Morpheme Acquisition Order

Definition:

Morpheme Acquisition Order (also called the Natural Order of Morpheme Acquisition) refers to the empirical finding that second language learners tend to acquire grammatical morphemes — small meaning-bearing units like verb endings, articles, and plural markers — in a broadly predictable sequence, largely regardless of their native language. This finding, established through studies by Roger Brown, Dulay and Burt, Bailey and colleagues in the 1970s, became foundational to Stephen Krashen’s Natural Order Hypothesis.

In-Depth Explanation

A morpheme is the smallest unit of meaning in a language. English grammatical morphemes include:

-ing (progressive aspect: “she is running“)
plural -s (“two cats“)
possessive ‘s (“the cat’s tail”)
articles (a, an, the)
copula be (“she is tired”)
auxiliary be (“she is running”)
irregular past tense (“he went“)
regular past tense -ed (“she walked“)
3rd person singular -s (“he runs“)

The Research

Roger Brown (1973) tracked the naturalistic acquisition of 14 grammatical morphemes in three L1-acquiring English-speaking children and found they acquired these morphemes in a strikingly similar order, regardless of their individual differences. This established a natural order for L1 English morpheme acquisition.

Heidi Dulay and Marina Burt (1974) applied the same methodology to L2 English learners — specifically, Spanish-speaking and Chinese-speaking children learning English. Despite their very different native languages, both groups showed a remarkably similar order of morpheme acquisition. The Contrastive Analysis Hypothesis would have predicted different acquisition orders for these two groups (since Spanish is much closer to English than Mandarin is) — but the similarities outweighed the differences.

A subsequent study by Dulay, Burt, and Krashen, and replication studies by Bailey, Madden, and Krashen (1974) on adult L2 learners, confirmed the general pattern for adults as well, though with some differences from the child L2 order.

The Approximate Order (English L2)

Research identified an approximate acquisition order for English morphemes:

Early acquired:

-ing (progressive)
Plural -s
Copula be (uninflected)

Mid acquired:

Article (a/the)
Irregular past tense
Regular past tense -ed
3rd person singular -s

Late acquired:

Possessive ‘s
Auxiliary be (inflected)

The third-person singular present -s (as in “he runs“) is notoriously late — even highly proficient L2 English speakers frequently omit it in rapid speech.

Krashen’s Natural Order Hypothesis

Krashen incorporated the morpheme acquisition order research into his Monitor Model as the Natural Order Hypothesis: grammatical structures are acquired in a predictable, natural order that cannot be altered by explicit instruction. You can teach 3rd-person -s all you like, but it will be acquired when the learner is ready — not because they’ve been drilled.

This hypothesis directly challenged the received wisdom of structural syllabuses (teach from simple to complex) — the “simple” to “complex” order in textbooks does not match the order in which items are actually acquired.

Criticism and Limitations

The morpheme acquisition order research was highly controversial and has been substantially revised:

Method criticism: The studies used obligatory contexts analysis (counting whether a morpheme appeared in contexts where it was grammatically required) — which measures accuracy in context, not acquisition per se
Acquisition vs. accuracy: A learner may use a morpheme accurately in some contexts but sporadically in others — obligatory contexts analysis blurs these distinctions
L1 effects are real: While L2 acquisition orders were more similar across L1 groups than CA predicted, L1 effects did exist — the CA vs. morpheme order debate was more nuanced than early formulations suggested
Individual variation: Many learners don’t follow the group trend — individual learning styles, instruction, and exposure patterns create real variation around the average order
English-specific: The morpheme studies focused heavily on English. Acquisition orders for L2 Japanese morphology, for instance, are substantially different and less clearly established

History

1973 — Roger Brown’s L1 morpheme studies.

Brown’s landmark longitudinal study of L1 acquisition in three children established that morphemes are acquired in a predictable order — the foundation for all subsequent morpheme acquisition research.

1974 — Dulay and Burt’s L2 studies.

Heidi Dulay and Marina Burt extended Brown’s methodology to L2 learners (Spanish and Chinese children learning English) and found a similar, cross-L1 acquisition order — directly challenging the Contrastive Analysis Hypothesis and establishing the concept of “creative construction” in L2 acquisition.

1974 — Bailey, Madden, and Krashen study adults.

Confirmed a similar morpheme acquisition order for adult L2 English learners, extending the finding beyond child learners.

1977–1985 — Krashen’s Natural Order Hypothesis.

Krashen incorporated morpheme order research into his Monitor Model, proposing a universal natural order for grammatical acquisition that instruction could not alter. This position remains influential and contested.

1980s–present — Critique and revision.

Researchers including Rosansky, Larsen-Freeman, and others critiqued the methodology and revised the conclusions, leading to more nuanced accounts of acquisition sequences in SLA — including developmental sequences research that went beyond morphemes to include syntactic and pragmatic structures.

