Intellectual disability (ID) recognized since ancient times; education and care developments began in the 1800s with definitional measures and assessment of intelligence.
Prevalence data (examples):
In 2016, $1.14\%$ of U.S. children aged 13–17 had a diagnosed ID; no significant change from the prior two years. (CDC data cited: Zablotsky, Black, & Blumberg, 2017).
Boys and children aged 8–12 years show higher prevalence; no race/ethnicity differences reported in rates. (Zablotsky et al., 2017).
Education eligibility (IDEA): about $6\%$ of the $6.7$ million U.S. students ages 3–21 served under ID eligibility. (National Center for Education Statistics, Institute of Education Sciences, & U.S. Department of Education, 2018).
International prevalence (2010–2015 meta-analysis by McKenzie et al. 2016):
Children/adolescents: $0.22\%$ to $1.55\%$ (US estimates on the high end).
Adults: $0.05\%$ (Australia) to $0.8\%$ (Canada).
Global prevalence varies slightly due to assessment and sampling methods.
References: Kamphaus & Walden (eds.) 2020; Assessment of disorders in childhood and adolescence. Guilford Publications.
Risk Factors and Causes
ID arises from a mix of biological and environmental risks across developmental stages (DSM-5, American Psychiatric Association, 2013).
Fragile X and Down syndrome are common genetic contributors; Down syndrome accounts for about $6$–$8\%$ of ID cases.
Other rarer genetic disorders (Kabuki syndrome, Schinzel–Giedion syndrome) also cause ID.
Perinatal risk factors include:
Traumatic birth and neonatal hypoxia (Modabbernia, Mollon, Boffetta, & Reichenberg 2016) leading to brain damage.
Postnatal/environmental risk factors include:
Toxin exposure, infectious diseases, seizures, head injuries/traumatic brain injury, severe social isolation, and severe neglect.
Early environmental trauma can increase ID risk; timing matters for developmental impact.
Clinical implication: clinicians should identify causes when relevant to tailor services, but multiple etiologies can co-occur and none guarantees ID.
Context: emphasize that ID etiologies refer to a constellation of interacting factors, not a single cause.
Comorbidity
Comorbidity with ID is common and clinically important because symptoms can co-occur and influence assessment and treatment.
Platt et al. (2018): large, nonclinical, representative US adolescent sample (6,000 youths 13–18) showed:
ID prevalence: 3.2% in the sample.
Among those with ID, 65.1% had symptoms suggestive of a comorbid mental illness.
After controlling socioeconomic factors (low income, low parental education, fewer biological caregivers), ID was comorbid with bipolar disorder, specific phobia, and agoraphobia.
ID associated with greater severity across anxiety disorders, ADHD, bipolar disorder, various phobias, and substance use disorders.
Specific comorbid disorders commonly discussed:
Autism Spectrum Disorder (ASD): ID-ASD comorbidity rates notable; CDC findings show varied prevalence across sites; among 8-year-olds, 31% had IQ ≤ 70, and 25% had IQ 71–85 (borderline for ID).
Among ASD samples, over 18% also had ID in larger ASD studies (Levy et al., 2010; Tonnsen et al., 2016).
Differentiating ID from ASD is challenging due to overlapping symptoms; practitioners should assess both to avoid misdiagnosis and to plan treatment.
Attention-Deficit/Hyperactivity Disorder (ADHD): ID comorbidity around 3.5% (vs. 3.9% in those without ADHD) (Neece et al., 2011; Platt et al., 2018).
Ahuja et al. (2013) found ADHD+ID cases have lower IQ than ID-alone and higher rates of oppositional defiant disorder and conduct disorder; ADHD symptoms present in both groups, but burden higher in comorbidity.
Earlier ADHD diagnosis is more common in children with ID than in typically developing peers (Neece et al., 2011).
Internalizing disorders (e.g., anxiety, depression): ID associated with higher rates; Rodas et al. (2017) found 10\% high-degree internalizing symptoms, 5\% clinically significant in 4–8-year-olds with ID (mother/father reports).
Platt et al. (2018): 13.2\% with major depressive episode or dysthymia; 6.0\% with generalized anxiety disorder vs. 12.7\% and 3.2\% in those without ID respectively.
