The overall goal is to characterize the autism-related phenotype (lesser variant) in non-autistic school-aged siblings of autistic probands for use in future genetic studies. Although language, social, and psychiatric abnormalities are known to exist in parents of autistics, most reports about deficits in school-aged autism sibs come from family history studies or studies with methodologic limitations. Some deficits (reading, spelling) identified in those reports have not been found in a methodologically sound, direct study of autism sibs. The specific aims are to: (1) estimate the frequency of disorders that may be genetically related to autism in school-aged siblings of idiopathic autistics compared to individually-matched 'normal' controls (sibs of Down syndrome probands to control for familial stresses related to the presence of a handicapped child); )2) characterize the lesser variant of autism for use in future genetic studies by testing all subjects in aspects of functioning (language, social, psychiatric) that are abnormal in non- retarded autistics; and (3) assess whether sibling morbidity is adequately explained by stress related to the presence of an autistic child in the family by examining the relationship between language, social, and psychiatric disorder in siblings, severity of proband behavior, and family stress. These aims stem from pilot findings of language formulation deficits and social deficits in autism sibs, who sometimes has ongoing or past psychiatric disorders. The language and social disorders in autism sibs appear to be milder versions of those seen in autism. Fifty autism sibs and 50 individually-matched controls will receive tests of (1) expressive and receptive grammar, semantic, and discourse ability; )2) social problem solving and peer relationships; (3) psychiatric disorder; and (4) the Family Environment Scale (a measure of perceived stress). Autism sibs will be ascertained from the Hopkins Autism Family Study. Thus, data interpretation will be facilitated by the large family study data set, including information about these sibs (reading, spelling, pre-, peri- and neonatal risk factors), their parents (based on direct cognitive, social, psychiatric, and language assessment), autistic sibling, (IQ, nature and severity of autistic symptoms) and extended family (family history). This study is important for defining the autism-related phenotype (necessary for genetic analyses) and will contribute to our understanding of abnormal mechanism in autism.