Fundamental Difference Hypothesis
The Fundamental Difference Hypothesis (FDH), proposed by Robert Bley-Vroman (1989, sharpened 1990), claims that child first-language acquisition and adult second-language acquisition are driven by qualitatively different mechanisms. Children acquire their L1 through a domain-specific language faculty, fixing the parametric values of Universal Grammar from exposure to input. Adults, by contrast, have lost access to that machinery and fall back on domain-general problem-solving applied to L2 input, sometimes supplemented by what they already know about their L1.
The Two Modes
| Child L1 | Adult L2 | |
|---|---|---|
| Primary mechanism | Domain-specific language faculty (UG) | Domain-general problem-solving |
| Route to grammar | Parameter-setting from input | Construction of a surrogate grammar by analogy, inference, and L1 transfer |
| Typical outcome | Convergence on native grammar | Variable, often incomplete, with fossilisation |
The hypothesis explains why adult learners can be vastly more intelligent and motivated than children yet routinely fail to reach native-like attainment: the cognitive resources they bring to the task are not the ones designed for it.
The 2009 Restatement
Bley-Vroman's "Evolving Context of the FDH" in Studies in Second Language Acquisition dropped the requirement that L1 acquisition use a fully encapsulated domain-specific module. The restated position focuses on qualitative differences in how child and adult learners process linguistic input, using overlapping but differently weighted cognitive resources. The core claim — that adult L2 is not the same kind of thing as child L1 — survives in weaker form.
Relation to the Critical Period Hypothesis
The FDH is a theoretical cousin of the critical-period hypothesis but makes a different move. CPH asks when the relevant machinery shuts down; FDH asks what is still available afterwards and how it works. The two are compatible and often cited together in age-of-acquisition research.
Empirical Status
The hypothesis is a frequent reference point rather than a tested theory. Work by Schwartz and Sprouse on Full Transfer/Full Access argues against it: if adult learners converge on native grammars for any phenomenon, UG must still be accessible. Emergentist and usage-based accounts (Nick Ellis, William O'Grady) reject the FDH's dualism altogether, arguing that the same learning mechanisms explain both L1 and L2 and that apparent differences are matters of input, attention, and prior linguistic knowledge rather than access to a special module.
References
- Bley-Vroman, R. (1989). What is the logical problem of foreign language learning? In S. Gass & J. Schachter (Eds.), Linguistic Perspectives on Second Language Acquisition. Cambridge University Press.
- Bley-Vroman, R. (1990). The logical problem of foreign language learning. Linguistic Analysis, 20(1–2), 3–49.
- Bley-Vroman, R. (2009). The evolving context of the Fundamental Difference Hypothesis. Studies in Second Language Acquisition, 31(2), 175–198.