The definition under Section 300.8 of the Individuals with Disabilities Education Act (IDEA) for intellectual disability is “significantly subaverage general intellectual functioning, existing concurrently with deficits in adaptive behavior and manifested during the developmental period, that adversely affects a child’s educational performance.” Clinically, the diagnostic criteria of intellectual disability in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), is very similar to IDEA: “a substantially below-average score on tests of mental ability or intelligence, and limitations in the ability to function in areas of daily life.”
In simpler terms, intellectual disability (ID) is diagnosed when a person has significant limitations in both intellectual functioning and adaptive behavior.
Intellectual functioning or intelligence refers to general cognitive capacity, such as learning, reasoning, and problem-solving. This is usually measured by an IQ test, where a below-average score can indicate a significant limitation in intellectual functioning. In general, scores of between 80 and 120 on an IQ test are considered average in the United States. Scores of 70 or 75 are sometimes given as the cutoff for determining possible intellectual disability.
For more information, see our full article Intellectual Disability 101.
