Schizophrenia is a mental disorder characterized variously by hallucinations, delusions, disorganized thinking or behavior, and flat or inappropriate affect. Symptoms develop gradually and typically begin during young adulthood. There is no objective diagnostic test; diagnosis is based on observed behavior, a psychiatric history that includes the person's reported experiences, and reports of others familiar with the person. For a formal diagnosis, the described symptoms need to have been present for at least six months or one month. Many people with schizophrenia have other mental disorders, especially mood, anxiety, and substance use disorders, as well as obsessive–compulsive disorder (OCD).