It pays to breast-feed — for babies. When they grow up, that steady diet of breast milk may boost their monthly income by up to 39 percent, according to a new report.

Researchers tracked down nearly 3,500 Brazilian adults who were enrolled in a study within days of their birth in 1982. Back in the ’80s, interviewers had asked their mothers how long their children were breast-fed and how old they were when they were introduced to other foods.

At the time, breast-feeding wasn’t associated with any particular socioeconomic class in Brazil, the study authors said. That was true of the families in this study — 21 percent of the children nursed for less than one month and 17 percent nursed for more than a year. (Among the rest, 26 percent were breast-fed for one to three months, 23 percent for three to six months, and 14 percent for six to 12 months.) Fast-forward to 2012. Those children are now 30-year-olds with jobs.

So the researchers, from the Federal University of Pelotas and the Catholic University of Pelotas, asked them how much income they earned in the previous month. They also administered an intelligence test to see if they could find a link between breast-feeding and adult IQ. (Many studies have found a correlation between breast-feeding and IQ in children.)

The average IQ of the adults was 98, and they completed an average of 11.4 years of school. The average monthly income of the 3,493 study participants was 1,000 Brazilian reals (about $311).

After adjusting for factors like parental education, family income at birth, genetic ancestry and mothers’ smoking and weight status, the researchers found a basically linear relationship between breast-feeding and monthly income — children who nursed for more than six months went on to earn significantly more than children who didn’t.

Compared to those who stopped breast-feeding by the time they were 1 month old, those who nursed for more than a year earned 28 percent more per month, according to the study. The benefits were even greater for people who had nursed for six months to one year — their monthly incomes were 39 percent higher than those in the baseline group who breast-fed for less than a month.

IQ followed a similar pattern. After adjusting for demographic factors, the researchers calculated that study participants who nursed for six to 12 months had 3.5 more IQ points than those who nursed for less than a month. People who were breastfed for more than a year gained 3.76 IQ points.

These higher IQs were the primary mechanism through which breast-feeding led to higher incomes, the study authors found.

Although they didn’t examine the biology of breast-feeding, the researchers noted that unlike infant formula, breast milk contains long-chain saturated fatty acids like DHA, which “are essential for brain development,” they wrote.