A Brazilian study has found that people who were breastfed are more intelligent as adults and can earn more for a living.

People breastfed as infants have higher intelligence scores in adulthood, and higher earnings.

And, critically, the socioeconomic status of mothers appear to have little impact on breastfeeding results, according to a paper published by The Lancet medical journal on Wednesday.

The study tracked the development of 3,500 newborns over 30 years.

"The effect of breastfeeding on brain development and child intelligence is well established," lead author Bernardo Lessa Horta of the Federal University of Pelotas in Brazil said in a statement.

What has been less clear, is whether the effects persist into adulthood, and whether a mother's socioeconomic status or education level played a bigger role in the outcome of previous studies than her choice to breastfeed or not.

"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," said Horta.

"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."