Reardon, Gray, & Melvin (2015) systematic review: anxiety disorders in 3–22\% of children with ID; cautions about measurement validity in ID populations.
Contextual considerations:
Environment can amplify or mask comorbid symptoms; trauma exposure is a key factor to assess.
Traumatic experiences are more common in youth with ID/ASD/ID comorbidity; higher maltreatment risk in ASD/ID groups (McDonnell et al., 2019; Byrne, 2018).
Trauma screening tools: TSCC (Briere, 1996), UCLA PTSD Reaction Index (Steinberg et al., 2004), Social Behavior Inventory (Gully, 2003).
Adults with ID show substantial undiagnosed mental illness risk (Peña-Salazar et al., 2018): 29.6% could be diagnosed with a mental illness that had not previously been documented.
Assessment, Diagnosis, and Classification
Core framework (AAIDD): ID is significant limitations in intellectual functioning and adaptive behavior, with onset before age 18.
DSM-5 (American Psychiatric Association, 2013) defines ID through intellectual deficits, with adaptive behavior deficits as central; standard score cutoff commonly used around 70 (M=100, SD=15 or 16).
Some state education regulations (IDEIA) allow considering measurement error; scores around 67–73 may qualify or disqualify depending on adaptive behavior and age of onset.
Practical diagnostic principles:
Intellectual deficit is best established using multiple measures; deficits in adaptive behavior are essential for functional impairment judgment.
IQ tests are not perfectly valid in isolation; adaptive behavior and history are crucial.
Table 13.1 (id-diagnosis examples): illustrative cases using intelligence test composites, adaptive behavior composites, onset age, and whether ID diagnosis is present (Yes/No).
Case 1: IQ 55, Adaptive 62, onset 27, ID: No.
Case 2: IQ 60, Adaptive 81, onset 2, ID: No.
Case 3: IQ 84, Adaptive 63, onset 7, ID: No.
Case 4: IQ 58, Adaptive 59, onset 2, ID: Yes.
AAIDD guidelines for cognitive assessment (Schalock et al., 2010):
Use global measures including diverse item types/factors of intelligence.
Valid IQ requires minimizing motor, sensory, emotional, or cultural biases.
If valid IQ cannot be obtained, rely on adaptive-based assessment and be cautious about subaverage functioning.
Implications for practice:
Screening measures are discouraged for ID diagnosis; rely on comprehensive, valid cognitive assessment when possible;
In extreme cases, adaptive measures may be used to supplement cognitive assessment.
Rule out nonintellective causes for low scores.
Role of clinical judgment: combine developmental history, test scores, adaptive behavior, and contextual information; avoid rigid cutoffs.
Writing diagnostic rationales: document the rationale behind ID diagnosis when composite scores differ; enables peer review and better justification.
Critique of rigid usage: rigid cutoff scores can misrepresent a child’s profile; be mindful of alternative explanations (language, culture, SES, test range, etc.).
Case example: Daniel Hoffman (Payne & Patton, 1981) illustrates misdiagnosis risk when relying solely on IQ scores.
Relationship between intelligence and adaptive behavior: historically modest correlations between IQ tests and Vineland AB scales; adaptive behavior adds unique information; teacher vs. parent ratings may differ in correlation strength (Kamphaus, 1987).
AAIDD criteria emphasize three domains for services need: Conceptual, Social, and Practical Skills; health and environment are to be systematically assessed.
Regression effects in ID diagnoses:
Intelligence tests show regression toward the normative mean; Spitz (1983) demonstrated mean changes over time in ID groups; potential to misclassify if relying on a retest alone.
Possible causes for retest gains: initial testing under suboptimal conditions, practice effects, cognitive development, or effective interventions.
If retest gains are observed, must corroborate with achievement and adaptive behavior scores.
The range of scores issue:
Many tests floor around 45–50; DAS is a notable exception with lower-range norms.
At very low levels, adaptive behavior becomes more critical than scholastic intelligence; some advocate using adaptive measures as primary for differentiating levels of disability.
The Bayley scales offer infant-level measures; debates about using infant scales with older children showing developmental delay.
