The term "Mental Disease or Disorder" refers to a set of disorders affecting a person's thinking, mood, behavior and communication which may be identified as such by the competent authority, for example, a court, doctors, nurses, social workers, psychologists etc. Under the law, a patient may be declared mentally incompetent for a specific limited period (usually one year) after which the competence can be extended by the court, taking into account the extent and nature of the mental disorder. Mental disease or disorder may also be a reference to a syndrome (a collection of symptoms) which may, in turn, identify the presence of an individual mental disorder or disease, or to a group of symptoms indicative of a mental disorder or disease but not necessarily including all the symptoms of that disease: e.g. manic depression may also be regarded as manic depressions, and so on.

Mental disease or disorder refers to any abnormality in the function of the brain and its functions, characterized by different degrees of abnormalities in the behavior of the affected person, whether manifested externally or internally (which is often called as the clinical manifestation of the disease). Mental disease can be either psychological, olfactory, physical, neurological and behavioral; some are more common than others. Mental disease can be separated into distinct groups: Schizophrenia, Manic Depression, Social Phobia, Post-Traumatic Stress Disorder, Agoraphobia, Obsessive-Compulsive Disorder, Personality Disorders, Cognitive disorders and Dysmorphic Disorder.

Some are still uncertain about the precise relationship between mental illness and criminal behavior; however, there are cases in which it is clearly visible that somebody has been subjected to severe mental disorder and has been subjected to criminal behavior, without having in any way undergone any type of therapy or any form of medication, at least from the police's point of view. Schizophrenia is one of the most common forms of mental illness and is characterized by hallucinations, delusions, paranoia and panic attacks. Psychopathic criminals have been known to display similar traits, as well. The causes of schizophrenia are still under debate, even though a recent study claims to have established a genetic factor (although this hasn't been proven yet). There is much controversy in the field of psychiatry, and the debate continues to rage on.