Most investigators agree that autistic disorders have a biological basis with a heterogeneous and varying etiology. Autopsy studies report as the most consistent finding a decrease in the number of Purkinje cells in the cerebellum. Neuroanatomic research as well underscores abnormalities in the posterior cranial fossa, or selective subcortical abnormalities of the limbic system or basal ganglia. Event-related potential studies reveal that autistic subjects are deficient in the processing of complex and new information, either when visssually or auditory stimulated. Hyperserotoninemia is found in 30% of the cases. The specificity of most findings is low, however.