Some argue adaptive behavior scales (e.g., Vineland) are more informative for intervention planning than IQ alone.
Selecting a score for ID diagnosis:
When there are discrepancies among composite scores (e.g., VC=78, V/P=65) and Full Scale=70, with adaptive composite around 70, ID is more likely.
Complex cases (e.g., VC=60, V/P=88, FSIQ=72, Adaptive=71) require clinical reasoning: consider language, acculturation, SES, and daily living skills.
Avoid rigid cutoffs; provide justification based on developmental history, language, acculturation, adaptive behavior, etc.
Limitations of intelligence tests for severe/profound ID:
Floor effects and limited differentiation among moderate, severe, and profound levels.
Adaptive behavior scales (Vineland AB) provide lower-bound and more actionable information for intervention planning.
In very severe cases, adaptive skills may better reflect functioning than intelligence tests; some advocate for using infant-like measures or content areas aligned with adaptive demands.
Regulatory concerns and ethics:
Some agencies require a recent IQ score to justify placement or benefits; overreliance on a single score can produce misdiagnoses.
AAIDD guidance emphasizes integrated assessment across adaptive behavior, emotional functioning, health, and environment.
Cautions against diagnosing ID from scores alone; document rationale; avoid using intelligence tests in isolation.
Assessment during treatment—overview: comorbidity considerations demand ongoing assessment, screening, and progress monitoring to tailor interventions.
Elements of a Comprehensive Assessment of Intelligence
History taking: essential to document onset prior to age 18; IQ/adaptive behavior tests alone cannot establish prior disability; caregiver interviews are crucial.
Intellectual and adaptive behavior assessment are necessary to document the two core deficits of ID.
Widely used measures (brief overview):
Individually administered intelligence tests provide the core diagnostic information; modern tests are designed to differentiate ID from psychiatric conditions and to capture cognitive profiles.
Tests discussed (as exemplars, not exhaustive):
Wechsler Intelligence Scale for Children—Fifth Edition (WISC-V): ages 6–16; 21 subtests; 10 primary; domains: Fluid Reasoning, Verbal Comprehension, Processing Speed, Visual Spatial, Working Memory; 60–90 minutes; examiner must be qualified; includes an iPad version.
Wechsler Abbreviated Scale of Intelligence—Second Edition (WASI-II): ages 6–90; 4- or 2-subtest forms; produces Overall Cognitive Ability, Crystallized Abilities, Nonverbal Fluid Abilities, Visual–Motor/Coordination; 15–30 minutes; screening utility; can rule out ID if scores in average range or higher.
Woodcock–Johnson Tests of Cognitive Abilities—Fourth Edition (WJ-IV-COG): ages 2 to 90+; 10 primary subtests (8 extended battery options); measures specific cognitive aspects; 60–90 minutes; part of a larger battery (achievement, oral language) with materials from Houghton Mifflin Harcourt; Canivez (2017) review.
Reynolds Intellectual Assessment Scales—Second Edition (RIAS-2): ages 3–94; 45 minutes; four primary tests covering Verbal and Nonverbal domains; intended to indicate risk for ID and is useful for individuals with mental illness.
Universal Nonverbal Intelligence Test—Second Edition (UNIT-2): nonverbal measure intended to approximate general intelligence; composite scores include Memory, Reasoning, Quantitative, Abbreviated/Standard batteries; useful for verbal impairment due to brain injury, cerebral palsy, autism with language challenges, or limited English speakers; entirely nonverbal format.
Adaptive Behavior Assessment:
Vineland Adaptive Behavior Scales, Third Edition (Vineland-3): domains include Daily Living Skills, Socialization, Communication; optional Motor Skills and Maladaptive Behavior; structured interview and/or rating forms; age range birth through 90 years; administration ~40 minutes for structured interview; scoring ~10 minutes; requires examiners with relevant master’s degree or higher; materials from Pearson.
Adaptive Behavior Assessment System—Third Edition (ABAS-3): domains: Social, Practical, Conceptual; forms completed by parents/caregivers across age bands; teachers complete forms for 2–21 years; self-report option for 16–89 years; rating forms ~15–20 minutes; includes planning tools and online formats; materials from Western Psychological Services.
