Intelligence and breast feeding

Studies examining whether breastfeeding in infants is associated with higher intelligence later in life include:
 * Horwood, Darlow and Mogridge (2001) tested the intelligence quotient (IQ) scores of 280 low birthweight children at seven or eight years of age. Those who were breastfed for more than eight months had verbal IQ scores 6 points higher (which was significantly higher) than comparable children breastfed for less time. They concluded "These findings add to a growing body of evidence to suggest that breast milk feeding may have small long term benefits for child cognitive development."
 * A 2005 study using data on 2,734 sibling pairs from the National Longitudinal Study of Adolescent Health "provide[d] persuasive evidence of a causal connection between breastfeeding and intelligence."
 * In 2006, Der and colleagues, having performed a prospective cohort study, sibling pairs analysis, and meta-analysis, concluded that "Breast feeding has little or no effect on intelligence in children." The researchers found that "Most of the observed association between breast feeding and cognitive development is the result of confounding by maternal intelligence."
 * The 2007 review for the AHRQ found "no relationship between breastfeeding in term infants and cognitive performance."
 * The 2007 review for the WHO concluded "Subjects who were breastfed experienced... higher performance in intelligence tests."
 * Two initial cohort studies published in 2007 suggest babies with a specific version of the FADS2 gene demonstrated an IQ averaging 7 points higher if breastfed, compared with babies with a less common version of the gene who showed no improvement when breastfed. FADS2 affects the metabolism of polyunsaturated fatty acids found in human breast milk, such as docosahexaenoic acid and arachidonic acid, which are known to be linked to early brain development.  The researchers were quoted as saying "Our findings support the idea that the nutritional content of breast milk accounts for the differences seen in human IQ. But it's not a simple all-or-none connection: it depends to some extent on the genetic makeup of each infant." The researchers wrote "further investigation to replicate and explain this specific gene–environment interaction is warranted."
 * In "the largest randomized trial ever conducted in the area of human lactation," between 1996 and 1997 maternity hospitals and polyclinics in Belarus were randomized to receive or not receive breastfeeding promotion modeled on the Baby Friendly Hospital Initiative. Of 13,889 infants born at these hospitals and polyclinics and followed up in 2002-2005, those who had been born in hospitals and polyclinics receiving breastfeeding promotion had IQs that were 2.9-7.5 points higher (which was significantly higher). Since (among other reasons) a randomized trial should control for maternal IQ, the authors concluded in a 2008 paper that the data "provide strong evidence that prolonged and exclusive breastfeeding improves children's cognitive development."