Fundamental Difference Hypothesis
The Fundamental Difference Hypothesis (FDH), articulated by Robert Bley-Vroman (1989, 1990), holds that adult second language acquisition is fundamentally different in nature from child first language acquisition. The difference is not merely one of degree — it is qualitative. Children acquire language through domain-specific mechanisms (Universal Grammar and the Language Acquisition Device); adults, having lost access to these mechanisms, must rely on general problem-solving abilities and explicit learning strategies.
The Argument
Bley-Vroman (1989) identified several characteristic differences between child L1 and adult L2 acquisition:
| Feature | Child L1 | Adult L2 |
|---|---|---|
| Guaranteed success | Universal — all children achieve full competence | Variable — most adults do not reach native-like proficiency |
| Fossilization | Does not occur | Common (see Fossilization) |
| Role of instruction | Not required | Often helpful or necessary |
| Role of affective factors | Minimal | Significant (see Affective Filter) |
| Role of correction | Little effect | Can be beneficial (see Corrective Feedback) |
| Sensitivity to input quality | Robust — acquisition proceeds with impoverished input | Fragile — input quality and quantity matter greatly |
Theoretical Basis
The FDH rests on the nativist assumption that L1 acquisition is guided by Universal Grammar — an innate system of linguistic principles and parameters. Bley-Vroman's claim is that UG ceases to be directly available after the critical period, forcing adults to compensate with:
- L1 knowledge — adults use their existing L1 grammar as a surrogate for UG
- General problem-solving — analogy, hypothesis testing, pattern recognition, memorisation
This creates a fundamentally different learning process: where children set parameters unconsciously through exposure, adults must consciously construct grammatical knowledge from input using cognitive strategies that were not designed for language acquisition.
The Ongoing Debate
The FDH generated extensive debate, structured around three positions on UG access in adult L2:
- No access (Bley-Vroman's strong position) — UG is unavailable; adults rely entirely on L1 transfer and general cognition
- Partial access — some UG principles remain active, but parameter resetting is constrained
- Full access — UG remains fully available; L2 difficulties stem from performance factors, not competence limitations (see Identity Hypothesis)
Bley-Vroman (2009) revisited the hypothesis, acknowledging that the picture is more nuanced than a binary distinction but maintaining the core claim that child and adult acquisition differ in fundamental ways.
Implications for Teaching
If the FDH is correct, several consequences follow for L2 pedagogy:
- Explicit grammar instruction is not just helpful but potentially necessary for adults
- Corrective Feedback serves a function in adult L2 that it does not serve in child L1
- The expectation of native-like attainment as a default outcome is unrealistic for most adult learners
- Teaching methods modelled on child L1 acquisition (e.g., pure immersion with no explicit focus) may be insufficient for adults