High IQ in Early Adulthood Is Associated with Parkinson’s Disease

BACKGROUND: High education level and high occupational complexity have been implicated as risk factors for Parkinson’s disease (PD). OBJECTIVE: The objective was to determine whether cognitive capacity, measured as IQ, in early adulthood is associated with the subsequent development of PD. METHOD: Data on IQ were retrieved from the Swedish Military Service Conscription Registry, comprising Swedish males who enlisted for military service in the period 1968–1993 (N = 1,319,235). After exclusion, 1,189,134 subjects in total were included in the present study. Individuals who later developed PD (N = 1,724) were identified using the Swedish National Patient Register and the Swedish Cause of Death Register. RESULTS: High education level was associated with PD. High IQ was associated with PD (p < 0.0001), both when analyzed as a continuous variable and when divided into three categories. The hazard ratio for the high IQ category compared to the low IQ category was 1.35 (95% confidence interval 1.17–1.55). Strong test results on the subtests, measuring verbal, logic, visuospatial and technical abilities, were also associated with PD. In a subgroup, smoking was inversely associated with PD, as well as with IQ. CONCLUSIONS: This study identifies high IQ to be a risk factor for PD.