Children who were breast-fed exclusively for the first three months of life or longer scored nearly six points higher on IQ tests at the age of 6 than children who weren't breast-fed exclusively, a new study has found.

The finding buttresses previous research that has suggested that children and adults who were breast-fed as infants scored better on IQ tests and other measures of cognitive development, such as thinking, learning and memory, the study authors said.

"Long and exclusive breast-feeding makes kids smarter," said lead researcher Dr. Michael S. Kramer, of McGill University and the Montreal Children's Hospital, in Canada.

Why breast-feeding might increase cognitive skills isn't clear, Kramer said. "It could be something in the milk, or it could be the physical contact between the mother and the baby," he said. "It could be the way the mother interacts with the baby during breast-feeding -- there is no way to know."

The one thing Kraemer is sure of is that it has nothing to do with differences between mothers. The women in the new study were all from the eastern European country of Belarus.

The findings are published in the May issue of the Archives of General Psychiatry.

For the study, Kramer's group randomly assigned 7,108 infants in Belarus to exclusive breast-feeding; another 6,781 infants received the usual practice of breast-feeding plus other foods.

When the children were 6.5 years old they were given a standard IQ test. Those children who were exclusively breast-fed scored, on average, 7.5 points higher in verbal intelligence, 2.9 points higher in nonverbal intelligence, and 5.9 points higher in overall intelligence.

In addition, their teachers said the breast-fed children had significantly better academic performance in both reading and writing, compared with children who weren't breast-fed exclusively.

Kramer thinks women should breast-feed exclusively for at least three, and if they can, six months, and try to continue breast-feeding for at least a year.

"For women in developed countries who can achieve exclusive breast-feeding for at least three months, their kids would benefit by about three or four IQ points," he said.

One expert thinks it's the nutrients in mothers' milk -- which aren't found in other foods -- that are essential for brain development and increased IQ.

"I'm not surprised because many studies have had similar results," said Dr. Ruth Lawrence, a professor of pediatrics at the University of Rochester School of Medicine, and a member of the American Academy of Pediatrics executive committee section on breast-feeding. "It's wonderful to have this very large study to confirm what we've known or thought for a long time."

Lawrence thinks that because mothers' milk contains certain amino acids not found in formula, it's better for infants' developing brains. These amino acids include omega three fatty acids and DHA (Docosahexaenoic acid), which are important for brain growth, she noted.

Human milk also contains cholesterol, while formula doesn't, Lawrence said. "We learned to fear cholesterol and yet cholesterol is very important for brain tissue, it's very important for nerve tissue," she said. "That's why human milk is a better nutrient to support brain growth."

Many professional organizations, including the American Academy of Pediatrics, recommend breast-feeding as the best way to improve infants' overall health and build their immune system. Breast-fed infants have fewer hospital admissions, ear infections, diarrhea, rashes, allergies and other medical problems than bottle-fed babies, according to the U.S. Food and Drug Administration.