Lag Effects in L2 Phonetic Training: Longer Intersession Intervals Compensate for Auditory Individual Differences

In cognitive psychology, longer intersession intervals are thought to enhance long‐term retention (i.e., lag effects). Recently, scholars have begun to test the generalizability of the lag effect to L2 vocabulary and grammar acquisition. Following their step, the present study not only replicated the lag effect but also provided a more fine‐grained insight into L2 phonetic acquisition, investigating how individual differences at both perceptual (auditory processing [AP]) and cognitive (working memory) levels moderate its impact. Sixty‐nine adult Mandarin learners of English participated, including two experimental groups—either a 1‐ or 7‐day intersession interval—receiving 45 min of high variability phonetic training (HVPT) on the English vowels [i]–[ɪ] (e.g., bean vs. bin) and a control group completing tests only. A pretest–posttest–delayed posttest design revealed two key findings. First, while both schedules yielded comparable immediate gains, only the 7‐day group demonstrated significantly more durable retention. Second, a significant aptitude‐treatment interaction emerged. While auditory acuity predicted short‐term gains in both groups, its predictive power for long‐term retention was confined to the 1‐day group. This suggests that a longer spacing interval might compensate for individual differences in AP, especially in long‐term retention, making phonetic training more accessible to a wider range of learners.