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In:Theoretical Issues in Second Language Research: Challenges and new directions
Edited by Junya Fukuta, John Matthews and Shigenori Wakabayashi
[Studies in Bilingualism 69] 2026
► pp. 1–17

Chapter 1
Are these hypotheses really hypothesis?

A critical examination of five classic ‘hypotheses’

 | Gakushuin University
 | Chuo University
This content is being prepared for publication; it may be subject to changes.
Abstract
This paper examines critically the validity of widely cited ‘hypotheses’ from the discipline of second language acquisition (SLA) research, focusing on whether they meet the criteria to be considered hypotheses that can anchor the inquiries, investigations, and findings in a discipline that pursues to reveal how second languages are acquired/learned and used. Although many of these hypotheses were proposed decades ago, they continue to serve as theoretical reference points in both empirical studies and pedagogical discussions in SLA. Therefore, reassessing their logical and empirical status is not a matter of historical critique but a necessary step toward clarifying the conceptual foundations on which current research still depends. First, drawing on definitions of scientific theories and hypotheses, we argue that a hypothesis should generate specific, empirically testable predictions and be subject to falsification. This paper then critically revisits five widely recognized and long-standing hypotheses in SLA research: the Input Hypothesis, Output Hypothesis, Interaction Hypothesis, Noticing Hypothesis, and Involvement Load Hypothesis. The paper further argues that, even though these hypotheses have been cited for a long time, their resilience is not based on continuous empirical scrutiny but rather on the lack of essential characteristics of scientific inquiry, which has kept them inherently difficult to reject. By reevaluating these foundational concepts, we call for a more systematic and transparent approach to defining and testing hypotheses in SLA research, aiming to enhance the theoretical and practical robustness of the field.
Article outline
  • 1.What constitutes a theory and what should be considered a hypothesis
  • 2.Input Hypothesis
  • 3.Output Hypothesis
  • 4.Interaction Hypothesis
  • 5.Noticing Hypothesis
  • 6.Involvement Load Hypothesis
  • 7.Further discussion
  • 8.Conclusion
  • Author queries
  • Notes
  • References
References (36)
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