Practical implications:
Adaptive behavior scales capture everyday functioning across multiple domains and are essential for determining need for supports and services.
The Vineland-3 and ABAS-3 provide complementary data to IQ, guiding intervention planning.
The Vineland lineage (Vineland Social Maturity Scale) and the move toward adaptive-based assessment reflect the field’s shift toward functional supports rather than purely cognitive metrics.
Personal anecdote (Lucy Kamphaus): illustrates the difference between IQ and adaptive behavior in real-world settings; underscores need for comprehensive assessment beyond IQ.
Levels of ID
Traditional criterion: standard score < 70 on intelligence testing has long been used to define ID (Flynn, 1985).
Flynn (1985) argued that a single cutoff is unstable due to sampling and norming differences; suggested that norms would need re-norming every 7 years to maintain a stable cutoff.
Contemporary practice generally uses a composite score around 70 as a cutoff, but clinicians are cautioned against rigid use; consider adaptive behavior, onset, and background information.
The differentiation of ID levels (mild, moderate, severe, profound) historically relied on both IQ and adaptive behavior; newer AAIDD approach emphasizes needs across domains, including health and environment, rather than fixed levels.
DSM-5 still uses levels (mild, moderate, severe, profound) for diagnostic description, with adaptive behavior functioning often guiding level determination.
Selecting a score when there are discrepancies across indices (e.g., VC vs V/P) requires clinical reasoning and context; do not rely solely on a single composite.
A case example demonstrates the complexity of decisions when some but not all scores fall in ID range; contextual variables (age, language, SES, acculturation) influence the diagnostic decision.
Conclusion: while a standard cutoff remains common, practitioners should integrate history, adaptive behavior, and test profiles to justify ID diagnoses.
Selecting a Score for ID Diagnosis
When a child has disparate index scores (e.g., VC=78, V/P=65) with Full Scale around 70 and adaptive scores around 70, an ID diagnosis is more plausible given evidence of both intellectual and adaptive deficits.
In ambiguous cases (e.g., VC=60, V/P=88, FSIQ=72, Adaptive=71), clinicians must weigh language background, acculturation, SES, learning opportunities, and adaptive functioning (e.g., Daily Living Skills on Vineland) to determine diagnosis.
Guidance for decision-making:
Do not apply rigid cutoffs; use developmental history, test scores, adaptive behavior, and contextual factors to justify the diagnostic decision.
Document the rationale for the diagnosis, enabling peer review and transparency.
If needed, explain decision to other professionals who know the child’s background to enhance defensibility.
Caution: use of intelligence tests in isolation for ID diagnosis is discouraged; administrative or regulatory requirements that mandate a recent IQ score should be balanced with holistic assessment.
Analogy: relying solely on IQ is like using only LDL cholesterol to assess heart disease risk; other factors (HDL, total cholesterol, triglycerides) are needed for a full assessment.
Regression and Range of Scores in ID Assessment
Regression toward the mean can affect retest IQ scores; example Spitz (1983): ID mean moved from 55 to 58 between ages 13 and 15.
Diagnostic implications:
A small score increase (e.g., 68 to 74) may reflect regression, practice effects, or developmental gains; may not indicate true diagnostic change.
If overall cognitive/achievement/adaptive behavior scores move with the retest (e.g., all above 70), ID diagnosis may be less likely; persistent deficits across domains support diagnosis.
Cumulative deficit phenomenon (Haywood, 1987): impoverished environments can lead to ongoing lower performance relative to peers even if cognitive abilities increase; observed in delayed MA relative to CA.
The cumulative effect means raw scores may increase with age, but standard scores lag behind due to accelerated normative expectations.
Implication: longitudinal interpretation must consider growth trajectories, not just retest scores.
The Range of Scores Issue
Floor effects: many tests do not differentiate well among moderate, severe, and profound ID; typical floor around 45–50; DAS is an exception with lower-range norms.
Adaptive behavior becomes crucial at lower functioning levels; Vineland AB scales can produce meaningful scores even when IQ is severely impaired.
