Second language (L2) anxiety has been proposed to play a causal role in L2 achievement. However, most studies have failed to acknowledge confounding variables that may be relevant to the study of anxiety and L2 achievement or to investigate the causal effect of L2 anxiety using longitudinal data. For these reasons, we investigated the effect of L1 reading achievement, L2 aptitude, and L2 anxiety as covariates on the growth of L2 reading achievement across three time points. We used the latent growth curve model (LGCM) to estimate the growth trajectory of US secondary school students’ L2 reading growth in Spanish (N = 307) over three school years. The findings showed that students’ L1 reading achievement and L2 aptitude strongly and significantly predicted L2 reading achievement growth. However, L2 anxiety did not predict L2 reading achievement growth. Findings suggest that growth in L2 reading achievement depends on the language-related skills used for L1 reading and the language skills that comprise L2 aptitude, but not on anxiety. Similar to past cross-sectional studies, L2 anxiety related only to initial levels of L2 reading achievement, suggesting that anxiety reflects students’ initial experience of L2 reading but not their L2 achievement.

Measurement invariance (MI) is essential to bolstering validity arguments behind psychometric instruments (Zumbo, 2007). Nonetheless, very few second language (L2) anxiety scales, including the most widely used L2 anxiety questionnaire—the Foreign Language Classroom Anxiety Scale (FLCAS; Horwitz et al., 1986)—have been tested for MI. The present paper seeks to address this deficiency in the literature (a) by demonstrating why this procedure is key to enhancing our understanding of the latent phenomenon in question, particularly in relation to different language learning contexts, (b) by outlining the main stages of MI testing with specific recommendations for L2 scale developers and users, (c) by providing commendable examples of the application of MI in applied linguistics research in order to illustrate the potential of this technique, and (d) by making a case for employing MI in future validation studies, thereby promoting methodologically sound research practices in the context of anxiety scales and elsewhere in applied linguistics.