The Global Landscape of Vocabulary Complexity Research: A Bibliometric Analysis (2015–2025)

Abstract. Vocabulary complexity remains a pivotal yet under-mapped construct within reading comprehension and language proficiency, where complex lexis often renders essential information inaccessible to large segments of the public. This study provides the first comprehensive quantitative science-mapping of vocabulary complexity research over the past decade, rigorously clarifying how the field has evolved and identifying where critical empirical gaps remain. A systematic bibliometric analysis was conducted on studies retrieved via Harzing’s Publish or Perish using the keyword “Vocabulary Complexity” for the 2015–2025 period (initial n=200). By shifting the analytical focus from traditional qualitative synthesis to macro-level bibliographic data, this research minimizes selection bias and provides an objective, data-driven overview. After excluding conference papers, books, and non-relevant items via PRISMA screening, the remaining 17 records were exported as a RIS file and analyzed with VOSviewer to generate keyword co-occurrence, overlay, and density visualizations, alongside annual publication trends and a ranking of the ten most-cited articles. The co-occurrence network revealed four thematic clusters: (1) morphological awareness, (2) syntactic complexity, (3) task complexity, and (4) lexical richness, which linked these internal linguistic features with external outcomes such as writing accuracy, reading comprehension, and EFL performance. Overlay and density maps illustrated a significant temporal shift from broad structural constructs toward applied centers such as “lexical complexity,” “vocabulary knowledge,” and “comprehension.” Annual publication outputs indicated a maturing field with cyclical peaks in 2018, 2020, and 2024. Citation analysis highlighted a decisive move from text-focused concepts of word difficulty toward multidimensional, pedagogical, and technology-mediated perspectives. Findings depict a rapidly consolidating research area were lexical and morphological complexity function as key predictors of learner proficiency. However, notable gaps persist in longitudinal designs, non-English and AI-mediated contexts, predictive modeling, and empirical work on threshold effects where increased complexity inadvertently impedes comprehension. Keywords: Bibliometric Analysis; Lexical Complexity; Morphological Awareness; Reading Comprehension; Vocabulary Acquisition