The content domain of infant tests (e.g., Bayley) has been proposed for very young or severely impaired individuals, but adult adaptation remains debated.
Some argue that adaptive behavior scales may be the most clinically useful for differentiating levels of ID and for planning interventions, more so than traditional IQ scores in severe cases.
The field generally agrees adaptive behavior should be central to diagnosis and treatment planning, especially for moderate to profound levels of disability.
Regulatory practice sometimes requires a recent IQ score; the AAIDD advocates a broader, more integrated diagnostic approach.
Assessment during Treatment
Given high comorbidity, ongoing assessment is essential to identify and monitor symptoms and to tailor interventions.
Screening for comorbidity: essential for early detection, using rating scales and interviews; not sufficient for diagnosis by itself.
Rating scales commonly used:
Behavior Assessment System for Children, Third Edition (BASC-3): caregiver/teacher/self-rating across internalizing/externalizing, anxiety, depression, daily living skills, etc.; ~10–30 minutes; helps inform intervention severity but not diagnostic conclusions.
Caution: results depend on normative samples; consider whether children with ID were represented in norming samples.
Screening limitations: measures validated for general populations may not be as valid for children with ID; clinicians should interpret results with caution and consider multi-method approaches.
Screening for anxiety with ID: Reardon, Gray, & Melvin (2015) identified 13 measures; some scales are broad (e.g., CBCL), others include accommodations for ID; samples vary in severity; more targeted research is needed.
Progress Monitoring:
Goal: track symptoms over time to assess intervention effectiveness and determine need for changes.
Measures should be sensitive to change and target specific behaviors; broad measures may miss meaningful progress.
Rating scales can be used for progress monitoring (e.g., BASC-3 Flex Monitor) with periodic administration (every 3–6 months, or longer, depending on the measure).
Practical constraints exist (teacher/patient time); frequent daily assessments may be impractical.
Alternative methods include progress-monitoring sheets and direct observation.
Direct Behavioral Observations:
Critical for understanding behavior in natural settings and informing FBA.
BASC-3 Student Observation System provides a structured observation tool; observations help identify functional aspects of behavior.
Observations must be used in conjunction with other measures; consider comorbidity in interpretations.
Multimethod approach:
Combine rating scales, interviews, observations, and performance-based assessments to form a comprehensive view.
Pay attention to context, including SES, caregiver support, and home/school environments, which influence behavior and test performance.
Traumatic experiences and ID:
Trauma screeners (TSCC, UCLA PTSD Index, Social Behavior Inventory) help disentangle trauma effects from ID-related behaviors.
Trauma exposure can influence presentation and comorbidity patterns; include trauma history in assessment when relevant.
Behavioral and Functional Assessments
Functional Behavioral Assessment (FBA):
Identifies function of problem behaviors (e.g., attention seeking, escape from demands, access to tangibles).
Involves data collection via observations and interviews to define antecedents, behavior, and consequences (ABC data).
More intensive and time-consuming than screening; particularly useful for targeted interventions.
For ID populations, FBA often yields valuable information about comorbid symptoms and environmental contributors.
When psychological symptoms are present, FBA can be complemented by screening to inform interventions and possible alternative treatments.
Evidence supports FBA effectiveness in reducing severe problem behaviors in ID (e.g., Wadsworth et al., 2015; Doehring et al., 2014).
Contextual factors for FBA:
Socioeconomic status, family structure, and caregiving arrangements influence behavior and intervention feasibility (Platt et al., 2018).
Contextual data improves function-based interpretations and informs whether behaviors stem from ID, environment, or dual influences.
Performance‑Based Assessment:
Assesses daily living skills through direct tasks and caregiver/teacher reports.
Sparrow et al. (2016) Vineland-3 and other tools assess daily living skills in real-life contexts.
Tasks might include: library book retrieval procedures, preparing meals, morning dressing, etc.
Observational protocols with explicit steps provide unbiased documentation of task performance; informs intervention planning.
Shepley et al. (2018) and Wynkoop et al. (2018) provide evidence for using performance-based measures in ID populations.
Measures and Key Concepts Summary (tests and domains)
IQ tests and domains (conceptual summary):
WISC-V: 6–16 years; 21 subtests; 10 primary; five domains; administration ~60–90 min; includes iPad version.
