As with other myths, this one is continuously passed around by word of mouth. Parents believe that if they give their child sugar, their child will run wild. What the parents do not realize is that the environment that the child is in, or even how the child is raised, is what causes this hyperactivity, not the sugar. Many times when children become hyper it is when they are attending a party or something that will excite them, hence the reason for the hyperactivity. Another cause may be that if the parent is allowing the child to consume sugar and they appear hyper, the parent is lenient in other ways and allows their child to run wild, therefore there is no control over the child.
As stated in the YouTube video posted, there have been experiments performed to test this theory and the results have been that the placebo effect is actually what plays a role in this belief. Parents who were told that their child had consumed sugar, when the child actually had not, reported that their child was more hyper. This is evidence that parents are influenced by the belief of the correlation between sugar and hyperactivity, when in reality there is no correlation between the two.
Sources:
Lawson, Timothy J. Scientific Perspectives on Pseudoscience and the Paranormal: Readings for General Psychology. Upper Saddle River, NJ: Pearson Prentice Hall, 2007. Print.
"YouTube - Does Sugar Cause Hyperactivity in Children?." YouTube - Broadcast Yourself. 19 Apr. 2010. Web. 29 June 2011.