Acquisition order refers to the empirically observed sequence in which learners acquire grammatical morphemes (and, by extension, grammatical structures) in a second language. Research beginning in the early 1970s showed that learners of English as a second or foreign language — regardless of their first language — tend to acquire specific grammatical morphemes in a remarkably consistent order, suggesting that acquisition follows internal developmental sequences rather than simply reflecting order of instruction or L1 influence.

In-Depth Explanation

The most influential acquisition order research examined English grammatical morphemes, finding a consistent sequence across learners from different L1 backgrounds. This order is central to Krashen’s Natural Order Hypothesis and suggests that internal developmental readiness — not instruction order or L1 transfer — determines when a given structure is acquired.

The morpheme acquisition order studies

The foundational discovery came from Roger Brown’s (1973) L1 acquisition research, which identified a consistent order for the acquisition of 14 grammatical morphemes in English-acquiring children. Shortly afterward, Dulay and Burt (1974) replicated comparable findings in L2 English learners — Spanish and Chinese speaking children, in separate studies — finding broadly similar morpheme acquisition orders across groups. Krashen (1982) synthesized this into his Natural Order Hypothesis: grammatical structures are “acquired” in a predictable sequence, and instruction cannot change this underlying order (though it may accelerate progress within it).

The English morpheme order (approximate)

Research studying English learners consistently finds an approximate ordering for English grammatical morphemes:

Earlier-acquired morphemes are not necessarily simpler in form, which is why Krashen argued the order reflects internal cognitive readiness rather than pedagogical difficulty. The -ing suffix, for example, is acquired before the plural -s despite both being simple suffixes.

Developmental sequences

Beyond morphemes, research on sentence processing and processability theory (Pienemann 1998) extended the acquisition order concept to complex syntactic structures. Processability Theory proposes that learners can only produce structures they have the procedural processing capacity to handle — and these capacities are acquired in a fixed developmental sequence. For Japanese: learners first acquire lexical structures, then phrasal agreement, then inter-phrasal agreement, then finally subordinate clause structures. This explains why certain grammatical forms are absent from learner output despite extensive instruction and input.

Implications for instruction

The acquisition order research has a provocative implication: instruction may not change the order. A learner who has not yet reached the developmental stage for a particular structure cannot produce it accurately — regardless of explicit instruction. This does not mean instruction is pointless; it means that instruction targeting a structure slightly ahead of the learner’s current stage (their ZPD, in Vygotsky’s terms) is more effective than instruction far beyond it. It also means that errors at certain stages are developmentally expected and not necessarily signs of methodological failure.

Criticism and qualification

The morpheme order studies have been criticized on methodological grounds (scoring methods, elicitation tasks, implicational scaling). The consistency of the ordering varies depending on task and context. Additionally, the studies focused on English, and generalizing to other target languages requires separate investigation. Nonetheless, the core finding — that acquisition follows internal developmental sequences that instruction cannot easily override — has been robust across multiple replications.

History

Roger Brown’s longitudinal study of three English-acquiring children (A First Language, 1973) established the morpheme acquisition order framework for L1 acquisition. Dulay and Burt (1973, 1974) extended the approach to L2 learners, and their findings were widely discussed as challenging behaviourist assumptions that acquisition followed instruction and L1 habit interference. Krashen incorporated the Natural Order Hypothesis as one of the five hypotheses in his Input Hypothesis model (1982). Subsequent research through the 1980s–90s examined developmental sequences in syntax, morphology, and pragmatics. Pienemann’s Processability Theory (1998) provided a more explicit computational cognitive mechanism for acquisition-order predictions. The subfield remains active, with research on developmental sequences in Japanese, German, French, and other languages.

Common Misconceptions

• “Acquisition order means you should teach grammar in that order.” The implication is actually the reverse: instruction cannot override developmental readiness, so forcing structures before the learner is ready may produce declarative knowledge without fluent acquisition.
• “If a learner makes errors on late-acquired structures, they’re not trying.” Errors on late-acquired morphemes are developmentally expected. Native-like accuracy on third-person singular -s, for example, takes much longer to acquire than its simple form suggests.
• “Natural order research means explicit grammar instruction is useless.” Krashen drew this radical conclusion, but most researchers take a more moderate view: explicit instruction accelerates progress through developmental stages and raises awareness, even if it cannot skip stages entirely.

Social Media Sentiment

Acquisition order research surfaces in online language-learning discussions primarily through Krashen’s Natural Order Hypothesis, which is invoked (sometimes oversimplified) in debates about grammar study vs. immersion. The argument that “you can’t force grammar” and that input-based methods respect natural acquisition sequences is popular among immersionist communities. Critics point out that the morpheme order studies were conducted on English specifically and don’t straightforwardly generalize to Japanese, where the developmental sequences have their own structure.

Last updated: 2026-04

Practical Application

• Don’t expect instant accuracy on late-acquired features: Japanese morphological accuracy (especially complex verb forms and particles) takes far longer to stabilize than learners or teachers expect. This is developmental, not motivational.
• Stage-appropriate practice matters: Rather than drilling structures far beyond current competence, focus on consolidating the structures you can currently produce inconsistently. Processability Theory suggests this is the productive edge.
• Immersion strategies do map onto developmental sequences: Immersion methods that provide massive input allow developmental sequences to unfold at their own pace — potentially faster than classroom-controlled exposure.
• Errors are diagnostic: Systematic errors at a stage (e.g., consistently omitting particles in certain contexts) indicate a developmental stage, not a random gap. Understanding the sequence helps diagnose where a learner is.

Related Terms

• Natural Order Hypothesis
• Input Hypothesis
• Processability Theory
• Interlanguage
• Behaviourism
• Zone of Proximal Development

See Also

• Sakubo – Japanese SRS App — Japanese study tool; SRS-based sentence exposure aligns with acquisition-order principles by providing contextual input across development stages.

Research / Sources

• Brown, R. (1973). A First Language: The Early Stages. Harvard University Press. — foundational L1 morpheme acquisition order research.
• Dulay, H. & Burt, M. (1974). Natural sequences in child second language acquisition. Language Learning, 24(1), 37–53. — L2 extension of morpheme order research.
• Pienemann, M. (1998). Language Processing and Second Language Development: Processability Theory. John Benjamins. — the cognitive computational framework extending acquisition order to syntax.