[From the diagnostic category "mental deficiency" (ICD-6) to the diagnostic category "disorder of intellectual...

The diagnostic category Disorder of Intellectual Development in the International Classification of Diseases (ICD)-11 had precursors in all previous ICD versions from ICD‑6 onwards. In the successive ICD versions up to ICD-11, there have been considerable modifications to this diagnostic category: changes in terminology, changes in the upper limit (intelligence quotient, IQ) and internal differentiation of the category, and, especially in ICD-10 and ICD-11, additions and clarifications to the diagnostic criteria for the category. Below-average intelligence has been a central feature from the outset, despite modified IQ thresholds in some ICD versions. These IQ thresholds are repeatedly relativized as mere guidelines. Inexactitudes due to, for example, communication problems or additional impairments, may play a role. The IQ thresholds should thus not be interpreted rigidly, e.g., the ICD-10 emphasizes that the assessment of intelligence should be based on comprehensive evaluations of abilities. The criteria established in ICD-11 should enable sufficiently reliable application of the diagnostic category of intellectual development disorder.