Association of Fluid Intelligence and Psychiatric Disorders in a Population-Representative Sample of US Adolescents.

Importance: Despite long-standing interest in the association of psychiatric disorders with intelligence, few population-based studies of psychiatric disorders have assessed intelligence.

Objective: To investigate the association of fluid intelligence with past-year and lifetime psychiatric disorders, disorder age at onset, and disorder severity in a nationally representative sample of US adolescents.

Design, Setting, And Participants: National sample of adolescents ascertained from schools and households from the National Comorbidity Survey Replication-Adolescent Supplement, collected 2001 through 2004. Face-to-face household interviews with adolescents and questionnaires from parents were obtained. The data were analyzed from February to December 2016. DSM-IV mental disorders were assessed with the World Health Organization Composite International Diagnostic Interview, and included a broad range of fear, distress, behavior, substance use, and other disorders. Disorder severity was measured with the Sheehan Disability Scale.

Main Outcomes And Measures: Fluid IQ measured with the Kaufman Brief Intelligence Test, normed within the sample by 6-month age groups.

Results: The sample included 10 073 adolescents (mean [SD] age, 15.2 [1.50] years; 49.0% female) with valid data on fluid intelligence. Lower mean (SE) IQ was observed among adolescents with past-year bipolar disorder (94.2 [1.69]; P = .004), attention-deficit/hyperactivity disorder (96.3 [0.91]; P = .002), oppositional defiant disorder (97.3 [0.66]; P = .007), conduct disorder (97.1 [0.82]; P = .02), substance use disorders (alcohol abuse, 96.5 [0.67]; P < .001; drug abuse, 97.6 [0.64]; P = .02), and specific phobia (97.1 [0.39]; P = .001) after adjustment for a wide range of potential confounders. Intelligence was not associated with posttraumatic stress disorder, eating disorders, and anxiety disorders other than specific phobia, and was positively associated with past-year major depression (mean [SE], 100 [0.5]; P = .01). Associations of fluid intelligence with lifetime disorders that had remitted were attenuated compared with past-year disorders, with the exception of separation anxiety disorder. Multiple past-year disorders had a larger proportion of adolescents less than 1 SD below the mean IQ range than those without a disorder. Across disorders, higher disorder severity was associated with lower fluid intelligence. For example, among adolescents with specific phobia, those with severe disorder had a mean (SE) of 4.4 (0.72) points lower IQ than those without severe disorder (P < .001), and those with alcohol abuse had a mean (SE) of 5.6 (1.2) points lower IQ than those without severe disorder (P < .001).

Conclusions And Relevance: Numerous psychiatric disorders were associated with reductions in fluid intelligence; associations were generally small in magnitude. Stronger associations of current than past disorders with intelligence suggest that active symptoms of psychiatric disorders interfere with cognitive functioning. Early identification and treatment of children with mental disorders in school settings is critical to promote academic achievement and long-term success.