My colleague Dr. Tom Robinson points out that sugar does not actually make children hyper. However, parents who believe that it does see what they expect. When researchers tell parents that their children have consumed sugar, the parents report hyperactive behavior even if the researcher was lying (We researchers are a sneaky bunch).
These sorts of mix-ups are common. Ever heard that if you have a night light in your infant’s room, you are putting your child at risk for myopia? Turns out that near-sighted parents (who of course pass along their lousy-vision genes) are more likely to put lights in their child’s room, and there is no direct effect of the light itself on children’s eyesight.