There are many reasons why doctors encourage new mothers to breastfeed their babies. Compared with babies who get formula, babies who are breastfed are less likely to die as a result of infections, sudden infant death syndrome or any other reason. The longer a mother nurses — and the longer she does so exclusively — the bigger the benefits, studies show.

A perceived benefit of breastfeeding is the possibility that it boosts a baby’s brain. A clinical trial involving more than 16,000 infants in Belarus who were randomly assigned to get either special support for breastfeeding (based on a program from the World Health Organization and UNICEF) or a hospital’s usual care found that babies in the first group scored an average of 7.5 points higher on a verbal IQ test and 5.9 points higher on overall IQ. Teachers, apparently, could tell the difference — children whose moms got extra help with breastfeeding got higher marks in school for both reading and writing.

That result is something of an outlier. In an analysis of 17 studies on breastfeeding and IQ, the four that were considered to be of highest quality — each had at least 500 participants and took a mother’s IQ into account, among other things — also found an association between breastfeeding and IQ, though the benefit was only 1.76 points, on average.

The latest data come from a study of about 7,500 Irish children who have been tracked since birth as part of the ongoing Growing Up in Ireland study. The results were published Monday by the journal Pediatrics.

Unlike the kids in Belarus, these kids were not randomly assigned to groups that got more or less help with breastfeeding. But researchers did their best to get around this problem by simulating random assignments. They identified pairs of children who seemed to be equally likely to be breastfed, based on factors like ethnicity, mother’s educational background and whether they had to spend time in the neonatal intensive care unit, except that one of them actually was and the other wasn’t.

Using this matching technique, the researchers found very little difference in cognitive development between the two groups of children. The only differences that were statistically significant were for 3-year-olds, and only between those who were exclusively breastfed for at least 6 months and those who didn’t nurse at all.

In the rough analysis, the children who were breastfed did better on tests of problem solving and hyperactivity. However, after the researchers accounted for other factors, the apparent benefits for problem solving were no longer statistically significant.