Interdependence Hypothesis
The Interdependence Hypothesis is Cummins's claim that academic and conceptual proficiency in a bilingual's two languages is not stored separately but draws on a single, shared cognitive base. First formulated in the late 1970s and elaborated across his subsequent career, the hypothesis underlies the case for first-language instruction in bilingual education and is one of the most consequential ideas to come out of SLA for school policy.
The Formal Claim
Cummins (1979, 1981) framed the hypothesis as a developmental prediction: to the extent that instruction in language X is effective in promoting proficiency in language X, transfer of this proficiency to language Y will occur, provided there is adequate exposure to language Y and adequate motivation to learn it. The two conditions matter. Transfer is not automatic; it requires both real exposure to the target language and the will to use it.
The claim cuts directly against the assumption, dominant in mid-twentieth-century bilingual policy, that time spent on the home language was time taken away from English (or whatever the dominant school language might be). If proficiencies share an underlying base, then literacy and conceptual development in the L1 build the very capacities that the L2 will draw on later.
The Iceberg Metaphor
Cummins's expository device for the hypothesis is the dual-iceberg model. Two languages appear as separate peaks above the waterline, distinct in surface form: vocabulary, pronunciation, grammar, and the mechanics of reading and writing. Below the surface, the two peaks merge into a single common underlying proficiency (CUP) that holds the cognitively demanding aspects of language use: literacy strategies, abstract reasoning, conceptual knowledge, and academic registers.
| Layer | Content |
|---|---|
| Above the surface (separate) | Pronunciation, surface grammar, basic vocabulary, accent, the mechanics of decoding |
| Below the surface (shared CUP) | Literacy strategies, conceptual knowledge, academic vocabulary, reasoning, world knowledge |
The contrast Cummins drew was with the separate underlying proficiency (SUP) model, in which the two languages are pictured as two balloons in the head competing for limited capacity. The CUP picture replaces the balloons with a single tank that either language can draw from and add to.
Evidence Cited in Support
The hypothesis grew partly out of empirical observations that puzzled the SUP picture:
- Children who arrive in a new country with strong L1 academic literacy typically reach age-appropriate L2 academic literacy faster than those who arrive without L1 schooling, even though the latter group may pick up conversational L2 more quickly.
- Bilingual education programmes that genuinely develop L1 literacy do not delay L2 literacy and often accelerate it once L2 exposure is sufficient.
- Skills strongly tied to literacy (reading comprehension, writing organisation, study strategies) transfer between languages with relatively little additional instruction.
- Skills tied to surface form (decoding, pronunciation) transfer less smoothly, consistent with their location above the iceberg.
These observations underlie the often-quoted distinction between BICS and CALP (basic interpersonal communicative skills versus cognitive academic language proficiency), which Cummins developed in the same period to explain why immigrant children often sound fluent within two years but take five to seven years to perform on grade level academically.
Implications for Education
The pedagogical consequences of the hypothesis have been far-reaching:
- Strong L1 development is treated as an investment in eventual L2 academic outcomes, not a competitor.
- Bilingual programmes that maintain L1 literacy through the primary years are predicted to outperform submersion programmes for academic outcomes once L2 exposure is sufficient.
- Translation, translanguaging, and cross-linguistic comparison are valid pedagogical moves, since they exploit the shared CUP rather than confusing two separate systems.
- L1 use in the L2 classroom is a resource for accessing CUP-level knowledge, not interference with L2 acquisition.
Criticisms
- Operational vagueness. Critics argue that the CUP construct is hard to measure directly and that the iceberg metaphor is more rhetorical than mechanistic.
- Range of transfer. Some studies find weaker transfer than the hypothesis predicts, particularly for typologically distant language pairs or for language pairs with very different writing systems.
- Confounded variables. Programmes that develop strong L1 literacy often differ from comparison programmes in many ways (teacher quality, parental engagement, community attitudes), making the causal contribution of L1 instruction hard to isolate.
- Political reception. Because the hypothesis underwrites a policy position on bilingual education, debates about the evidence have not always been clean of political pressure in either direction.
Place in the Field
Despite the criticisms, the Interdependence Hypothesis remains the dominant theoretical justification for L1-supported bilingual education, and the CUP model has held up better than most alternatives in subsequent neuroimaging and cognitive work showing substantial overlap in how the brain handles bilinguals' two languages. The framework also provides the conceptual anchor for the Threshold Hypothesis, with which it forms a paired account of when and how bilingualism helps or hinders cognitive development.
References
- Cummins, J. (1979). Linguistic interdependence and the educational development of bilingual children. Review of Educational Research, 49(2), 222–251.
- Cummins, J. (1981). The role of primary language development in promoting educational success for language minority students. In Schooling and Language Minority Students: A Theoretical Framework. California State Department of Education.
- Cummins, J. (2000). Language, Power and Pedagogy: Bilingual Children in the Crossfire. Multilingual Matters.
- Cummins, J. (2008). Teaching for transfer: Challenging the two solitudes assumption in bilingual education. In N. Hornberger (Ed.), Encyclopedia of Language and Education, Vol. 5 (pp. 65–75). Springer.