form of focus on form (attention to linguistic structure, whether through explicit instruction, feedback, or task design) accelerates acquisition and helps learners achieve accuracy that immersion alone may not provide — especially for non-salient grammatical features

COUNTER-ARGUMENTS

• Krashen's Monitor Model criticism: Stephen Krashen's influential distinction between "acquisition" (unconscious) and "learning" (conscious), and his Input Hypothesis (i+1), have been criticized by Kevin Gregg (1984) and others as unfalsifiable — the theory cannot specify in advance what constitutes "comprehensible input" at exactly i+1, and the acquisition/learning distinction resists empirical testing. Despite this, Krashen's framework remains widely cited in language teaching
• UG access in L2: Whether adult second-language learners have access to Universal Grammar (full access, partial access, or no access) is a central theoretical debate — Lydia White argued for full UG access, Harald Clahsen and Pieter Muysken for partial access, and usage-based theorists reject the premise entirely. Divergent L2 outcomes compared to uniform L1 success remain the core puzzle

IMAGES

BIBLIOGRAPHY

1. Corder, S | 1967 | "The Significance of Learners' Errors" | International Review of Applied Linguistics | ∅ | 5::161–170 | Pit | ∅ | doi:10.1515/iral.1967.5.1-4.161 | ∅ | ∅ | ∅
2. Dörnyei, Zoltán | 2005 | ∅ | The Psychology of the Language Learner | ∅ | ∅ | Erlbaum | ∅ | doi:10.1177/13621688070110040802 | ∅ | ∅ | ∅
3. Ellis, Nick C | 2002 | "Frequency Effects in Language Processing" | Studies in Second Language Acquisition | ∅ | 24.2::143–188 | ∅ | ∅ | doi:10.1017/s0272263102002024 | ∅ | ∅ | ∅
4. Gass, Susan M.; Larry Selinker. . | 2013 | ∅ | Second Language Acquisition: An Introductory Course | ∅ | ∅ | Routledge | 4th | doi:10.1111/j.1540-4781.2010.01032.x | ∅ | ∅ | ∅
5. Hartshorne, Joshua K., Joshua B | 2018 | "A Critical Period for Second Language Acquisition" | Cognition | ∅ | 177::263–277 | Tenenbaum, and Steven Pinker | ∅ | doi:10.1016/j.cognition.2018.04.007 | ∅ | ∅ | ∅
6. Johnson, Jacqueline S.; Elissa L | 1989 | "Critical Period Effects in Second Language Learning" | Cognitive Psychology | ∅ | 21::60–99 | Newport | ∅ | ∅ | ∅ | ∅ | ∅
7. Krashen, Stephen D. | 1982 | ∅ | Principles and Practice in Second Language Acquisition | ∅ | ∅ | Pergamon | ∅ | ∅ | ∅ | ∅ | ∅
8. Lantolf, James P.; Steven L | 2006 | ∅ | Sociocultural Theory and the Genesis of Second Language Development | ∅ | ∅ | Thorne | ∅ | ∅ | ∅ | ∅ | Oxford University Press
9. Lenneberg, Eric H. | 1967 | ∅ | Biological Foundations of Language | ∅ | ∅ | Wiley | ∅ | ∅ | ∅ | ∅ | ∅
10. Long, Michael H | 1981 | "Input, Interaction, and Second-Language Acquisition" | Annals of the New York Academy of Sciences | ∅ | 379::259–278 | ∅ | ∅ | ∅ | ∅ | ∅ | ∅
11. Norris, John M.; Lourdes Ortega | 2000 | "Effectiveness of L2 Instruction: A Research Synthesis and Quantitative Meta-Analysis" | Language Learning | ∅ | 50.3::417–528 | ∅ | ∅ | ∅ | ∅ | ∅ | ∅
12. Selinker, Larry | 1972 | "Interlanguage" | International Review of Applied Linguistics | ∅ | 10::209–231 | ∅ | ∅ | ∅ | ∅ | ∅ | ∅
13. Swain, Merrill | 1985 | "Communicative Competence: Some Roles of Comprehensible Input and Comprehensible Output in Its Development" | Input in Second Language Acquisition | ∅ | ∅ | In , ed | ∅ | ∅ | ∅ | ∅ | Susan M; Gass and Carolyn G; Madden, 235 253; Newbury House
14. VanPatten, Bill; Jessica Williams, eds. . | 2020 | ∅ | Theories in Second Language Acquisition | ∅ | ∅ | Routledge | 3rd | ∅ | ∅ | ∅ | ∅

CROSS-REFERENCE INDEX

• Language and Thought → ZG_3_05 — Language and Thought
• Syntax → ZG_3_09 — Syntax
• Code Switching → ZG_4_10 — Code Switching
• Neurolinguistics → ZG_5_08 — Neurolinguistics
• Learning and Memory → T_3_03 — Learning and Memory

Last updated: March 12, 2026

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