The longer you're breastfed, the more likely you are to have increased intelligence in adulthood, more years spent in formal education, and higher adult earnings, according to a new study that tracked almost 3,500 newborns over 30 years of their life.

It's well-known that breastfeeding offers many short-term benefits for a child, including reduced risk of death and disorder due to infectious diseases. And while the long-term benefits are far more ambiguous, several studies have revealed a link between intelligence and breastfeeding. 

So researchers, led by Bernardo Lessa Horta from the Federal University of Pelotas in Brazil, decided to investigate this further by following a large cohort of children from birth right into adulthood.

"The effect of breastfeeding on brain development and child intelligence is well established, but whether these effects persist into adulthood is less clear," Lessa Horta said in a press release."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."

The team started their study back in 1982, and gathered data on 3,493 newborn infants born in Pelotas, Brazil. Over the course of three decades, they were divided in five groups, depending on how long they breastfed as babies, and at an average age of 30, their IQ was tested, and their income and education levels were recorded. Several variables that could contribute to IQ, income and education were accounted for, including family income at birth, their parents' education level. smoking during pregnancy, birthweight, and genomic ancestry. 

"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," says Lessa Horta. "Previous studies from developed countries have been criticised for failing to disentangle the effect of breastfeeding from that of socioeconomic advantage, but our work addresses this issue for the first time."

The results have been published in an open access article in The Lancet Global Health.

Interestingly, the team found that the longer a person was breastfed, the better the results in later life were. And the difference was pretty huge. If a child had been breastfed for an entire year, they scored on average four more IQ points, had almost another year of formal education, and earned an extra 341 reais a month - which is about about a third more than the average income level in Brazil - than those who had breastfed for less than a month.

But beyond 12 months, and the benefits in later life dropped off significantly - in adulthood, these people were on average poorer, and less educated.

So what's going on here? Lessa Horta told Sarah Boseley at The Gurdian that some people think it's not the breast milk that's giving the children the benefits in later life, but the actual parenting techniques of the mothers doing the breastfeeding. "Some people say it is not the effect of breastfeeding but it is the mothers who breastfeed who are different in their motivation or their ability to stimulate the kids," he said.

But his team is going after a theory that could be more easily tested - that the long-chain saturated fatty acids (DHAs) found in breast milk, which studies have shown are essential for proper brain development, could be at play. 

Of course, while the science behind this study may be fine, it's bound to be controversial, and upsetting for those who are unable to breastfeed their children. Dean Burnett at The Guardian has an interesting rundown of the issues associated with this kind of research, which is well worth a read.