Whether a mother was rich or poor, or had high or low social status, made no difference to the results, it said.
The findings drew a mixed response from outside commentators, a sign of scientific caution about what influences IQ.
“Our study provides the first evidence that prolonged breastfeeding not only increases intelligence until at least the age of 30 years,” said Bernardo Lessa Horta of Federal University of Pelotas in Brazil, who led the probe.
“(It) also has an impact both at an individual and societal level by improving educational attainment and earning ability.”
In The Lancet Global Health journal, Horta’s team analysed data from a study of local children who were born in 1982.
Information on breastfeeding was compared to IQ test results at the average age of 30 years, as well as the educational achievement and income of 3,493 participants.
The researchers divided the group into five categories, based on the length of time they were breastfed as infants.
They took into account 10 “social and biological variables” that might affect IQ.
These included family income at birth, parental schooling, maternal smoking during pregnancy, maternal age, birthweight and how the baby was delivered.
Breastfeeding led to increased adult intelligence, longer schooling and higher adult earnings, regardless of family background, the results suggested.
“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,” said Horta.
The longer a child was breastfed, the greater the benefits, the investigation also found.
An individual breastfed for at least a year as a baby gained a full four IQ points, had 0.9 years more schooling, and an income of 341 Brazilian reals (98 euros, $104) higher per month at the age of 30, compared to those breastfed for less than one month, the study found.
Breast milk is rich in long-chain saturated fatty acids (DHAs) which are essential for brain development, Horta suggested.
“The amount of milk consumed (also) plays a role,” he added.
In a comment carried in the same journal, Erik Mortensen of the University of Copenhagen said the findings had important public health implications.
“However, these findings need to be corroborated by future studies designed to focus on long-term effects and important life outcomes associated withbreastfeeding.”
A positive note was struck by leading British expert Colin Michie, Britain’s Royal College of Paediatrics and Child Health.
One of a string of inquiries into breastfeeding in recent years, the study was “very powerful” as it was exceptionally large and long-term, he said.
But, he cautioned, “breastfeeding is one of many factors that can contribute to a child’s outcomes.”
Given the many benefits of breastfeeding, it was essential for health watchdogs to encourage the practice among mums, he said. (AFP)