Intellectual Disability

Intellectual disability (ID) is a lifelong developmental disability defined by significant impairments in intelligence and functioning. The diagnosis of ID is based on three criteria: (1) significant limitations in intellectual functioning; (2) significant limitations [Page 814]in adaptive behavior as expressed in conceptual, social, and practical adaptive skills; and (3) onset before age 18 years. The use of an age criterion distinguishes ID from cognitive impairments that occur at later points in life, such as traumatic brain injury, mental illness, or dementia. This definition of ID was established by the American Association on Intellectual and Developmental Disabilities (AAIDD) terminology and classification committee. This entry examines the assumptions underlying the diagnosis of ID, severity levels, prevalence levels, causes, treatment options, and health care concerns.