Children whose mothers breast-fed them longer and did not mix in baby formula scored higher on intelligence tests, the researchers in Canada and Belarus reported. About half the 14,000 babies were randomly assigned to a group in which prolonged and exclusive breast-feeding by the mother was encouraged at Belarussian hospitals and clinics.

The mothers of the other babies received no special encouragement. Those in the breast-feeding encouragement group were, on average, breast-fed longer than the others and were less likely to have been given formula in a bottle.