Risperidone is an important medication used to treat children with psychiatric illnesses or neurodevelopmental disorders, such as autism. Despite excellent symptom control, the potential for side effects is worrisome. Treating these disorders is difficult because not everyone responds the same way to the same risperidone dose. One reason for this is genetic differences in how people break down the drug. Understanding these differences will help clinicians choose a dose and better predict the response so patients will be treated successfully with a lower risk for side effects. This study will research these genetic differences in children with psychiatric or neurodevelopmental disorders. Hypothesis: The inter-patient variability in risperidone pharmacokinetics and exposure, adverse events, and clinical response in patients with psychiatric or neurodevelopmental disorders is associated with identifiable pharmacogenetic factors, such as CYP2D6 single nucleotide polymorphisms (SNPs).