Repetitive behavior disorders are prevalent among people with severe mental retardation. These disorders can interfere significantly with an individual’s daily functions. This trial is part of a long-term project that has studied the biologic basis of and possible treatments for repetitive behavior disorders. The trial will evaluate the effectiveness of two medications, a selective serotonin reuptake inhibitor (SSRI) and an atypical antipsychotic, in treating repetitive behavior disorders in people with mental retardation.