Autism, originally described by Kanner (1943), is among the most severe of neuropsychiatric disorders. It is a pervasive developmental disorder affecting social, communicative, and compulsive/repetitive behaviors characterized by stereotypic complex hand and body movements, craving for sameness, and narrow repetitive interests. Individuals with autism spectrum disorders (ASD) are characteristically heterogeneous and show marked variability in their response to interventions. Studies of behavioral and psychopharmacological interventions document approximately 1/3 of ASD participants fail to respond to targeted treatments. Efforts to evaluate the specificity of treatment effects are important to inform conceptualizations about the disorder, identify behavioral phenotypes, and to aide clinical decision making.