Most research on the prevalence, distribution, and psychiatric comorbidity of intellectual disability (ID) relies on clinical samples, limiting the generalizability and utility of ID assessment in a legal context. This study assessed ID prevalence in a population-representative sample of US adolescents and examined associations of ID with socio-demographic factors and mental disorders.
Data were drawn from the National Comorbidity Survey Adolescent Supplement (N = 6256). ID was defined as: (1) IQ ⩽ 76, measured using the Kaufman Brief Intelligence Test; (2) an adaptive behavior score ⩽76, and (3) age of onset ⩽18 measured using a validated scale. The Composite International Diagnostic Interview assessed 15 lifetime mental disorders. The Sheehan disability scale assessed disorder severity. We used logistic regression models to estimate differences in lifetime disorders for adolescents with and without ID.
ID prevalence was 3.2%. Among adolescents with ID, 65.1% met lifetime criteria for a mental disorder. ID status was associated with specific phobia, agoraphobia, and bipolar disorder, but not behavior disorders after adjustment for socio-demographics. Adolescents with ID and mental disorders were significantly more likely to exhibit severe impairment than those without ID.
These findings highlight how sample selection and overlap between ID and psychopathology symptoms might bias understanding of the mental health consequences of ID. For example, associations between ID and behavior disorders widely reported in clinical samples were not observed in a population-representative sample after adjustment for socio-demographic confounders. Valid assessment and understanding of these constructs may prove influential in the legal system by influencing treatment referrals and capital punishment decisions.
General Scientific Summary
Current definitions of intellectual disability (ID) are based on three criteria: formal designation of low intelligence through artificial problem-solving tasks, impairment in one's ability to function in his/her social environment, and early age of onset. In a national population sample of adolescents, the majority of those with ID met criteria for a lifetime mental disorder. Phobias and bipolar disorder, but not behavior disorders, were elevated in adolescents with ID. Findings highlight the need to consider how behavioral problems are conceptualized and classified in people with ID.