Many studies report that children who were breast fed perform better in tests of intellectual competence than those who were not.^1–3 This association is seen for full term infants and more strongly for those who are preterm or of low birth weight. The IQ advantage for full term infants is small, roughly three to four points, but it is remarkably consistent across studies. It is unclear, however, whether this difference reflects a direct nutritional advantage or a difference in socioenvironmental factors that are more favourable in women who breast feed.

Women who breast feed are more likely to provide their child with a more enriched and cognitively stimulating environment than those who do not breast feed, which could contribute to their children's better cognitive performance. Several studies have reported a positive relation between mother's IQ and the decision to breast feed.1 A study in this week's BMJ by Der and colleagues assesses the impact of controlling for maternal intelligence and other confounding factors on this …