Is There a Direct Causal Effect of Education on Dementia? A Swedish Natural Experiment on 1.3 Million Individuals

Abstract

Background: Education is related to dementia risk. Researchers in public health often treat education as a modifiable risk factor, implying that prolonging education may reduce dementia incidence. This reasoning rests on the largely untested assumption that education has causal effects on dementia risk. Methods: We used variance in education induced by a compulsory schooling reform, which extended Swedish primary school (Folkskola) from 6 to 7 years as a tool for studying direct causal effects of education on dementia risk. The reform was implemented gradually in school districts and had minor spillover effects. We studied 18 birth cohorts (1920-1937), excluding individuals with unknown reform status, those who died, emigrated, or received dementia diagnosis prior to 65 years of age. The outcome was dementia diagnosis in the National Inpatient and Cause of Death Registers. The reform allowed us to study direct effects of prolonged education with high validity and statistical precision. In the main analyses, we employed sex-stratified Cox survival models with stratified baseline hazards at the school-district level, used chronological age as the time scale, and dummy-coded cohort effects. Findings: 1,341,842 individuals were included. The reform did not affect the risk of dementia diagnosis in a statistically significant way. The estimates and associated confidence intervals (women: HR = 1.01; 95% CI = 0.97-1.04; men: HR = 1.02; 95% CI = 0.99-1.06; all: HR = 1.01; 95% CI = 0.98-1.04) indicate that any direct effect is likely of a negligible or very small magnitude. Sensitivity analyses targeting pre-post changes, differences in healthcare-seeking behavior, and impact of exposure misclassification left results essentially unaltered. Interpretation: Longer education, at least in the absence of sizable spillover effects to adult socio-economic factors, cannot be uncritically assumed to substantially reduce the risk of moderate to severe dementia. Funding Statement: FORTE (2013-2277) and Swedish Research Council (446-2013-7189) to ML. Declaration of Interests: The authors declare no competing interests. Ethics Approval Statement: The study was approved by the Regional Ethical Review Board in Stockholm (Case number: 2010/1185-31/5).