Clinicians and researchers alike have questioned and studied the origin of schizophrenia for decades. Since the early 1900s, a genetic hypothesis has been suggested. This article reviews studies on a possible genetic cause of schizophrenia by use of various methods of comparison: twins, adoption, family history, family studies, and studies of biologic markers. Most researchers conclude that schizophrenia is likely caused by a genetic predisposition coupled with environmental influences.