Intellectual and Developmental Disabilities: Definitions, Diagnostic Limitations, and Cross-System Classification

Intellectual and Developmental Disabilities (IDDs) represents a broad, cross-disciplinary construct applied in literature across healthcare, education, and social policy to describe lifelong neurodevelopmental conditions, which emerge during the developmental period. However, the absence of IDD as a formal diagnostic category within major nosological systems has contributed to conceptual ambiguity, inconsistent assessment practices, and challenges in interdisciplinary communication. The authors of this paper critically examine the conceptual foundations of IDD, distinguishing the umbrella term from the core diagnosis of intellectual disability (ID) and related developmental conditions such as Global Developmental Delay (GDD) and specific developmental disorders (SDDs). Drawing on contemporary diagnostic frameworks, the authors evaluate the limitations of the Diagnostic Statistical Manual-5th Edition-Text Revision (DSM-5-TR) in identifying psychiatric comorbidities in individuals with IDDs and highlights the role of the Diagnostic Manual-Intellectual Disability-2nd Edition (DM-ID-2), as a necessary diagnostic companion. The Cognition-Conation-Affect-Sensation (CCAS) model is introduced as an integrative lens for understanding the multidimensional functioning of individuals with IDDs beyond deficit-based interpretations. A comparative analysis of DSM-5-TR and the International Classification of Diseases-11th Revision (ICD-11) classifications further clarifies areas of alignment and divergence in the conceptualization of ID, GDD, and developmental delays (DDs). Overall, the authors argue that conceptual clarity, developmentally informed diagnostics, and systems-sensitive frameworks are essential for accurate diagnosis, effective intervention planning, and equitable support across clinical and educational settings.