Individual differences in language learning and teaching: Growing pains at the theory-methods interface

The role of individual differences (IDs) in second language acquisition (SLA) is well established, with two dedicated journals, a biannual conference, book series, handbooks, and a professional organization all focused on this area. However, these structural and visible signs of growth may present an overly optimistic view of the domain's maturity. As we argue in this paper, a closer look at IDs in SLA reveals significant concerns in theory, methods, and their interface, with construct validity emerging as a critical issue. Despite a large body of empirical research, relatively few IDs have been sufficiently theorized or rigorously validated, leading to exploratory analyses, susceptibility to HARKing, and other questionable research practices (see Hiver & Al-Hoorie, 2020; Isbell et al., 2022). Methodologically, while recent efforts at scale validation (e.g., Botes et al., 2021; Teimouri, 2018) are commendable, a lack of systematic validation and insufficient attention to construct clarity undermine the field's theoretical robustness and empirical reliability (Papi & Teimouri, 2024; Sudina, 2021, 2023a). Furthermore, as shown in other domains of SLA (e.g., Plonsky, 2023), research on IDs often overlooks understudied and underserved populations, limiting both its generalizability and social utility. This paper addresses these critical issues and their impact on the field's contributions to theory and practice, offering specific, actionable recommendations to guide future research. Educational relevance: This study highlights the critical importance of construct validity in Individual Differences (IDs) research within Second Language Acquisition (SLA). We argue that failing to validate constructs rigorously leads to theoretical ambiguities, unreliable findings, and misleading conclusions. In addition, replicating studies without ensuring construct validity risks reinforcing conceptual flaws rather than advancing scientific knowledge. By prioritizing construct validation in SLA, educators, researchers, and policymakers can rely on more accurate assessments of learner differences, leading to better-informed language learning interventions and teaching strategies.