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.