In a recent study published in the Proc. Natl Acad. Sci., scientists have found out that breastfeeding can indeed boost an infant's brain development and intelligence, provided the baby carried at least one copy of a 'C' allele for a gene called FASD2, which is involved in the genetic control of fatty acid pathways.

In children who carried at least one copy of a 'C' allele for FASD2 , those who were breast-fed generally had a higher IQ than those who were not: by an average of 6.4 IQ points in the New Zealand study, and by 7.0 IQ points in the British one. By contrast, children carrying two 'G' alleles had roughly the same IQ irrespective of their diet. About 10% of the population is thought to be 'GG'.

Just another evidence that it isn't nature vs nurture; more like nature via nurture.