Does more education cause lower BMI, or do lower-BMI individuals become more educated?
More educated adults have lower average body mass index (BMI). This may be due to selection, if adolescents with lower BMI attain higher levels of education, or it may be due to causation, if higher educational attainment reduces BMI gain in adulthood. We test for selection and causation in the National Longitudinal Survey of Youth 1979, which has followed a representative US cohort from age 14–22 in 1979 through age 47–55 in 2012. Using ordinal logistic regression, we test the selection hypothesis that overweight and obese adolescents were less likely to earn high school diplomas and bachelor's degrees. Then, controlling for selection with individual fixed effects, we estimate the causal effect of degree completion on BMI and obesity status. Among 18-year-old women, but not among men, being overweight or obese predicts lower odds of attaining higher levels of education. At age 47–48, higher education is associated with lower BMI, but 70–90% of the association is due to selection. Net of selection, a bachelor's degree predicts less than a 1 kg reduction in body weight, and a high school credential does not reduce BMI.