The presence of a chronic illness in one "identical" twin, but not both twins, can help distinguish genetic from environmental contributions and define the disorder more precisely. Kates et al. (p. 539) studied brain structure in monozygotic (identical) twins who were concordant or discordant for the narrow phenotype for autism to identify anatomic abnormalities related to autism. They also studied a normal comparison group. Within twin pairs, the discordant twin pairs diverged in the volumes of gray and white matter in the cerebellum, whereas concordant twin pairs had convergent cerebellar volumes. Both the discordant and concordant pairs showed between-twin similarity in gray and white matter volumes in the cerebrum. However, in the discordant pairs, both the twins without autism and the affected co-twins had lower white matter volumes than healthy subjects in the frontal, temporal, and occipital lobes. These abnormalities in the twins without autism confirm autism’s genetic substrate and the concept of a broader autism phenotype. The broader phenotype was supported by language or social impairment in the nonautistic twin in eight of the nine discordant pairs. F1