Psychiatric comorbidity and intellectual disability

Abstract

Abstract 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 U.S. 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; and 2) an adaptive behavior score ≤76, measured using a validated scale. The Composite International Diagnostic Interview assessed fifteen 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%, and was more common among those with specific phobia (OR = 1.66, 95% C.I. 1.02, 2.68), bipolar disorder (OR = 7.24, 95% C.I. 2.10-24.99), after adjusted for demographic and clinical covariates. Among those with Axis I psychiatric disorder, adolescents with ID and mental disorders were significantly more likely to exhibit severe impairment than those without ID, across a broad range of disorders. 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.