Is education causally related to better health? A twin fixed-effect study in the USA.

BACKGROUND: More years of schooling is generally associated with better health. However, this association may be confounded by unobserved common prior causes such as inherited ability, personality such as patience, or early family circumstances. The twin fixed-effect approach can potentially address this problem by cancelling these factors between twin pairs. The purpose of this study is to identify the causal effects of education on health and health behaviours using a twin fixed-effect approach.
METHODS: We used twin data from the National Survey of Midlife Development in the United States, 1995-1996. The study population included 302 male [55.6% monozygotic (MZ) and 44.4% dizygotic (DZ)] and 387 female twin pairs (47.3% MZ and 52.7% DZ). A range of health outcomes [perceived global, physical and mental health, body mass index (BMI), waist circumference, waist-hip ratio, number of depressive symptoms] and health behaviours (smoking and physical activity) were examined among twin pairs who were discordant on years of schooling.
RESULTS: Among MZ twins, more years of education was associated with better perceived global health. For all other health outcomes/behaviours, the point estimates of the effect of education in the fixed-effect analyses suggested a weak protective association. Among DZ male twins, each additional year of schooling lowered the prevalence of smoking by 32% [odds ratio (OR): 0.68, 95% confidence interval (CI): 0.48-0.97] in the fixed-effect analysis.
CONCLUSION: The widely reported associations between schooling and health outcomes/behaviours may not reflect causal relationships in every instance. Although low statistical power may explain some of the null associations, our twin fixed-effect analyses suggest that at least some cases of the education/health relationship reflect confounding by unobserved third variables.