Identity Hypothesis

The Identity Hypothesis (also known as the L1 = L2 Hypothesis) proposes that first and second language acquisition are essentially the same process, governed by the same underlying mechanisms. If Universal Grammar drives L1 acquisition, it also drives L2 acquisition — the innate Language Acquisition Device remains accessible regardless of age. This position stands in direct opposition to the Fundamental Difference Hypothesis.

Evidence For

Several lines of research support the hypothesis:

• Similar developmental sequences — L2 learners, regardless of L1 background, acquire certain morphemes and structures in a predictable order that broadly parallels L1 acquisition. The morpheme order studies of Dulay and Burt (1974) were foundational here, showing that children from different L1 backgrounds followed similar acquisition orders for English grammatical morphemes.
• Similar error types — L2 learners produce developmental errors (e.g., overgeneralisation of past tense: goed, breaked) that resemble L1 errors, suggesting reliance on the same underlying mechanisms rather than L1 transfer alone.
• Universal patterns — features like the silent period, formulaic speech in early stages, and structural simplification appear in both L1 and L2 development.
• Natural Order Hypothesis — Krashen's claim that grammatical structures are acquired in a predictable order in both L1 and L2 is consistent with the identity position.

Evidence Against

The differences between L1 and L2 acquisition are also substantial:

• Variable outcomes — all children achieve L1 competence; most adult L2 learners do not reach native-like proficiency
• Fossilization — L2 development can plateau permanently; L1 development does not
• L1 influence — Language Transfer is a significant factor in L2 that has no parallel in L1 acquisition
• Acquisition order is not identical — while there are broad similarities, the morpheme orders for L1 and L2 are not the same. Articles and copula be tend to be acquired earlier by L2 learners than L1 learners; irregular past tense is acquired later
• Age effects — the Critical Period Hypothesis suggests that the mechanisms available for language acquisition change with age

Current Standing

The strong version of the Identity Hypothesis — that L1 and L2 acquisition are identical processes — has few contemporary advocates. The evidence for Language Transfer, Fossilization, and age effects is too robust. However, the weaker version — that the same cognitive architecture underlies both L1 and L2 acquisition, even if the processes differ in important ways — remains influential and underpins nativist approaches to SLA that argue for continued Universal Grammar access in adult learners.

The hypothesis is best understood not as a standalone theory but as one pole of a continuum, with the Fundamental Difference Hypothesis at the other. Most current SLA theory occupies the middle ground.