Why are educated adults slim-Causation or selection?

More educated adults tend to have lower body mass index (BMI) and a lower risk of overweight and obesity. We contrast two explanations for this education gradient in BMI. One explanation is selection: adolescents with high BMI are less likely to plan for, attend, and complete higher levels of education. An alternative explanation is causation: higher education confers lifelong social, economic, and psychological benefits that help adults to restrain BMI growth. We test the relative importance of selection and causation using data from the National Longitudinal Survey of Youth, 1997 cohort (NLSY97), which tracks self-reported BMI from adolescence (age 15) through young adulthood (age 29). Ordinal regression models confirm the selection hypothesis that high-BMI adolescents are less likely to complete higher levels of education. Selection has primarily to do with the fact that high-BMI adolescents tend to come from socioeconomically disadvantaged families and tend to have low grades and test scores. Among high-BMI girls there is also some evidence that educational attainment is limited by bullying, poor health, and early pregnancy. About half the selection of high-BMI girls out of higher education remains unexplained. Fixed-effects models control for selection and suggest that the causal effect of education on BMI, though significant, accounts for only one-quarter of the mean BMI differences between more and less educated adults at age 29. Among young adults, it appears that most of the education gradient in BMI is due to selection.