IQ and Mental Illness: The Evidence Shows a Complex Inverse Relationship
The evidence demonstrates that lower IQ is associated with increased risk of most psychiatric disorders, particularly schizophrenia and depression, while higher IQ does not correlate with increased mental illness—in fact, the opposite is true. 1
The Core Relationship: Lower IQ Increases Psychiatric Risk
The most robust population-based evidence comes from a 27-year Swedish cohort study of 50,087 males, which found:
- Lower premorbid IQ significantly increases risk for schizophrenia, severe depression, and other nonaffective psychoses 1
- Risk for schizophrenia is spread across the entire IQ range, with even average IQ showing increased risk compared to high IQ 1
- Notably, there is NO association between IQ (high or low) and bipolar disorder, suggesting distinct neurodevelopmental pathways 1
Bidirectional Effects in Schizophrenia
The relationship between IQ and schizophrenia operates in both directions, though asymmetrically:
- Lower intelligence causally increases schizophrenia risk (OR = 1.62) more strongly than schizophrenia causes intelligence decline (OR = 1.06) 2
- This bidirectional relationship is confirmed through Mendelian randomization analysis, supporting that low premorbid IQ is both a cause and consequence of schizophrenia 2
- Patients with schizophrenia display both impaired premorbid intelligence AND progressive intelligence decline 2, 3
The Borderline IQ Vulnerability
Individuals with borderline IQ (70-84) face particularly elevated psychiatric risk:
- 53% of young adults with borderline IQ meet diagnostic criteria for a psychiatric disorder, compared to only 12% with IQ ≥85 (OR = 6.2) 4
- This group shows excess risk specifically for ADHD and anxiety disorders 4
- The association persists even after adjusting for birth weight and socioeconomic status 4
Disorder-Specific Patterns
Schizophrenia
- 10-20% of children with early-onset schizophrenia have IQs in the borderline to intellectually disabled range 5
- Positive symptoms (hallucinations, delusions) are associated with IQs greater than 85, while negative symptoms correlate with lower IQ and brain damage 5
- Intelligence decline is most severe in schizophrenia compared to other psychiatric disorders 3
Bipolar Disorder
- No causal association exists between intelligence level and bipolar disorder risk 1, 2
- However, bipolar I disorder shows intelligence decline similar to schizophrenia, while bipolar II resembles major depression 3
- Brain abnormalities in offspring of bipolar patients are less pronounced than in offspring of schizophrenia patients, despite similar IQ reductions 6
Depression
- Lower IQ increases risk of severe depression requiring hospitalization 1
- Intelligence decline in major depression is less severe than in schizophrenia but more than in healthy controls 3
Clinical Implications
The critical pitfall is assuming higher IQ protects against or correlates with mental illness—the evidence shows the opposite. Lower cognitive functioning, whether premorbid or acquired, represents a significant vulnerability factor for most psychiatric disorders except bipolar disorder 1, 4.
When assessing psychiatric risk:
- Premorbid IQ below 85 signals substantially elevated risk for schizophrenia, depression, and anxiety 1, 4
- Cognitive testing should be part of comprehensive psychiatric evaluation, as intelligence decline may indicate disease progression 3
- Educational attainment serves as cognitive reserve, protecting against intelligence decline in schizophrenia and bipolar disorder 3
The neurodevelopmental hypothesis is supported by smaller intracranial volume in offspring of schizophrenia patients, indicating stunted early brain development independent of IQ 6.