Does breastfeeding have an effect on intelligence?

A young mother under my care is concerned because she has decided not to breastfeed and was told by several people that formula feeding will cause the baby not to be "as smart." Is this based on real science?

Studies during the last decade have shown breastfeeding to be associated with higher intelligence in young children. Better controlled and more recent studies have shown that this association is probably not proof of causation, because mothers who decide to breastfeed tend to be better educated and more socioeconomically advantaged than those who decide to use formula. After controlling for these differences, the effect tends to disappear.