Breastfeeding is linked to intelligence, new research suggests.
Scientists have discovered that there is a direct correlation between the size of a mammals brain – including that of humans – and the amount of time they suckle.
The longer the period, the bigger the brain.
The findings are likely to add further weight to the "breast is best" argument although the researchers said they were not sure whether it was the energy that milk supplied or the nutrients that made the difference.
"It supports the theory that breast feeding and brain development are associated with each other," said study author Professor Robert Burton at Durham University.
"What we don't know at the moment is if there is anything in mother's milk that helps the brain to grow.
"Its components have been a hot topic among scientists for some time.
"At the moment we cannot say for sure that formula milk is not an adequate replacement."
Prof Barton and his team studied 128 mammal species – including humans – to see if there was a link between how much time was "invested" in their upbringing and the size of their brain.
They found that the longer the gestation period and the longer the suckling directly affected the size of the brain.
For example, in comparison, a deer, which are about the same body weight as humans, are only pregnant for seven months with a suckling period of up to six months.
This results in a brain six times smaller than that of a human.
"In general terms it does support the idea that 'breast is best' and that the World Health Organisation's guideline that you should breast feed for 18 months to two years are correct," said Prof Barton.
Three years ago a study of almost 14,000 children found those who were breastfed did significantly better in IQ tests.
Another study published earlier this month showed breastfed babies do better at reading, writing and maths at the ages of five, seven, 11 and 14.
Breast milk has long been known to boost babies' immunity – helping them fight ear infections, stomach bugs and even asthma.
As few as three per cent breastfeed for the recommended six months, and only a third are still doing so after a week.
The new findings are published in Proceedings of the National Academy of Sciences.