WASI-II: 6–90 years; 4- or 2-subtest forms; provides Overall Cognitive Ability and domain scores; 15–30 min; screening utility; can rule out ID if scores are average+.
WJ-IV-COG: 2–90+ years; 10 primary subtests; 60–90 min; part of a broader battery.
RIAS-2: 3–94 years; 45 min; Verbal and Nonverbal indices; screen for ID risk; designed for people with mental illness.
UNIT-2: nonverbal; useful for language-impaired populations (e.g., autism) or limited English speakers; full batteries available.
Adaptive behavior measures:
Vineland-3: birth–90 years; five domains including Daily Living Skills, Socialization, Communication; structured interview and rating forms; administration ~40 min (interview), 10 min (rating); Pearson materials.
ABAS-3: birth–89 years; three domains; parent/teacher/self-report across multiple age bands; 15–20 min; planning tools included; Western Psychological Services.
Key concepts and terminology:
Adaptive behavior vs. intelligence: adaptive behavior focuses on daily living, socialization, and practical skills; many cases show only partial overlap with IQ.
Levels of ID (mild, moderate, severe, profound) historically tied to both IQ and adaptive functioning; AAIDD emphasizes supports needs across domains; DSM-5 retains levels tied to adaptive function; practice varies by setting.
Heuristic for interpretation: avoid single-score decisions; integrate history, adaptive behavior, and cognitive profiles.
Do not rely on IQ alone to diagnose ID; integrate adaptive behavior and developmental history; explain diagnostic rationale in writing for peer-review and accountability.
Be mindful of the limits of intelligence tests, especially for severe/profound ID; adapt assessment strategies accordingly (e.g., use adaptive behavior measures as primary indicators when appropriate).
Consider environmental and trauma factors as potential contributors to behavioral presentations and comorbidity; implement trauma-informed assessment as needed.
Use multi-method approaches (rating scales, interviews, observations, performance tasks) to identify comorbidity and to guide intervention planning.
Be cautious about regulatory requirements that demand a recent IQ score; advocate for comprehensive assessment that informs a person-centered treatment plan.
Early identification and access to diagnostic, prevention, and intervention services are critical for better long-term outcomes; limited service provision remains a major barrier in many contexts.
Mathematical and Conceptual References (LaTeX snippets)
Intelligence criterion for ID (typical):
extStandardscore≈70(M=100,SD=15extor16)
In practice, a standard score around 70 or lower is used as a diagnostic threshold, subject to adaptive behavior and onset age.
IQ formula historically used in ratio IQs (less common today):
extIQratio=CAMA×100
Diagnostic criteria and domain considerations (adaptive behavior domains per AAIDD):
Lesion/trauma screening tools (examples): TSCC, UCLA PTSD Reaction Index, Social Behavior Inventory; used to screen trauma-related symptoms in children with ID.
Example case scores (illustrative, from Table 13.1):
Case 1: IQ composite=55,Adaptive composite=62,Age of onset=27,ID=No
Case 4: IQ composite=58,Adaptive composite=59,Age of onset=2,ID=Yes
Connections to Foundational Principles and Real-World Relevance
Historical context: ID concepts date back to ancient times; modern assessment integrates cognitive and adaptive domains consistent with contemporary disability models.
Foundational principle: intelligence is a robust construct but not sufficient alone to determine functional disability; adaptive behavior and environmental context are essential to the diagnosis and service planning.
Real-world relevance: accurate ID diagnosis enables access to services, supports, and interventions that can improve daily living, educational outcomes, and overall quality of life; misdiagnosis can deny needed supports or promote inappropriate placements.
Ethical implications: avoid stigmatization and ensure fair assessment across diverse populations; transparency in diagnostic reasoning; advocate for comprehensive, person-centered evaluation rather than rigid cutoffs.
Key References (selected from the transcript)
Zablotsky et al. (2017); McKenzie et al. (2016); Platt et al. (2018); Neece et al. (2011); Rodas et al. (2017); Reardon et al. (2015); Tonnsen et al. (2016); Levy et al. (2010); Tonnsen et al. (2016).