The situation was different in Belarus at the time, he said, with less routine encouragement for the practice. Kramer said how breast-feeding may make children more intelligent is unclear.

"It could even be that because breast-feeding takes longer, the mother is interacting more with the baby, talking with the baby, soothing the baby," he said. "It could be an emotional thing. It could be a physical thing. Or it could be a hormone or something else in the milk that's absorbed by the baby."

Previous studies have shown babies whose mothers breast-fed them enjoy many health advantages over formula-fed babies.

These include fewer ear, stomach or intestinal infections, digestive problems, skin diseases and allergies, and less risk of developing high blood pressure, diabetes and obesity.

The American Academy of Pediatrics recommends that women who do not have health problems exclusively breast-feed their infants for at least the first six months, with it continuing at least through the first year as other foods are introduced.