But researchers have sought to determine whether it was the breast-feeding that did it, or that mothers who prefer to breast-feed their babies may differ from those who do not.

The design of the study randomly assigning babies to two groups regardless of the mothers' characteristics was intended to eliminate the confusion.

"Mothers who breast-feed or those who breast-feed longer or most exclusively are different from the mothers who don't, "Dr. Michael Kramer of McGill University in Montreal and the Montreal Children's Hospital said in a telephone interview.