Intellectual disabilities are characterized by below average scores on tests of intellectual functioning as well as functional limitations in areas such as communication, self-care, and social situations. Intellectual disability is also known as cognitive disability or mental retardation. The definition of mental retardation/intellectual disability introduced in the Manual on Definition, Classification, and Support Systems in Mental Retardation defines intellectual disability as "... characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and practical adaptive skills. This disability originates before age 18."
The causes of intellectual disabilities are varied. Biomedical factors that contribute to intellectual disabilities include maternal health and genetic disorders. Social factors include the nature of family and social interactions as well as a lack of access to health care and parental neglect. The behavioral risk factors include behaviors that contribute to limited functioning, such as parental drug use. Educational factors identify accessibility to educational experiences that support adaptive skills such as inadequate family support and/or special education. Intellectual disabilities may manifest as deficits in one or more of the eight primary domains of human cognitive ability: language, reasoning, memory and learning, visual perception, auditory reception, idea production, cognitive speed, and knowledge and achievement.
The three essential elements that are used to diagnose and classify individuals within service provision systems include significant limitations in intellectual functioning, behavioral restrictions in adapting to ecological demands, and identification of symptoms before age 18. The diagnosis of an intellectual disability cannot be made solely on the basis of Intelligence Quotient (IQ) scores. The second element in the diagnosis and classification of intellectual disabilities includes deficits in adaptive functioning. Adaptive functioning refers to an individual's ability to respond to and cope with daily environmental demands. Different classifications for intellectual disabilities have emerged over the years. The most common classification scheme was based on IQ levels and was classified as mild (IQ 55-70), moderate (IQ 40-55), severe (IQ 25-40) and profound (IQ below 25). The classification systems used in schools were educable, trainable, severe and profound. Based on the intensity of support, intellectual disabilities are classified as intermittent, limited, extensive and pervasive.
Rehabilitation for persons with an intellectual disability serves to improve their skills and their level of functioning. Therapies such as special education, occupational therapy, speech therapy, physical therapy, vocational training, ADL training, social skills training and other allied forms of treatment that focus on education, rehabilitation, or habilitation improve productivity and thus improve the quality of life of persons with intellectual disabilities.