Breastfed babies are more likely to turn into well-educated and higher-earning adults, according to a major long-term study.

Researchers in Brazil have followed nearly 6,000 babies from birth for the past three decades, enabling them for the first time to get an idea of the long-term effects of breastfeeding. Nearly 3,500 of them, now 30-year-old adults, accepted an invitation to be interviewed and sit IQ tests for the purpose of the study. Those who had been breastfed proved to be more intelligent, had spent longer at school and earned more than those who had not been. And the longer they were breastfed as a baby, the better they tended to be doing.

It is already known that breastfeeding can increase a child’s IQ by a small amount. The question that Dr Bernardo Lessa Horta from the Federal University of Pelotas in Brazil wanted to answer was whether this translated into greater intelligence and better prospects as an adult.

“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,” he said.

It is not just the age of the participants that makes this study unusual. Horta says it is free of the major complication of most breastfeeding studies because, when it began in 1982, it was not just the more affluent and educated mothers who breastfed in Brazil. Breastfeeding was not limited to one socio-economic group. It was, he says, evenly distributed across the social classes. So the higher achievers at the age of 30 did not come from better-off homes.

They found that all the breastfed babies had greater intelligence, as measured by a standard IQ test, had spent more years in education and had higher earnings. But the longer they had been breastfed, the greater the benefits. Children who had been breastfed for 12 months had an IQ that was four points higher than those breastfed for less than a month, had nearly a year’s more schooling and earned around US104 (HK$806) a month more – about a third more than the average income level.

The research now published in the Lancet Global Health journal.