["TESOL Quarterly, EarlyView. ", "\nAbstract\nQuantitative meta‐syntheses focusing on the psychometric properties of measurement tools in second and foreign language (L2) individual differences (ID) research have drawn scholars' attention to the quality of data collection instruments (Plonsky & Derrick, 2016; Sudina, 2023). In many primary L2 ID studies systematically reviewed to date, little attention has been given to instrument validity, which may lead to unsubstantiated inferences and unjustifiable conclusions. Although some attempts have been made to raise awareness of the importance of validity evidence for certain constructs (e.g., language learning motivation; see Al‐Hoorie, Hiver, & In'nami, 2024; Papi & Teimouri, 2025), an overarching theoretical and methodological validation framework that L2 ID researchers could rely on appears to be missing. In our theoretical paper, we aim to provide scaffolding for measurement in L2 ID studies by drawing on argument‐based validation (ABV) practices in other areas of applied linguistics (Chapelle, 2021; Kane, 2013; for a recent synthesis, see Zhang & Aryadoust, 2022). For illustrative purposes, we used a quantitative measure with documented psychometric characteristics. Thus, we hope to provide empirically based guidelines for L2 ID researchers on reporting information about the validation process and validity evidence for their psychometric tools.\n"]