DSM-5 Diagnostic Criteria for Intellectual Disability
The DSM-5 requires three core criteria to diagnose intellectual disability: (1) deficits in intellectual functioning confirmed by both clinical assessment and standardized testing, (2) deficits in adaptive functioning that result in failure to meet standards for personal independence and social responsibility, and (3) onset during the developmental period. 1
The Three Required Criteria
Criterion A: Intellectual Deficits
The first criterion requires identifying deficits in intellectual functioning, specifically in:
- Reasoning and problem solving
- Planning and abstract thinking
- Judgment in novel situations
- Academic learning
- Learning from experience
This criterion must be confirmed by BOTH clinical assessment AND standardized intelligence testing 1. However, the DSM-5 explicitly de-emphasizes reliance on IQ scores alone, noting that IQ test scores are merely "approximations of conceptual functioning" and may be insufficient to assess real-world reasoning and practical skills 1. Individual neuropsychological profiles are more clinically useful than a single IQ composite score 1.
Criterion B: Adaptive Functioning Deficits
The second criterion requires deficits in adaptive functioning across three specific domains:
Conceptual domain: Reading, writing, language, mathematics, knowledge, memory, problem solving, and judgment in novel situations 1
Social domain: Awareness of others' thoughts and feelings, social judgment, interpersonal communication skills, and social problem solving 1
Practical domain: Self-management across life settings including personal care, responsibilities, transportation, finances, school, and daily activities 1
The individual must fail to meet standards for personal independence and social responsibility across multiple environments (home, school, work, community) 1. Importantly, standardized adaptive behavior scales alone do not capture all functional domains—clinical assessment is mandatory 1.
Criterion C: Developmental Period Onset
The deficits must have onset during the developmental period 1. The age of presentation depends on severity:
- Severe ID: Delayed motor, language, and social milestones may be identified in the first 2 years of life
- Mild ID: May not be identified until school age when academic learning difficulties become apparent 1
Critical Diagnostic Nuances
Special Considerations by Age
For children under 5 years: IQ measures are not considered reliable. Use the diagnosis of "Global Developmental Delay" instead, defined by significant limitations in two or more developmental domains 1
For individuals over 5 years: When assessment is rendered difficult or impossible due to sensory/physical impairment, communicative difficulties, locomotor disability, or severe behavioral/psychiatric problems, use "Unspecified Intellectual Disability" 1
Assessment Pitfalls to Avoid
Performance on standardized testing can be significantly underestimated by:
- Cultural and linguistic minority status (testing instruments may lack sensitivity) 1
- Motivation, cooperation, interest, temperament, and behavior at time of testing
- Physical and mental health status
- Test setting and examiner attitude
- Communication, sensory, and motor factors 1
Clinical training and judgment are essential to interpret test results appropriately, particularly for individuals with severe ID or language impairment where test reliability is lower 1.
Severity Determination
Unlike previous DSM editions, severity is NOT determined by IQ scores but rather by adaptive functioning levels across the three domains (conceptual, social, practical) 1. This represents a fundamental shift from a psychometric disability model to a clinical disorder model 2.
Ongoing Reassessment Required
A diagnosis of intellectual disability requires continued reassessment—federal law mandates re-evaluation at least every 3 years in school-aged children 1. The condition is not static; adaptive functioning can improve with interventions or worsen with progression of certain genetic disorders 1.