This proposal represents the third step of a quantitative approach to identifying genetic loci that contribute to the susceptibility for autistic spectrum disorders. It seeks to provider further validation of a new measure of autistic spectrum deficits, the Social Reciprocity Scale (Constantino et al., 2000), which, in studies involving over 900 children, has proven feasible for use in large-scale epidemiologic samples. The Social Reciprocity Scale (SRS) is unique in that it is designed to quantify autistic traits across the entire spectrum from unaffected to severely affected (referred to as the "broader autism phenotype). It therefore is useful for identifying extreme discordant sib pairs in the population, which is potentially important for gene mapping studies in autism since such pairs provide the highest degree of statistical power for identification of quantitative trait loci. Although SRS scores are highly genetically influenced (Constantino and Todd, submitted) and correlate well with clinical diagnoses of autistic spectrum disorders (Constantino et a., 2000), the instrument has never yet been directly compared with established diagnostic instruments for autism. This is a proposal to validate the Social Reciprocity Scale with the Autism Diagnostic Interview-Revised (ADI-R) among clinical and research subjects with and without pervasive developmental disorders at the Washington University School of Medicine. If the SRS compares favorably with the ADI-R, the SRS could feasibly be used to study the genetics of autism by examining the genetic structure of autistic traits in whole populations.