A new study provides some of the best evidence to date that breast-feeding can make children smarter, an international team of researchers said on Monday.

　　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.

　　At 3 months, 73 percent of the babies in the breast-feeding encouragement group were breast-fed, compared to 60 percent of the other group. At 6 months, it was 50 percent versus 36 percent.

　　In addition, the group given encouragement was far more likely to give their children only breast milk. The rate was seven times higher, for example, at 3 months.

　　The children were monitored for about six and a half years.

　　The children in the group where breast-feeding was encouraged scored about 5 percent higher in IQ tests and did better academically, the researchers found.

　　Previous studies had indicated brain development and intelligence benefits for breast-fed children.

　　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.