Mental, or psychological, illnesses and disorders occur in or relate to the mind. Mental disorders differ from physical (related to the body) disorders because there are usually no physical symptoms of a mental disorder (such as a broken arm or an upset stomach) for a doctor to observe. Mental disorders originate in the mind of an individual and display themselves outwardly through a person's behavior or emotions. When behavior or emotions are deemed "abnormal," a mental illness might be at the root of the problem.

In a world in which cultural and social differences are abundant, particularly as one moves from one country or region to another, it is very difficult to define what is "normal" behavior over what is "abnormal" behavior. However, abnormal behavior has come to be identified through the presence of different coexisting characteristics or conditions:

Infrequency is one facet of abnormal behavior...