Intellectual disability (ID), also called intellectual development disorder (IDD), general learning disability, or mental retardation (MR), is a generalized neurodevelopmental disorder characterized by significantly impaired intellectual andadaptive functioning. It is defined by an IQ score under 70 in addition to deficits in two or more adaptive behaviors that affect everyday, general living. Once focused almost entirely on cognition, the definition now includes both a component relating to mental functioning and one relating to individuals’ functional skills in their environments. As a result of this focus on the person’s abilities in practice, a person with an unusually low IQ may not be considered intellectually disabled. Intellectual disability is subdivided intosyndromic intellectual disability, in which intellectual deficits associated with other medical and behavioral signs and symptomsare present, and non-syndromic intellectual disability, in which intellectual deficits appear without other abnormalities. Down syndrome and fragile X syndrome are examples of syndromic intellectual disabilities.
Intellectual disability affects about 2–۳% of the general population. 75–۹۰% of the affected people have mild intellectual disability. Non-syndromic or idiopathic cases account for 30–۵۰% of cases. About a quarter of cases are caused by a genetic disorder.Cases of unknown cause affect about 95 million people as of 2013.
The terms used for this condition are subject to a process called the euphemism treadmill. This means that whatever term is chosen for this condition, it eventually becomes perceived as an insult. The terms mental retardation and mentally retarded were invented in the middle of the 20th century to replace the previous set of terms, which were deemed to have become offensive. By the end of the 20th century, these terms themselves have come to be widely seen as disparaging, politically incorrect, and in need of replacement. The term intellectual disability is now preferred by most advocates and researchers in most English-speaking countries. As of 2015, the term “mental retardation” is still used by the World Health Organization in the ICD-10 codes, which have a section titled “Mental Retardation” (codes F70–F79). In the next revision, the ICD-11 is expected to replace the term mental retardation with eitherintellectual disability or intellectual developmental disorder, which the DSM-5 already uses. Because of its specificity and lack of confusion with other conditions, the term “mental retardation” is still sometimes used in professional medical settings around the world, such as formal scientific research and health insurance paperwork.