Abstract Understanding how second language (L2) proficiency develops in instructional settings remains a central concern within applied linguistics. This two-year longitudinal study examines English language development in a higher education context in Mexico, addressing key methodological and empirical gaps. We tracked L2 development—operationalized as changes in the TOEFL ITP® scores—across three time points and examined a wide range of variables, including the often-neglected factor of socio-economic status (SES). Results from linear mixed effects modelling and sensitivity analyses showed statistically significant, though small, gains in proficiency. Baseline proficiency emerged as a strong predictor of learning trajectories, with lower-proficiency students showing greater relative improvement. SES, contact with English, and age of onset were the most consistent predictors of L2 performance. Mediation analysis indicated that SES influences proficiency both directly and indirectly, primarily through contact with English. However, much of the SES effect remained unexplained, suggesting broader systemic influences. These findings shed light on hidden disparities in language education and offer insights for educational policy, highlighting the need to address structural barriers to equitable L2 development.
Khabbazbashi et al. (Fri,) studied this question.