Obese children tend to have higher IQs than thin or average-weight children, according to a nationwide study of 20,000 children. But researchers say there's no evidence that overfeeding will make kids smarter. The obesity-intelligence link was an unexpected result of tests given to 20,137 children in a study aimed at uncovering the causes of cerebral palsy. The study was sponsored by the National Institute of Neurological and Communicative Disorders and Stroke. Researchers discovered the heaviest 10 percent of the children did better on IQ tests and measures of speech, language and hearing than did average weight or lean children, at least through age 8. So far, researchers have no explanation for the findings. For the purposes of research, children were classified as lean, average or obese based on the ratio of weight to height. The skinniest 10 percent were listed as lean, the heaviest 10 percent as obese and the 80 percent in between as average. From 1959 through 1969, more than 20,000 pregnant women were enrolled in the federally funded project after promising to bring their children back to participating institutions for regular examinations. The University of Minnesota was among the institutions taking part in the study, which ended in the 1970s. Researchers analyzing the results began finding that obese youngsters had IQs averaging 10 points higher than lean children at age 4, and five points higher at age 7. So far, researchers have no explanation for the findings. Dr. Robert A. Ulstrom and Dr. Robert O. Fisch, University of Minnesota pediatrics professors who took part in the project subdivided the youngsters by sex, race and socioeconomic status. It turned out that those factors had no effect on the obesity findings. "We had thought that perhaps huskier and fatter children were from higher economic level homes, but that didn't pan out and neither did anything else," Ulstrom said. Researchers also ruled out the possibility that the heavier youngsters are forced to develop intellectual abilities because they are not included in group physical activities. "There isn't that type of ostracism at that age," Ulstrom said.