One-third of American mothers do not breastfeed their newborns, and three-quarters of those who do breastfeed introduce formula before their babies reach 6 months. Most studies of infant feeding conclude that breast milk is superior to infant formula in nearly all situations. Yet, many of these studies suffer from the weakness of selection bias, that is, they do not account for differences between mothers who choose to breastfeed and those who don't—differences that might affect qualities attributed to breastfeeding, such as intelligence. A new study of siblings, which reduces this selection bias, provides persuasive evidence of a causal connection between breastfeeding and intelligence.
University of California, Berkeley researchers Eirik Evenhouse, Ph.D., and Siobhan Reilly, Ph.D., supported in part by the Agency for Healthcare Research and Quality (HS00086), analyzed data from the first wave (1994) of the National Longitudinal Study of Adolescent Health. They examined the relationship between breastfeeding history and 15 indicators of physical health, emotional health, and cognitive ability among 16,903 adolescents, including 2,734 sibling pairs. Nearly all of the correlations found in the between-family model became insignificant in the within-family model. The exception was a persistent positive correlation between breastfeeding and cognitive ability, that is, siblings who were breastfed had higher cognitive ability than those who were not. The findings were similar whether breastfeeding was measured as a yes/no or in terms of duration.
The effect was large enough to matter, and it was lasting, persisting into adolescence. This significant correlation provides the strongest nonexperimental evidence to date that breastfeeding improves cognitive ability. The results also suggest, however, that many of the other long-term effects of breastfeeding have been overstated. The researchers call for sibling studies with larger sample sizes.
See "Improved estimates of the benefits of breastfeeding using sibling comparisons to reduce selection bias," by Drs. Evenhouse and Reilly in the December 2005 HSR: Health Services Research 40(6), pp. 1781-1802.