Interdependence Hypothesis

The Interdependence Hypothesis (also called the Developmental Interdependence Hypothesis), proposed by Jim Cummins (1979), states that a learner's development of competence in a second language is partially a function of the competence already developed in the first language. Literacy skills, academic knowledge, and cognitive strategies developed in L1 transfer to L2 — the two languages are not stored in separate mental compartments but draw on a Common Underlying Proficiency (CUP).

The Claim

Cummins (1979) argued that beneath the surface differences between languages lies a shared cognitive/academic proficiency. This CUP means that:

• Concepts learned in L1 do not need to be relearned in L2 — a child who understands fractions in Vietnamese does not need to relearn the concept in English, only the English labels
• Literacy skills (reading strategies, text structure awareness, inferencing) transfer across languages
• Academic language proficiency (CALP) is interdependent — developing it in L1 supports its development in L2
• The transfer is bidirectional, though in practice L1 → L2 transfer is more commonly studied

The Iceberg Metaphor

Cummins used the dual-iceberg metaphor: two languages appear separate on the surface (different pronunciation, vocabulary, grammar), but below the surface they share a common operating system — the CUP. This shared proficiency includes conceptual knowledge, cognitive strategies, metalinguistic awareness, and literacy skills.

Evidence

• Bilingual education research — studies consistently show that children educated through their L1 in early years perform as well or better in L2 academic tasks than children placed directly in L2-medium instruction. This is paradoxical unless L1 skills transfer to L2.
• Cross-linguistic transfer of reading skills — reading comprehension ability in L1 predicts reading comprehension in L2, even when the two languages use different scripts (e.g., Spanish-English, Japanese-English).
• Metalinguistic awareness — bilingual children show heightened metalinguistic awareness, suggesting that working in two languages develops the shared underlying proficiency.

Connection to BICS and CALP

The Interdependence Hypothesis is inseparable from Cummins' distinction between BICS and CALP:

• BICS (Basic Interpersonal Communicative Skills) — conversational fluency, acquired relatively quickly (1–2 years), context-embedded
• CALP (Cognitive Academic Language Proficiency) — the decontextualised, cognitively demanding language required for academic success, takes 5–7 years to develop

The Interdependence Hypothesis specifically concerns CALP: academic language proficiency, once developed in L1, provides the foundation for L2 academic language. BICS, being more surface-level and context-dependent, is less dependent on cross-linguistic transfer.

Connection to the Threshold Hypothesis

Cummins' Threshold Hypothesis is a companion claim: L1 proficiency must reach a minimum threshold before the benefits of bilingualism (including cross-linguistic transfer) can be realised. If L1 development is interrupted too early — for example, by premature submersion in L2-only education — neither language may develop adequate CALP, leading to what has been termed subtractive bilingualism.

Implications for Education

• L1 literacy should be maintained, not abandoned, when learners enter L2-medium education
• Submersion programmes that ignore L1 undermine the very resource (L1 academic proficiency) that supports L2 development
• Bilingual and transitional programmes that develop L1 CALP before or alongside L2 produce better long-term outcomes
• For adult SLA, the hypothesis supports activating L1 academic knowledge as a resource rather than treating L1 as interference