The government advises breastfeeding for first six months
More evidence is being put forward that breastfed babies eventually become more intelligent than those who are fed with formula milk.
Canada's McGill University found breastfed babies ended up performing better in IQ tests by the age of six.
But the researchers were unsure whether it was related to the breast milk itself or the bond from breastfeeding.
The study of nearly 14,000 children is the latest in a series of reports to have found such a positive link.
However, one problem has been that some of the research has struggled to identify whether the findings were related to the fact that mothers from more affluent backgrounds were more likely to breastfeed and it was factors related to the family circumstances that was really influencing intelligence.
But the latest study attempted to take this into account by following the progress of children born in hospitals in Belarus, some of which ran breastfeeding promotion schemes to boost rates across all groups.
They found that those who breastfed exclusively for the first three months - with many also continuing to 12 months - scored an average of 5.9 points higher on IQ tests in childhood.
Teachers also rated these children significantly higher academically than control children in both reading and writing, the Archives of General Psychiatry reported.
Lead researcher Professor Michael Kramer said: "Long-term, exclusive breastfeeding appears to improve children's cognitive development."
But he added: "Even though the treatment difference appears causal, it remains unclear whether the observed cognitive benefits of breastfeeding are due to some constituents of breast milk or are related to the physical and social interactions inherent in breastfeeding."
Fatty acids found in breast milk are thought to boost intelligence, but the report said the physical and emotional aspect of breastfeeding may lead to permanent changes to brain development.
The researchers also suggested breastfeeding may increase verbal interaction between mother and child, which in turn could aid their development.
Nonetheless Professor Kramer said more efforts should be made to promote breastfeeding.
In England, the government recommends mothers breastfeed exclusively for the first six months.
But research shows while three quarters start off breastfeeding, just one in four are still doing it by six months.
Rosie Dodds, of the National Childbirth Trust, said: "This research certainly increases the evidence about the impact of breastfeeding.
"And I think what we now need is more effort put into supporting it."