DEFINITION

• Intellectual disability (ID), or intellectual developmental disorder, is a disorder originating during the developmental period that is characterized by significant limitations in both:
  • Intellectual functioning (i.e. general mental capacity such as learning, reasoning, and problem solving) and
  • Adaptive behavior (i.e. conceptual, social, and practical adaptive skills)

• ID is classified under the Neurodevelopmental Disorders section of the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5)[1].
  • DSM-5 does not use intelligence quotient (IQ) cutoffs for severity level of ID (mild, moderate, severe, profound), but rather impairments in conceptual, social, and practical domains.
    
    • For more information on how DSM-5 handles this cutoffs, please see the DSM-5 itself[1].
• ID replaces the DSM-IV term "mental retardation" (MR)[2].
  • In this prior multiaxial system, the diagnosis of mental retardation was placed on Axis II.
  • IQ cutoffs for mental retardation in DSM-IV were: mild (IQ 50-55 to ~70), moderate (IQ 35-40 to 50-55), severe (IQ 20-25 to 35-40), and profound (IQ < 20-25).