CHICAGO (AP) - Breast-fed babies may grow up to be smarter adults, according to research that bolsters the evidence linking nursing and intelligence.

Most previous studies did not measure breast-feeding's effects on IQ into adulthood, and the few that did so ignored factors such as parents' education and social status, said the researchers, who took such variables into account.

In their study of 3,253 Danish men and women, the more babies were breast-fed through 9 months of age, the higher they scored on intelligence tests in their late teens and 20s. Breast-feeding past 9 months had no additional effect on scores.

The link can probably be explained by the effect of nutrients in mothers' milk on the developing brain and benefits from the close physical and psychological relationship breast-feeding involves, researchers said. Mothers who take time to breast-feed may spend more time interacting with their youngsters throughout childhood, which also could affect intelligence, the researchers said.

In the study, mothers had been questioned about their breast-feeding practices when their youngsters were a year old.

Those who had been breast-fed for seven to nine months scored an average of about six points higher on IQ tests than those whose mothers said they nursed for less than one month.

That gap ``is not the difference between an Einstein and a mentally retarded child,'' said study director June Machover Reinisch. But she said it could be the difference ``between normal and bright-normal, or bright-normal and superior.''

Reinisch is director emeritus of the Kinsey Institute for Research in Sex, Gender and Reproduction.

The study appears in Wednesday's Journal of the American Medical Association.

The American Academy of Pediatrics recommends that babies be breast-fed exclusively for their first six months, citing evidence that breast milk is nutritionally superior, reduces the incidence of infection and may enhance mental development.

Few participants in the latest study had been bottle-fed exclusively, though about 1,000 had nursed for less than a month.

The study was funded in part with grants from the National Institutes of Health.

Malla Rao, a scientist at the National Institute of Child Health and Human Development, said that the study overlooked factors that could help explain the results - such as whether participants had dropped out of school - but that the findings agree with those of most previous studies.