Intellectual Disability (ID), formerly called mental retardation, is characterized by significant limitations in both intellectual functioning (reasoning, learning, problem-solving) and adaptive behavior (conceptual, social, and practical skills used in everyday life). ID originates before age 22 and is diagnosed through standardized IQ testing (typically scoring below 70–75) combined with adaptive behavior assessments such as the Vineland Adaptive Behavior Scales. Severity levels — mild, moderate, severe, and profound — describe the degree of support needed. ID can co-occur with autism; when both are present, the individual may qualify for a broader range of waiver services. Causes include genetic conditions (Down syndrome, Fragile X), prenatal exposure to toxins, birth complications, and environmental factors.
Intellectual disability is a qualifying condition for virtually every HCBS waiver and state developmental disability system in the country. An ID diagnosis often opens doors to services that an autism diagnosis alone may not, including ICF/IID eligibility and self-determination programs.