Breast feeding “is best for a brainy baby,” “unlocks IQ,” and “links to higher IQ,” said the headlines earlier this month. The Daily Mail (6 November) explained, “Breast feeding really does make babies brainier, a major study suggests. British researchers have found that mother's milk in the first few months of life can boost children's IQ by seven points. This applies in nine cases out of 10, where the youngster inherits a common but newly identified ‘brain boosting' gene.”

The research purporting to show that breast means brains was published by the US journal Proceedings of the National Academy of Sciences (PNAS 2007; 0:0704292104v1-0). The researchers, from King's College London, Duke and Yale universities in the US, and the University of Otago, New Zealand, were interested in finding a genetic variable which mediated the effects of breast feeding. In two birth cohorts, they found an “association between breast feeding and IQ . . . moderated by a genetic variant in FADS2, a gene involved in the genetic control of fatty acid pathways.” If babies had this variant—and 90% did—then breastfeeding produced a …