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This study examined how restricted and repetitive behaviors and interests (RRBs) developed over time in a sample of children with autism spectrum disorders (ASD). One hundred ninety-two children referred for a diagnosis of autism at age 2, and 22 children with nonspectrum development disorders were evaluated with a battery of cognitive and diagnostic measures at age 2 and subsequently at ages 3, 5, and 9. Factor analysis of the RRB items on the Autism Diagnostic Interview–Revised revealed two RRB factors at each wave of data collection, one comprising “repetitive sensorimotor” (RSM) behaviors and the other “insistence on sameness” (IS) behaviors. For children with ASD, RSM scores remained relatively high over time, indicating consistent severity, whereas IS scores started low and increased over time, indicating worsening. Having a higher nonverbal intelligence (NVIQ) at age 2 was associated with …