A total of 1,104 studies investigating serotonin levels in children with autism have been reviewed. Among these studies, 59% reported elevated serotonin levels (hyperserotonemia) in children with autism. In 25% of the studies that identified high serotonin levels, a direct relationship was found between serotonin elevation and core symptoms of autism.

In cases with high serotonin levels, neurocognitive impairments (cognitive skill deficits), mood disorders, behavioral problems, verbal and nonverbal communication disorders, speech regression, and difficulties in initiating and maintaining social interactions were found to be associated with autism.

A direct correlation has been identified between elevated serotonin levels and the severity of these autism-related issues.