When they controlled for the mother’s age, education, marital status, family income and other factors, they found that breast-feeding for six months or more was associated with better performance in all four academic skills, but only in boys.

The lead author, Wendy H. Oddy, an associate professor of nutrition at the University of Western Australia, said the reasons for the sex difference were unclear. “We think boys tend to be more vulnerable to stress,” she said. “Female hormones might have a protective effect.”

The study, published in the January issue of Pediatrics, had a number of strengths. It followed children prospectively, and had a large sample. Data was collected close to the time the women stopped nursing. But the scientists were unable to adjust for length of maternity leave, partner support and other factors.

“The results add to the strong evidence that breast-feeding as long as possible is beneficial for child health,” Dr. Oddy said, “but particularly for brain development.”