Insistence on Sameness (IS) is a core feature of autism, characterized by compulsive adherence to routine, and stereotyped, repetitive behaviors. This study will test two hypotheses about abnormalities in brain function that may contribute to IS, using functional magnetic resonance imaging (fMRI) and behavioral studies. While the cognitive model of impaired set-shifting implicates dorsolateral prefrontal cortex (PFC) and dorsal striatum, altered reward processes implicate ventral striatum and ventrolateral PFC. The first hypothesis is that the brain systems responsible for changing from repetitive behaviors to more flexible ones are impaired. Alternatively, individuals with autism may not recognize or respond to external cues or rewards intended to promote changes in behavior. The study will use tests that require individuals to change from one set of behaviors to another, based on reward, to examine behavioral flexibility in autism. Understanding the brain systems involved in changing behavior and their dysfunction will ultimately help guide treatment for this understudied yet disabling aspect of autism; further, investigating reward processes in this context has broad relevance, given the reduced impact of social and other cues that are typically used to reinforce and support adaptive behavior in autism.