Language proficiency & transfer
Common Underlying Proficiency & Linguistic Interdependence
A bilingual person does not run two separate engines, one per language. On the surface, two languages can look unrelated because their sounds, spellings, and grammar differ. Below the surface, though, the thinking, conceptual knowledge, and literacy abilities that power both languages draw on one shared system. A useful picture is an iceberg with two peaks above the waterline: the peaks look distinct, yet they rest on a single submerged mass. Because that base is shared, an idea worked out or a reading strategy mastered in one language is not locked inside it. Reasoning, background knowledge, and how-to-read-and-write skills become available to the other language as well, so a learner builds on them rather than rebuilding them from scratch.
Where it comes from
Jim Cummins developed this account across two foundational works. In Cummins (1979) he advanced the developmental interdependence hypothesis: how far a learner advances in a second language is shaped, in part, by the level of proficiency already reached in the first language when intensive exposure to the second begins. In Cummins (1981) he described the shared base as a common underlying proficiency, arguing that although two languages differ on the surface, their cognitive and academic dimensions operate from one interconnected store and therefore transfer across languages. That same work is closely associated with the dual-iceberg image and the principle of teaching for transfer. Wright (2019) synthesizes these ideas for classroom teachers of emergent bilinguals and serves here as a secondary source.
Why it matters for emergent bilingual students
This concept positions a student's home language as intellectual capital, not a barrier to get past. An emergent bilingual who has reasoned through fractions in Spanish, or who can already read closely and draw inferences in the home language, brings competencies that surface in English as soon as the English forms are in place. The learner is not at zero in the content; they are looking for the English to express understanding they already hold. It also helps explain why continuing to grow the home language can strengthen English rather than draining time from it, since both draw on the same underlying proficiency. When a teacher invites a student's full linguistic repertoire into the lesson, English can develop more readily while the student's identity and home language are affirmed at the same time. This aligns with the 2026 Texas ELPS premise that emergent bilinguals' existing knowledge and home language are resources every teacher draws on.
In your classroom
Plan lessons that teach for transfer rather than treating every concept as brand new in English. Open by surfacing what students already know about the topic in their home language: a quick home-language preview, partner talk in the L1, a bilingual key-terms list, or a cognate study (for example, family/familia, important/importante). Then supply the English needed to express that existing understanding, and name the cross-language connection out loud so students notice it. In a Texas ELPS setting, this is also how you deliver linguistic accommodations: leverage prior knowledge and the home language as a scaffold, differentiate by proficiency level across listening, speaking, reading, and writing, and treat home-language support as a bridge into English, not a detour.
Common misconception to avoid
A widespread but mistaken belief is that any home-language use in class steals time from English, so the room should be English-only ("maximize English exposure and the rest takes care of itself"). The interdependence research points the other way: a well-developed home-language foundation can support English and academic growth because both languages draw on one underlying proficiency. Banning the home language removes a powerful scaffold and sets aside knowledge the student already has, which tends to slow learning rather than speed it. A related error is reading a learner who is quiet in English as lacking ideas, when the ideas are present in the L1 and only the English wording is still emerging. The goal is additive: grow English alongside a maintained home language, not swap one for the other.
Research basis
How far a learner develops in a second language depends in part on the level of proficiency already reached in the first language when intensive second-language exposure begins (the developmental interdependence hypothesis).
Cummins, J. (1979). Linguistic interdependence and the educational development of bilingual children. Review of Educational Research, 49(2), 222-251. https://doi.org/10.3102/00346543049002222 [link]
Two languages operate from a single common underlying proficiency: their surface features differ, but their cognitive and academic dimensions are interconnected and transfer across languages, so developing the primary language promotes educational success in the second language.
Cummins, J. (1981). The role of primary language development in promoting educational success for language minority students. In California State Department of Education (Ed.), Schooling and language minority students: A theoretical framework (pp. 3-49). Evaluation, Dissemination and Assessment Center, California State University, Los Angeles.
Teacher-preparation synthesis of the dual-iceberg model, common underlying proficiency, and teaching for transfer for educators of emergent bilinguals.
Wright, W. E. (2019). Foundations for teaching English language learners: Research, theory, policy, and practice (3rd ed.). Caslon.
Written in our own words and grounded in Wright’s Foundations for Teaching English Language Learners and the primary theorists; reviewed by an independent SLA specialist and an adversarial citation audit.
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