Elsevier

Annals of Epidemiology

Volume 26, Issue 1, January 2016, Pages 71-76.e3
Annals of Epidemiology

Original article
Instrumental variable approaches to identifying the causal effect of educational attainment on dementia risk

https://doi.org/10.1016/j.annepidem.2015.10.006Get rights and content

Abstract

Purpose

Education is an established correlate of cognitive status in older adulthood, but whether expanding educational opportunities would improve cognitive functioning remains unclear given limitations of prior studies for causal inference. Therefore, we conducted instrumental variable (IV) analyses of the association between education and dementia risk, using for the first time in this area, genetic variants as instruments as well as state-level school policies.

Methods

IV analyses in the Health and Retirement Study cohort (1998–2010) used two sets of instruments: (1) a genetic risk score constructed from three single-nucleotide polymorphisms (SNPs; n = 7981); and (2) compulsory schooling laws (CSLs) and state school characteristics (term length, student teacher ratios, and expenditures; n = 10,955).

Results

Using the genetic risk score as an IV, there was a 1.1% reduction in dementia risk per year of schooling (95% confidence interval, −2.4 to 0.02). Leveraging compulsory schooling laws and state school characteristics as IVs, there was a substantially larger protective effect (−9.5%; 95% confidence interval, −14.8 to −4.2). Analyses evaluating the plausibility of the IV assumptions indicated estimates derived from analyses relying on CSLs provide the best estimates of the causal effect of education.

Conclusions

IV analyses suggest education is protective against risk of dementia in older adulthood.

Section snippets

Methods

The Health and Retirement Study (HRS) is a national, longitudinal study of individuals 50 years of age or older and their spouses. The first survey wave was collected in 1992, with biennial interviews (or proxy interviews for decedent participants) available through 2010. New cohorts were added in 1993, 1998, 2004, and 2010. We used follow-up data from 1998–2010 and includes individuals from all enrollment cohorts except 2010. Survey response rates ranged from 70% to 82%, and retention rates

Results

Table 1 shows the demographics of the analytic sample used for the two IV analyses. Using the 1980 census, 5% sample in the first stage, compulsory schooling laws (CSLs), school to work laws (CSL-w), and school characteristics were significantly associated with years of schooling. F statistics, an indication of the strength of the instruments for the excluded instrument, were well above 10 (Table 2). The linear regression coefficients for the effects of CSLs and school characteristics on years

Discussion

The IV estimates indicate that education lowers dementia probability. These results are consistent with previous IV literature reporting a protective effect of education on cognitive function [10], [11], [12]. Education may be an indicator of cognitive reserve, with higher educational attainment indicating larger reserve to delay the onset of dementia [27], [28].

In the IV analyses, we used state of birth and year of birth to classify the compulsory schooling required for each participant. There

Acknowledgments

E.J.T.T.’s work was funded by the National Institutes of Health (NIH) grants AI113251 and ES020337. S.E.G.’s work was supported by the Intramural Research Program of the Eunice Kennedy Shriver National Institute of Child Health and Human Development

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    None of the authors declare any conflicts of interest.

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