Controlled trials find no effect on behaviour; the belief is largely in the parent's expectation.
What the evidence shows
Almost every parent 'knows' that a birthday cake turns children wild. The belief is so strong it feels self-evident, and it shapes how families manage sweets.
A meta-analysis of 23 controlled, blinded trials (Wolraich, Wilson & White, 1995) found sugar had no measurable effect on children's behaviour or cognition — including in children thought to be 'sugar-sensitive.' A striking follow-up showed that parents who merely believed their child had been given sugar rated them as more hyperactive, even when the child got a placebo. The behaviour people attribute to sugar is better explained by the exciting settings where sweets appear (parties, holidays) and by expectation. The claim is refuted.
Sources
Wolraich, M. L., Wilson, D. B., & White, J. W. (1995). The effect of sugar on behavior or cognition in children: A meta-analysis.
JAMA, 274(20), 1617–1621
Across 23 blinded trials, sugar had no significant effect on children's behaviour or cognitive performance.
DOI: 10.1001/jama.1995.03530200053037 →