Operationalization, validation, and universalization of the 2025 IDA definition of dyslexia

In October, 2025, following an international survey and review of the 2002 working definition of dyslexia, the International Dyslexia Association promulgated a revised definition stating that “Dyslexia is a specific learning disability characterized by difficulties in word reading and/or spelling that involve accuracy, speed, or both and vary depending on the orthography…..Underlying difficulties with phonological and morphological processing are common but not universal, and early oral language weaknesses often foreshadow literacy challenges.” We first describe how the moderately-correlated and hence dissociable dimensions of word reading accuracy and speed can be operationalized in any orthography (alphabetic or non-alphabetic), then present converging evidence for the validity of a dyslexia subtyping approach based on the distinction between selective deficits in word reading accuracy and word reading speed. We summarize multiple studies (in two languages with very different orthographies) showing true accuracy/speed subtypes in the strict double dissociation sense of selective impairment on one dimension alongside normative levels of performance on the other. We repeatedly find substantial numbers of specific accuracy-only disabled and speed-only disabled subtypes, as well as a doubly disabled accuracy-plus-speed subtype. We also find double dissociations on reading measures not used to define the subgroups as well as cognitive-linguistic variables. In line with the IDA definition’s reference to “morphological processing” and “early oral language weaknesses…”, the accuracy-only subgroups display a broad range of language weaknesses–primarily phonological and morphological awareness (but intact RAN), whereas the speed-only subgroups show impaired RAN alone with language skills intact. These findings call for a reappraisal of current interventions which continue to treat dyslexics as a single homogeneous group.