Breastfeeding produces not only healthier babies but brighter children, according to a study that could reignite calls for mothers to abandon the bottle.
As little as four weeks of breastfeeding for a new-born infant has a "positive and significant effect" on brain development up to secondary school and beyond, the study says.
Children who had been breastfed consistently outperform their formula-fed peers at ages 5, 7, 11 and 14, in tests of reading, writing and mathematics, researchers from the University of Oxford and the Institute for Social and Economic Research at the University of Essex found.
One of the paper's co-authors, Maria Iacovou, a social scientist at the Institute for Social and Economic Research, said the health benefits of breast milk were widely known and understood, but the benefits for cognitive development were less clear.
"The issue was that while it looked as though breast feeding did have an effect on cognitive development, no one knew if that was just because the type of mother more likely to breastfeed in the first place was more likely to nurture brighter children, or whether there was a true causal link," she said.
The study compared breastmilk-fed children with formula-fed "twins" who were equivalent in all other observable respects, and found a link between breast milk and cognitive development, said Iacovou.
"Breast milk has well-known health benefits and now we can say there are clear benefits for children's brains."
Despite the findings, she said, she still supported mothers who decided that breastfeeding wasn't for them.
"It really, really wasn't my intention to make any mother feel guilty," she said.
"All this talk about bringing up children would sometimes seem to have us think that the child is the only thing that matters.
"Mothers are people too and have feelings and if you don't want to breastfeed your baby ... you are not going to cause it harm. They would just do a little bit less well.
"I think we have a lot of challenges to change our culture and attitudesto breastfeeding, but it's likely tohappen through a gradual process of normalisation.
"Don't pressurise women who don't want to breastfeed but we should start focusing more on those women who do want to and try to help them and make it more normal for everyone."