It's a parenting rule as steadfast as the laws of gravity: if a kid eats a sugary treat, they'll be bouncing off the walls in no time. But according to science, that's not so. Multiple studies have shown that sugar actually has no effect on a child's behavior. Parents only think it does because of two classic fallacies: false association and confirmation bias. The false association comes into play when parents see kids going crazy at birthday parties and on Halloween. They incorrectly point to the sweets the kids have eaten as the reason for the mayhem, when in fact, fun occasions like parties and holidays make children excitable anyway. But once that association is in place, confirmation bias rears its head: parents notice the times that their children get hyper with sweets, but ignore the times they get hyper without them. Though it's true that a child with low blood sugar can get an energy boost from sugar—or any food, for that matter—it's not so with regular blood sugar. The body will either use the sugar for normal energy needs or store it as fat.
The science bears this out. A 1994 study in the Journal of Abnormal Child Psychology found that mothers who had been told their sons had ingested sugar rated them as more hyperactive and were more likely to criticize them, even though many of the boys only had an artificial sweetener. Another study in 1994 looked at sugar consumption over a period of three weeks and found that the amount of sugar in a child's diet had no effect on their intellect or behavior. Then in 1995, researchers performed a meta-analysis of 16 studies on the subject and found consistently negative results, showing that the sugar high is indeed a myth. Learn more about sugar's effects (or lack thereof) with the videos below.