Common Misconceptions

“The acquisition order is fixed and universal.”

While Dulay and Burt (1974) and subsequent research identified a broadly consistent order for English grammatical morphemes, the order shows meaningful variation across learners depending on L1 background, learning context, and task type. The “natural order” is a statistical tendency, not an invariable sequence.

“Teaching should follow the acquisition order.”

This is a tempting but unsupported conclusion. Research shows that instruction can accelerate acquisition of morphemes even when they are taught “out of order” — the key factor is learner readiness, not strict adherence to a predetermined sequence. The Teachability Hypothesis addresses when instruction is most effective.

“If a learner produces a morpheme correctly, they’ve acquired it.”

Consistent accuracy in obligatory contexts is the standard criterion, not occasional correct use. Most researchers require 80-90% accuracy in obligatory contexts before considering a morpheme “acquired.” Sporadic correct usage may reflect monitoring or formulaic production rather than genuine acquisition.

“The acquisition order is the same for all languages.”

The well-known English morpheme order (progressive -ing > plural -s > copula be > articles > past irregular > third person -s) is specific to English. Other languages have their own developmental sequences. Japanese, for example, has a different grammatical system where particles and verb conjugations follow distinct acquisitional patterns.

Criticisms

The morpheme acquisition order research has been criticized on methodological and theoretical grounds. The original studies (Dulay & Burt, 1974; Bailey, Madden, & Krashen, 1974) relied on cross-sectional data using the Bilingual Syntax Measure — a single elicitation instrument that may conflate processing difficulty with acquisition order. Longitudinal studies have shown more individual variation than the cross-sectional “average order” suggests.

The theoretical explanation for why morphemes follow a particular order remains incomplete. Krashen attributed it to natural, input-driven processes; others have argued that the order reflects processing complexity, perceptual salience, communicative value, or L1 transfer — explanations that are difficult to disentangle empirically. The research has also been criticized for overrepresenting English and Spanish while doing little to establish whether comparable “natural orders” exist for typologically different languages like Japanese, Arabic, or Mandarin.

Social Media Sentiment

Morpheme acquisition order receives relatively little direct discussion in online language learning communities, though its implications are frequently present in debates about grammar instruction timing. Reddit discussions about “when to learn X grammar point” implicitly engage with developmental sequence research — experienced community members often advise focusing on high-frequency, early-acquired structures first.

The concept is more discussed in applied linguistics and teacher training circles than in self-study communities. When it does appear, it is often in simplified form — “there’s a natural order to grammar acquisition” — used to argue either for or against explicit grammar instruction.

Practical Application

Don’t force late-acquired forms early — If a grammar point resists acquisition despite repeated study and practice, it may be developmentally premature. Continue exposure through comprehensible input and return to explicit study later.
Prioritize high-frequency morphemes — English articles, progressive -ing, plural -s, and copula be are both early-acquired and high-frequency. For Japanese, basic particles (は, が, を, に) and て-form precede complex conjugations.
Use errors as diagnostic information — Persistent morpheme errors indicate where the learner is in the developmental sequence, not that instruction has failed. Adjust expectations rather than intensifying drill on structures the learner is not ready to acquire.
Supplement natural order with explicit attention — While the natural order exists, explicit instruction can accelerate acquisition when timed to learner readiness. Focus on form is most effective when it targets structures the learner is developmentally ready to acquire.

Related Terms

Research

Brown, R. (1973). A First Language: The Early Stages. Harvard University Press.

Established the L1 morpheme acquisition order in English — the baseline against which L2 morpheme studies were compared.

Dulay, H., & Burt, M. (1974). Natural sequences in child second language acquisition. Language Learning, 24(1), 37–53.

The foundational L2 morpheme acquisition order study — showed cross-L1 consistency in acquisition order, challenging Contrastive Analysis.

Bailey, N., Madden, C., & Krashen, S. D. (1974). Is there a ‘natural sequence’ in adult second language learning? Language Learning, 24(2), 235–243.

Extended morpheme acquisition order findings to adult L2 learners — key input to Krashen’s Natural Order Hypothesis.

Krashen, S. D., Sferlazza, V., Feldman, L., & Fathman, A. (1976). Adult performance on the SLOPE test: More evidence for a natural sequence in adult second language acquisition. Language Learning, 26(1), 145–151.

Further evidence for the morpheme acquisition order in adults and its implications for the Natural Order Hypothesis.

Larsen-Freeman, D. (1975). The acquisition of grammatical morphemes by adult ESL students. TESOL Quarterly, 9(4), 409–419.

Important replication and critique of morpheme order research — identified methodological issues and added nuance to acquisition order claims.

Mikey Does