The American Association on Intellectual and Developmental Disabilities defines intellectual disability (ID) as the following:
- Substantial limitations in intellectual functioning
- Significantly limitations in adaptive behavior consisting of conceptual, social, and practical skills;
- Originating before age 18 ( AAIDD, 2008)
- Intellectual disability is concerned with IQ measures, intellectual functioning, language development and social behavior.
The severity of ID has been summed up in the following table:
| Severity | IQ Range | % of MR | Characteristics |
| Mild | 52-68 | 89 | Usually live and work independently within regular community. Often have families. |
| Moderate | 36-51 | 6 | Capable of some semi-independence at work and in residence. As adults, many work in supportive environments and live with relatives or in community residences. |
| Severe | 20-35 | 3.5 | Capable of learning some self care skills and are not totally dependent. Some adults are able to work in supportive environment and live with relatives or community residences. |
| Profound | 19 and below | 1.5 | Capable of learning some basic living skills but will require continual care and supervision. Often exhibit multiple handicap |