"They tend to be smarter. They tend to be more invested in their babies. They tend to interact with them more closely. They may be the kind of mothers who read to their kids more, who spend more time with their kids, who play with them more," added Kramer, who led the study published in the journal Archives of General Psychiatry.

The researchers measured the differences between the two groups using IQ tests administered by the children's pediatricians and by ratings by their teachers of their school performance in reading, writing, math and other subjects.

Both sets of scores were significantly higher in the children from the breast-feeding promotion group. The study was launched in the mid-1990s. Kramer said the initial idea was to do it in the United States and Canada, but many hospitals in those countries by that time had begun strongly encouraging breast-feeding as a matter of routine.