The Diagnostic and Statistical Manual of Mental Disorders (DSM), which lists types of mental illness as recognized by the American Psychiatric Association, contains more than 400 different mental disorders. Any given disorder may exhibit different symptoms in different people. It is no surprise then that the signs of mental illness cover a wide range and combination of behaviors. Several behaviors can be considered common signs of mental illness, including extreme or unexplained anxiety, delusions, and severe moods.
Psychologists use the DSM to determine what, if any, disorder the mental health patient has. In order to be considered a mental illness, the behaviors in question must meet certain criteria. The symptoms must either be such that they keep the person from functioning normally in society or cause the person significant distress. Deviant behavior, behavior that varies from social norms, is not considered one of the signs of mental illness unless it fits a specific disorder listed in the DSM.
One of the common signs of mental illness is severe or unexplained anxiety. The DSM defines anxiety as a state of mental and physical arousal accompanied by fear and a vague sense of disaster. Most people just know it when they feel it. If the anxiety has a clear source, such as an upcoming test or a change in life circumstances, it is considered to be a normal reaction. If, however, the anxiety is triggered by small happenings or has no known cause, it may be a sign of mental illness.
Delusions are another of the common signs of mental illness. When a person holds a false belief and does not waver from that belief even when proven wrong, the person is delusional. For example, many schizophrenics believe that someone is out to get them or that they are constantly under surveillance. Hallucinations are like delusions of the senses. Patients may taste, see, or most commonly hear things that aren't there.
Another of the common signs of mental illness is severe mood or severe changes in mood. Mood disorders include depressive disorders and bipolar disorders. These mood changes can reach one of two extremes. Depression is a sense of despair and hopelessness usually marked by low energy, appetite, and libido. Mania is the opposite, marked by high energy and an inflated self-image.
Signs of mental illness can appear in all areas of a person's life. Everything from impulsiveness to the inability to read at a level that is considered normal for a particular age group can be a sign of mental illness. It is easy to over-scrutinize normal behaviors until they seem to be on the point of mental illness. The best way to know if someone has a mental illness is to speak to a psychologist or other mental health professional.