Decoding and vocabulary improvements mediate sustained gains in reading comprehension: Evidence from a randomised controlled trial of a multicomponent reading intervention.

Reading comprehension is a foundational academic skill that underpins learning across subject areas and predicts long‐term educational success (Snow, ). The Simple View of Reading (SVR; Gough & Tunmer, ) and the Reading is Language model (RiL; Snowling & Hulme, ) provide influential frameworks for understanding how comprehension develops. Both models propose that reading comprehension reflects the combined contributions of word decoding and oral language comprehension. A substantial body of evidence supports the predictive roles of both components across languages, including English (Catts, Adlof, & Weismer, ; Keenan, Betjemann, & Olson, ; Salceda, Alonso, & Castilla‐Earls, ) and more transparent orthographies such as Dutch, Finnish, Greek and Norwegian (Caravolas et al., ; de Jong & van der Leij, ; Florit & Cain, ; Kendeu, Papadopoulos, & Kotzapoulou, ; Lervåg, Hulme, & Melby‐Lervåg, ; Müller & Brady, ). Cross‐linguistic research highlights the role of orthographic depth in shaping this balance. In opaque orthographies such as English, decoding is relatively difficult to master and strongly predicts comprehension in the early years, whereas in transparent orthographies decoding is mastered earlier, allowing language comprehension to play a larger role from a younger age (Caravolas, Lervåg, Defior, Seidlová Málková, & Hulme, ; Caravolas et al., ; Florit & Cain, ; García & Cain, ; Leppänen et al., ; Verhoeven, van Leeuwe, & Vermeer, ; Wimmer & Goswami, ). Even in English, however, this balance shifts developmentally: decoding constrains comprehension early on, but as fluency develops, language comprehension becomes the primary driver (Adlof, Catts, & Lee, ; Kim, Wagner, & Foster, ; Tilstra, McMaster, Van den Broek, Kendeou, & Rapp, ). For struggling readers, this trajectory is disrupted. Persistent decoding difficulties limit fluency and restrict the cognitive resources available for higher‐order comprehension processes (Leach, Scarborough, & Rescorla, ; Ouellette & Beers, ). Longitudinal research shows that decoding difficulties often persist into later grades, constraining comprehension longer than for typically developing peers (Catts, ; Torppa et al., ). Decoding also remains a significant predictor of comprehension growth in poor readers after it ceases to be predictive in more skilled readers (Lervåg & Hulme, ). Lervåg et al. () further demonstrated a curvilinear relationship, with decoding exerting the strongest effects for children with the lowest skills and weakening once sufficient accuracy and fluency are achieved.