Infants breast-fed for nine months grew up to be significantly more intelligent than infants breast-fed for one month or less, according to a study published in today's Journal of the American Medical Association.

Supporting the conclusion that breast-feeding improved intelligence, researchers found a strong "dose effect" -- a gradual improvement based on the number of months of breast-feeding up to nine months, when the effect ended.

Results from the study, of more than 3,000 young men and women from Copenhagen, Denmark, strongly support the long suggested, but unproved, conclusion that breast-feeding makes babies not only healthier, but smarter.

As the first study to measure the effects of breast-feeding into the subjects' late teens and twenties, it could become an authoritative statement in the emotional debate over how to encourage nursing. Today, the practice is most common among white and wealthier women and least common among minority and poorer women.

"We are really quite certain that what we are seeing here is the effect of the duration of breast-feeding on an individual's intelligence," said June Machover Reinisch of the Kinsey Institute for Research in Sex, Gender and Reproduction, one of the study's authors. "The question that remains is what exactly is the aspect of breast-feeding that results in the greater intelligence."

"The evidence is growing that breast-feeding is among the most important lifelong benefits a mother can give to her child," she said.

Although public health officials, and even infant formula producers, recommend breast-feeding as the best way to nourish an infant for the first six to 12 months, most American babies are bottle-fed during much of their infancy.

The proportion of mothers who begin breast-feeding has been increasing in recent years to almost 70 percent, but a study by the infant formula industry found that only 31 percent of all infants are still being breast-fed at 6 months. Because of previously studied health effects, the American Academy of Pediatrics recommends one year of breast-feeding as optimal.

Studies have shown that many women never breast-feed or stop quickly because of a wide range of ambivalent or negative signals from society. Researchers have identified cultural biases against the practice, serious difficulties experienced by some mothers when they return to work, limited availability of training in how to breast-feed, and the sometimes aggressive advertising and promotion of the infant formula industry. About half of all the formula used in the United States is purchased for poor women through the federal Women, Infants and Children program (WIC).

The JAMA findings could also have an impact on the infant formula industry. A new type of formula, supplemented with two beneficial compounds found in breast milk but not in traditional formula, has recently come onto the market. Makers of the new formula believe their studies have shown the added ingredients could be responsible for some of the benefits of breast milk on intelligence, although the Food and Drug Administration did not evaluate that issue when it approved the formula.

While some previous studies have suggested an association between breast-feeding and improved intelligence, the new study appears to provide the strongest indication of an effect.

The research, funded primarily by the National Institutes of Health, used two large groups of Danish men and women who had been studied since their mothers were pregnant with them between 1959 and 1961. When the children were one year old, the mothers were questioned about how long they breast-fed their babies.

One group of 973 men and women was given a Wechsler Adult Intelligence Scale test, an intensive one-on-one assessment, while the other sample of 2,280 men received intelligence tests when they entered the Danish military. Testing of both groups occurred in the 1970s and 1980s. In a recent analysis of the data for the JAMA study, researchers found that in both groups, those breast-fed nine months scored significantly higher than those breast-fed for less than one month.

Other studies have shown a correlation between breast-feeding and scores on intelligence tests, but some of that relationship disappeared when complicating factors were taken into account. For instance, the children of mothers who are better educated and wealthier would be expected to be healthier and to score higher on intelligence tests however they were fed as infants. Their mothers would be statistically less likely to smoke, to be overweight and to have large families -- all associated with less healthy children who do less well on intelligence tests.

But the new Danish study took into account 13 similar factors related to the mother's health, wealth and behavior when analyzing the difference between the scores of more breast-fed and less breast-fed young adults. The differences held up after they were factored in.

Some experts yesterday cited one possible weakness in the study, in its reliance on the mothers' recollections of how long they had breast-fed their children. Those recollections, these experts said, may not be entirely accurate.

Researchers said the results were potentially more meaningful than earlier studies that looked at intelligence in young children, which is less stable and more difficult to measure with confidence.

Nonetheless, some of the earlier studies also showed significant results. A recent study, led by Malla Rao of the National Institute of Child Health and Human Development, found that babies born small but at term and breast-fed for six months scored an average of 11 points higher on intelligence tests than those breast-fed for 3 months. The IQ tests were given when the children were 5.

The JAMA report offers three possible explanations for the association between breast-feeding and higher scores. The first is that two fatty acids associated with the development of nerve cells and the brain are present in breast milk but absent from infant formula and cow's milk. The two, docosahexaenoic acid (DHA) and arachidonic acid (ARA), have been shown in some experiments to improve eyesight and some motor responses in infants and young children.

According to author Reinisch, these two compounds are among hundreds found in breast milk but not in substitutes. Other substances found in breast milk have been shown to improve a child's respiratory and gastrointestinal health as well.

The authors also suggest that the mother-infant bond can be deepened through breast-feeding, and that that contact may affect the child's intellectual development. They report as well the hypothesis that how long a mother breast-feeds is an "indicator of the interest, time and energy that the mother is able to invest in the child during the whole upbringing period."