I always assumed intellectual disability was defined purely by IQ falling below a certain cutoff but I recently heard that the diagnosis involves more than just a test score. What are the actual criteria and how much does IQ matter versus other factors like adaptive functioning? And has the definition changed over time because I have seen older terms used that seem to have been replaced?
The diagnosis is more nuanced than a simple IQ cutoff and that distinction matters a lot in practice. The current clinical definition requires three things to be present simultaneously: significantly below average intellectual functioning, which is generally defined as an IQ score around 70 or below, significant limitations in adaptive functioning across conceptual, social, and practical domains, and onset during the developmental period before age 18. The adaptive functioning component is crucial because it means a low IQ score alone is not sufficient for diagnosis. Someone scoring 68 on a cognitive assessment but functioning independently in daily life, maintaining relationships, and holding employment would not necessarily meet criteria. The IQ threshold itself has also been refined over time to account for measurement error, with clinicians now typically considering a range around the cutoff rather than a hard line. The terminology has shifted significantly too, with intellectual disability replacing older terms like mental retardation which was formally retired in clinical and legal contexts over the past two decades 
The adaptive functioning requirement is the part most people do not know about and it is genuinely important 
A diagnosis is not just about what you score on a test, it is about how that cognitive profile actually affects your ability to navigate daily life. Two people with identical IQ scores can have very different functional outcomes and the diagnostic criteria are designed to capture that difference 