Babies who are breast-fed score higher on intelligence tests than formula-fed babies, regardless of their mother's IQ, education and other socioeconomic factors, according to a major study released yesterday.

Breast-fed infants tested on average 5.3 IQ points higher, according to the study, published in the American Journal of Clinical Nutrition. After 15 factors such as maternal smoking and education, birth weight, birth order and family income were weighted and subtracted, the researchers still found that breast-fed babies tested 3.1 IQ points higher than formula-fed babies.

The analysis, which covered more than 7,000 children in 11 studies, also found that babies breast-fed eight weeks or less showed little of the reported positive cognitive effects of breast-feeding. For those breast-feeding more than eight weeks, the improvement increased until it reached a plateau at 20 weeks.

"When you do a meta-analysis, you are looking to see . . . whether researchers are finding similar results or not," said James Anderson, a professor of internal medicine and clinical nutrition at the College of Medicine of the University of Kentucky, Lexington, and lead researcher of the study. "What we found is that the results were fairly consistent, and that gives us a higher level of comfort with the findings."

The link between breast-feeding and brain development has been well established in recent years, but the reasons for it remain controversial. Some researchers believe the link is based on the fact that well-educated, wealthier women breast-feed far more than poor and less-educated women.

But others believe there are chemicals in breast milk that encourage brain development, and that those chemicals are absent in formula. In particular, extensive research is underway into the effects of several Omega-3 fatty acids--such as docosahexaenoic acid (DHA) and arachidonic acid (AA)--that are prevalent in breast milk but absent in infant formula. These fatty acids are known to be important in the development of the infant brain.

The new analysis was partially funded by a company that produces DHA from algae, Martek Biosciences Corp. of Columbia. DHA from Martek and other companies is used to supplement infant formula sold abroad, but that supplementation is not allowed in the United States, pending a decision by the Food and Drug Administration. Other funding for the meta-analysis came from the independent, nonprofit HCF Nutrition Foundation.

Previous research on this subject has produced mixed results. In May, the journal Pediatrics published research that concluded that the observed difference in IQ between breast-fed and formula-fed babies was the result of genetic and socioeconomic factors alone.