Objective: This study examines the influence of cognitive function and change in cognitive function during adolescence on schizophrenia and bipolar disorders in adulthood and explores possible mechanisms for any associations.
Method: A cohort of 6923 men born in Copenhagen, Denmark in 1953, who had completed assessments of cognitive performance (measured by a total test score that combines verbal, arithmetic and spatial functions) at ages 12 and 18, were followed from 1972 until 2002. Psychiatric outcomes (hospital admissions for schizophrenia and bipolar disorders) were obtained from the Danish Psychiatric Register.
Results: During the follow-up period 133 of the men had a discharge diagnosis of schizophrenia or schizophrenia spectrum disorder. Cognitive function measured at both ages 12 and 18 years was inversely associated with these disorders (unadjusted hazard ratio (HR) per SD of cognitive function at age 12: 0.87 (95% CI: 0.75,1.05) and at age 18: 0.72 (95% CI: 0.60,0.86)). Adjustment for birth weight, social circumstances during childhood, indicators of social integration and educational attainment at age 18 years changed these associations only slightly. When cognitive function measured at ages 12 and 18 were entered in the same model only the latter was inversely associated with risk of schizophrenia or spectrum disorder. Decline in cognitive function between ages 12 and 18 was associated with an increased risk of schizophrenia spectrum disorder (HR: 1.32 (95% CI: 1.01,1.75)). Cognitive function at both ages also showed an inverse association with bipolar disorders, though the association was imprecisely estimated due to small numbers (N=16).
Conclusions: Low cognitive function at ages 12 and 18 years and cognitive decline between these two ages were associated with increased risk of schizophrenic disorders. This suggests that reduction in cognitive function during childhood may be an early symptom of these outcomes.