Intellectual disability is a disorder marked by below average intellectual function and a lack of skills necessary for independent daily living. The condition begins in the developmental period.
The general mental abilities that are examined to diagnose intellectual disability include reasoning, problem solving, planning, abstract thinking, judgment, learning from instruction and experience, and practical understanding. These abilities are measured using individually administered tests of intelligence by a trained clinician. Additionally, people with intellectual disability may struggle with the skills needed to function in daily life, such as communication, social participation, and independent living without ongoing support.
While previous versions of the DSM defined severity of intellectual disability by IQ score, severity is now defined by the ability to meet the demands of daily life, as compared with peers. Severity of intellectual disability is categorized as mild, moderate, severe, or profound. Education, job training, support from family, and individual characteristics such as motivation and personality can all contribute to the ability of individuals with intellectual disability to adapt to the demands of everyday life.
Other behavioral traits associated with intellectual disability (but not deemed criteria for a diagnosis) include aggression, dependency, impulsivity, gullibility, passivity, self-injury, stubbornness, low self-esteem, low frustration tolerance, and high risk of suicide. It is common for people with intellectual disability to have co-occurring mental, neurodevelopmental, medical, and physical conditions. For example, other mental disorders and epilepsy are three to four times higher in people with intellectual disability than in the general population. If a genetic condition has caused the intellectual disability, a person may also have the characteristic physical features of that condition (as in Down syndrome).
Intellectual disability affects about 1 percent of the population, and prevalence for severe intellectual disability is approximately six per 1,000 people, according to the DSM-5.