Synergy across multiple systems: a PPCT-based case study of heritage language maintenance in a Thai Chinese family

Thailand is a key Southeast Asian site for examining Chinese heritage language (CHL) maintenance. Although prior heritage language (HL) research has shown that transmission is rarely determined by a single factor, the mechanisms through which multiple influences jointly produce divergent outcomes remain insufficiently specified. Drawing on the bioecological Process-Person-Context-Time (PPCT) framework, this qualitative-dominant case study integrates interviews with descriptive questionnaire measures to explain why, under a broadly shared family language policy promoting Chinese learning, only the youngest of six fourth-generation sisters in a Thai Chinese family developed sustained, advanced Mandarin proficiency. Findings show that her CHL trajectory resulted from the cumulative alignment of high-quality proximal processes, generative personal characteristics, multi-level contextual coordination, and favourable historical and life-course timing. The study demonstrates that CHL maintenance depends less on static inputs than on sustained, meaningful proximal processes regulated by contextual coordination, temporal continuity, and learners’ active agency. It advances a process-oriented account of HL maintenance and illustrates the explanatory value of the PPCT framework for explaining within-family differentiation in HL trajectories.