Types of Intellectual Disability
Intellectual disability is classified into four severity levels—mild, moderate, severe, and profound—based on adaptive functioning deficits across conceptual, social, and practical domains, not IQ scores alone. 1, 2
Severity-Based Classification
The primary classification system divides intellectual disability by severity of adaptive functioning impairment:
Mild Intellectual Disability: Individuals demonstrate difficulties with abstract thinking, academic learning, and executive function but can achieve independence in self-care and practical skills with appropriate supports. 1
Moderate Intellectual Disability: Marked limitations in conceptual skills with language and academic abilities significantly below age expectations; individuals require ongoing support for daily living activities but can participate in social and vocational activities with assistance. 1
Severe Intellectual Disability: Conceptual skills are limited, with minimal understanding of written language or numerical concepts; individuals require substantial daily support for all activities of daily living and close supervision. 1
Profound Intellectual Disability: Very limited conceptual skills with dependence on others for all aspects of daily physical care, health, and safety; communication is primarily nonverbal and individuals require 24-hour care. 1
Age-Specific Diagnostic Categories
For children under 5 years, use "Global Developmental Delay" rather than intellectual disability, as IQ testing is unreliable in this age group. 1, 2 This diagnosis requires significant limitations in two or more developmental domains (motor, language, social, cognitive). 1
For individuals 5 years and older where assessment is impossible due to sensory/physical impairments, severe communication difficulties, or co-occurring severe psychiatric disorders, use "Unspecified Intellectual Disability." 1, 2 This category acknowledges that the degree of impairment cannot be reliably determined despite clear evidence of intellectual disability. 1
Syndrome-Specific Types
Beyond severity classification, intellectual disability can be categorized by specific genetic or medical etiologies, each with distinct cognitive and psychiatric profiles:
Deletion 22q11.2 (DiGeorge Syndrome): Borderline to mild intellectual disability with verbal IQ higher than performance IQ; strengths in language abilities but weaknesses in receptive language, abstract reasoning, and visuospatial skills; 30% develop psychotic symptoms. 1
Smith-Magenis Syndrome (deletion 17p11.2): Moderate intellectual disability (range mild to severe) with weaknesses in short-term memory, visual-motor coordination, and sequencing; characteristic sleep disturbance with inverted circadian melatonin rhythm and self-injurious behaviors. 1
Lesch-Nyhan Syndrome (HPRT deficiency): Mild to moderate intellectual disability with chronic compulsive self-injurious behavior. 1
Critical Diagnostic Distinctions
The severity classification must be based on adaptive functioning across three domains—conceptual (academic skills, reasoning, problem-solving), social (interpersonal skills, social judgment), and practical (self-care, job responsibilities, money management)—not on IQ scores. 1, 2 This represents a fundamental shift from historical practice where IQ ranges determined severity categories.
Intellectual disability must be differentiated from specific learning disorders, communication disorders, autism spectrum disorder, and major/mild neurocognitive disorders before confirming diagnosis. 2 Unlike specific learning disorders that affect isolated academic domains, intellectual disability involves global impairment across multiple cognitive and adaptive areas. 1
Common Pitfalls in Classification
Never rely solely on IQ scores for diagnosis or severity determination—adaptive functioning determines severity level. 2 A person with IQ of 65 but strong adaptive skills in practical and social domains may function better than someone with IQ of 55 and poor adaptive skills. 1
Do not assume intellectual disability is static—cognitive and adaptive skills can improve substantially with early intervention, treatment of comorbidities (epilepsy, sensory impairments), and environmental supports, sometimes to the point where diagnosis is no longer appropriate. 1, 2 Conversely, progressive genetic disorders like Rett syndrome may cause worsening over time. 1
Consider cultural and linguistic factors, as standardized tests may significantly underestimate abilities in minority populations. 1, 2 Performance can also be affected by motivation, cooperation, physical/mental health status, and testing environment. 1