Children who are breast-fed for longer go on to become more intelligent, educated and successful adults, a study has found.
Researchers analysed breastfeeding data on almost 3,500 babies who were given IQ tests when they reached the age of 30 and provided information on educational attainment and income.
Dr Bernardo Lessa Horta, from the Federal University of Pelotas in Brazil, said: "Our study provides the first evidence that prolonged breastfeeding not only increases intelligence until at least the age of 30 years but also has an impact both at an individual and societal level by improving educational attainment and earning ability.
"What is unique about this study is the fact that, in the population we studied, breastfeeding was not more common among highly educated, high-income women, but was evenly distributed by social class."
Participants were divided into five groups based on the length of time they were breastfed as infants. The researchers took account of factors that might influence IQ, such as genetics, birthweight, parental schooling and whether or not the mother smoked during pregnancy.
Breastfeeding generally was found to increase adult intelligence, length of schooling and adult earnings. But the longer a child was breastfed, up to a period of one year, the greater the benefits turned out to be.
Someone who had been breastfed for at least a year gained four more IQ points, on average, at the age of 30 than a person who had been breastfed for less than a month.
He or she also had 0.9 more years of schooling and earnings that were higher by a third of the average income.
Dr Horta added: "The likely mechanism underlying the beneficial effects of breast milk on intelligence is the presence of long-chain saturated fatty acids (DHAs) found in breast milk, which are essential for brain development.
"Our finding that predominant breastfeeding is positively related to IQ in adulthood also suggests that the amount of milk consumed plays a role."
The research is published in The Lancet Global Health journal.
Dr Colin Michie, chairman of the Royal College of Paediatrics and Child Health's nutrition committee, said: " There have been many studies on the link between breastfeeding and IQ over the years with many having had their validity challenged.
"This study however looks at a number of other factors including education achievement and income at age 30 which, along with the high sample size, makes this study a very powerful one.
"It's widely known that breastfed babies are better protected against chest and ear infections, are at less risk of sudden infant death and are less likely to become obese, but it's interesting to see that the benefits of breastfeeding for a prolonged period of time not only benefit the baby in the early years, but also translate into increased intelligence and improved earning ability later in life.
"It is important to note that breastfeeding is one of many factors that can contribute to a child's outcomes, however this study emphasises the need for continued and enhanced breastfeeding promotion so expectant mothers are aware of the benefits of breastfeeding.
"Furthermore, once mothers have given birth, we must ensure they are properly supported to continue breastfeeding for as long as they are able to."