The truth: More than 20 studies show that sugar has no effect on behavior, says Mark Wolraich, M.D., a chief of pediatrics at the University of Oklahoma in Oklahoma City. Rather, it's guilt by association: Kids tend to eat gobs of sugar at parties, where they'd go wild anyway. It may also be a matter of perception. In one study, moms who were told their children had eaten sugar rated them as very hyper compared to kids who ate an artificial sweetener, even though they actually all ate the sweetener. Bottom line: While limiting sugar's a great idea for your child's health, you'll have to find another way to keep her from bouncing off the walls.