The connection between working memory (WM) and the breadth of vocabulary knowledge (BVK) in foreign language young learners remains underexplored, particularly with respect to how these constructs co-develop across the primary school years. Although growth in WM has been linked to early gains in BVK, the directionality and temporal dynamics of their association are not well understood. Utilizing a cross-lagged panel design, this study tracked the development of WM and BVK in 158 young learners from grade 1 to grade 5. Results unveiled lagged associations between WM and BVK, suggesting that working memory serves as a valuable indicator for future BVK acquisition, while also indicating that accumulated BVK may, in turn, exert an influence on WM. These findings highlight a complex, bi-directional relationship between WM and BVK throughout primary school students’ formative years, in line with the transactional model.
