Subaverage intellectual ability that is present from birth or infancy and is manifested by abnormal development, learning difficulties, and problems in social adjustment. A standardized intelligence test is a common method of identification. Individuals with IQ scores of 53–70 are usually classified as having mild intellectual disability and are able to learn academic and prevocational skills with some special education. Those with scores of 36–52 are classified as having moderate intellectual disability and are able to learn functional academic skills and undertake semiskilled work under supervised conditions. Those in the severe (21–35) and profound (below 21) ranges require progressively more supervision or full-time custodial care. Intellectual disability can be caused by genetic disorders (such as Down syndrome), infectious diseases (such as meningitis), metabolic disorders, poisoning from lead, radiation, or other toxic agents, injuries to the head, and malnutrition.
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