A large study of breastfeeding found that it raises children's IQs and improves their academic performance.
The study, which is published in the Archives of General Psychiatry, analysed 14,000 children up to the age of six-and-a-half years.
Researchers at McGill University discovered that performance in IQ tests as well as reading and writing ability was significantly better in children who had been breastfed exclusively for an extended period.
Lead investigator Dr Michael Kramer, professor of paediatrics, epidemiology and biostatistics at McGill, commented: "Our study provides the strongest evidence to date that prolonged and exclusive breastfeeding makes kids smarter."
The researchers noted that the observed benefits could be due to a constituent of breast milk or the "physical and social interactions inherent in breastfeeding".
Alternatively, they suggest that the increased verbal interaction between mother and child during breastfeeding may improve the infant's cognitive development.