Improved Estimates of the Benefits of Breastfeeding Using Sibling Comparisons to Reduce Selection Bias

Eirik Evenhouse, University of California, San Francisco
Siobhan Reilly, Mills College

We use sibling comparisons to reduce selection bias in measurement of the benefits of breastfeeding. Using data on 2,734 sibling pairs in the 1994 Add Health survey, we examine the relationship between breastfeeding history and fifteen indicators of physical health, emotional health, academic performance, and cognitive ability. For each indicator, we estimate a within-family model to see whether differences in siblings’ outcomes correlate with differences in the siblings’ breastfeeding histories. Nearly all of the correlations found in a between-family model become statistically insignificant in our within-family model. The notable exception is a persistent positive correlation between breastfeeding and cognitive ability. These findings hold whether breastfeeding is measured in months or a Yes/No variable. They offer persuasive evidence of a causal connection between breastfeeding and intelligence. However, they also suggest that non-experimental studies of breastfeeding overstate some of the other long-term benefits, even with the inclusion of controls for socioeconomic characteristics.