The proposed study will build on an ongoing NIMH-funded study (R01 MH 067005 "A population-based twin study of autism in California" PI: Joachim Hallmayer). The existence of this population-based study of twins, with at least one twin with autism, will provide unprecedented opportunities to examine the relative impact of genetic and nongenetic factors on brain anatomy and chemistry in autism. Specifically, 80 same-sex autism twin pairs comprised of 40 monozygotic (identical) and 40 dizygotic (fraternal) twin pairs, will be randomly recruited from the ongoing study. These twin pairs will be compared to 40 group-matched typically developing same-sex twin pair controls (20 monozygotic and 20 dizygotic). Experimental measures include high resolution anatomical brain scans, cognitive and behavioral measures, and neurochemical measures. The overarching goal is to understand how clinical features of autism may be linked to neurobiological measures in autistic individuals, thus allowing the identification of clinical or biological endophenotypes that will lead to a better characterization and understanding of this disorder.