Language learning aptitude and working memory in L2 acquisition: The role of proficiency and structure difficulty

Research on language aptitude has empirically demonstrated its facilitative role in second language learning (L2) outcomes. However, relatively few studies have examined how these effects evolve with increasing L2 proficiency, and even fewer have explored the role of linguistic structure difficulty as a moderating factor. This thesis investigates whether the influence of language aptitude remains stable or changes in learners at different proficiency levels. It also examines the impact of linguistic structure difficulty and addresses the long-standing question of the interface between implicit and explicit knowledge by analysing the relationship between aptitude and knowledge of targeted morphosyntactic structures. Eighty-six L1 Croatian learners of English completed the LLAMA aptitude battery and a serial reaction time task to assess aptitude for explicit and implicit learning, as well as forward digit span and operation span tasks to assess phonological and executive working memory, respectively. Reading and listening proficiency was assessed via the Oxford Placement Test, while speaking proficiency was operationalized as complexity, accuracy and fluency measured in an oral production task. L2 knowledge of selected English morphosyntactic structures of varying difficulty – articles, passive, and the simple past – was measured using a self-paced reading task, elicited imitation, and gap-fill tests. The thesis explores the componential structure of language aptitude and working memory, concluding that both implicit and explicit aptitude are multicomponential constructs. Notably, implicit aptitude is shown to differ from explicit aptitude with its components pulling in opposite directions and is reconceptualized as a cognitive proclivity. The findings reveal that the facilitative effects of language aptitude shift with proficiency: explicit aptitude is more relevant at lower proficiency levels, while implicit aptitude becomes increasingly important at advanced levels. Additionally, the study demonstrates a bidirectional interface between implicit and explicit aptitude and L2 knowledge. This means that implicitly learned knowledge can lead to explicit knowledge, while explicitly learned knowledge can facilitate the development of implicit knowledge. The moderating role of structure difficulty is confirmed, highlighting its influence on the effects of working memory and explicit aptitude, though not implicit aptitude. In sum, the findings confirm facilitative effects of both explicit and implicit aptitude. Moreover, they position aptitude as a dynamic concept. The evidence for a dynamic interface between explicit and implicit knowledge highlights the interplay between learning processes, suggesting that the traditional question of whether the interface exists may be outdated.