U.S. and Danish researchers say breastfeeding a baby may increase the child's intelligence later in life. The scientists say the longer a mother nurses, the smarter her child is likely to be up to a point.

A long-term study of more than 3,200 Danish men and women links nursing and intelligence. "It's a strong association. It holds for all the groups that we looked at," said psychologist June Reinisch of the Kinsey Institute at Indiana University, who collaborated with Danish researchers in the study. "So it means that the longer you breastfeed up to a certain point, the stronger the effect is on intelligence," she added.

The results published in the Journal of the American Medical Association show that those who had been nursed for nine months did the best on intelligence tests as teenagers or young adults. Their scores were an average of six points higher than those who consumed breast milk for less than one month. The scores increased as nursing duration increased, but showed no improvement after nine months.

At the Illinois international headquarters of La Leche League, a nursing advocacy group, lactation consultant Carol Huotari says the study is yet one more to show the long-term benefit of breastfeeding. "A lot of the articles show that there are benefits while the baby is very small, but this is another one that shows that the advantages go on into adulthood," she noted.

What might cause breastfeeding's impact on intelligence? June Reinisch and her Danish colleagues suggest it could be one of three possibilities: a nutrient in mother's milk not found in cow's milk, the physical and psychological contact between a mother and child during feeding, or some unidentified factor.

Ms. Reinisch is betting on the first choice - something in the milk. She bases her view on a previous study that compared the mental development of babies who had been fed either formula or breast milk from bottles. "Then at 18 months they looked at developmental scores and found that the children fed on donor breast milk had better scores than the ones who were fed on standard formula," she said.

University of California pediatrician Caroline Chantry agrees that mother's milk could make the difference. "There are things in breast milk - growth factors, etc. - that do influence brain development," said Dr. Chantry. "So it's entirely possible that it is the breast milk, and there are an increasing number of studies that support the fact that breast milk probably does affect brain development."

Dr. Chantry's own new study of 2,300 infants shows another breastfeeding benefit. She told a Baltimore pediatrics conference recently that babies who consume breast milk exclusively for six months are less likely to suffer from respiratory disorders than those breastfed for four months. This includes pneumonia, wheezing, and recurrent colds and ear infections.

Her research differs from the intelligence study in that few of the Danish infants had been exclusively raised on breast milk. Still, she says, the two investigations point to the same conclusion: "The benefits of breastfeeding are dose-responsive. In general, the more, the better."

Dr. Chantry's study on nursing's respiratory benefits supports World Health Organization and the American Academy of Pediatrics guidelines recommending exclusive breastfeeding for six months.

The duration had been a matter of debate. The WHO had originally recommended only partial breastfeeding after four months because poor mothers are more likely to have nutritional deficiencies, particularly iron, that would deprive their infants. Although the WHO has changed its view, it recommends further research on the issue.

However, it still calls for total breastfeeding to last two years, while U.S. professional pediatric guidelines suggest one year.