Abnormal behavior includes actions, thoughts, and feelings that are harmful to the person and/or to others. Historically, the causes of abnormal behavior have been explained by supernatural theories, biological theories, and psychological theories. Contemporary psychologists view abnormal behavior as a natural phenomenon with both biological and psychological causes. The term insanity is a legal term with several different meanings.

Anxiety disorders, characterized by excessive anxiety, include the following: (1) phobias, which are intense and unrealistic fears; (2) general anxiety disorders, which are characterized by free-floating anxiety; (3) panic anxiety disorders, which involve attacks of intense anxiety; (4) posttraumatic stress disorders, a reaction to the stress of war, assault, or other trauma; and (5) obsessive-compulsive disorders, characterized by persistent, anxiety-provoking thoughts and by urges to repeatedly engage in a behavior.

Somatoform disorders are conditions in which the individual experiences symptoms of health problems that are psychological rather than physical in origin. One type of somatoform disorder is referred to as somatization disorder. This disorder involves multiple minor symptoms of illness that indirectly create a high risk of medical complications; hypochondriasis is characterized by excessive concern with health. Another type of somatoform disorder is the conversion disorder. This involves serious specific somatic symptoms without any physical cause; somatoform pain disorders involve pain without any physical cause.

In the various types of dissociative disorders, there is a change in memory, perception, or identity. For example, dissociative amnesia and dissociative fugue are characterized by memory loss that has psychological rather than physical causes. Individuals experiencing depersonalization feel that they or their surroundings have become distorted or unreal. Individuals who exhibit dissociative identity disorder (multiple personality) appear to possess more than one personality in the same body. The recent increase in the diagnosis of dissociative identity disorder has stirred controversy.

Affective disorders are disturbances of mood. The individual experiencing major depression is deeply unhappy and lethargic. In the condition known as bipolar affective disorder, periods of mania alternate irregularly with periods of severe depression.

Schizophrenia involves three major areas of abnormality: delusions and hallucinations, disorganized thinking emotions and behavior, and reduced enjoyment and interests. The major types of schizophrenia include (1) paranoid schizophrenia, in which the individual holds false beliefs or delusions - usually of grandeur and persecution - that seriously distort reality; (2) disorganized schizophrenia, which is characterized by extreme withdrawal from normal human contact, fragmented delusions and hallucinations, and a shallow "silliness" of emotion; and (3) catatonic schizophrenia, which is marked by stupors during which the individual may remain in the same posture for long periods of time. Psychologists believe that schizophrenia may have biological causes such as deterioration of the cortex and abnormal prenatal development. Psychologists also view stress as a trigger for the disorder in those who are genetically predisposed to schizophrenia.

Personality disorders are thought to result from personalities that developed improperly during childhood rather than from breakdowns under stress. Schizoid personality disorders are characterized by a loss of interest in proper dress and social contact, a lack of emotion, and an inability to hold regular jobs. The antisocial personality frequently violates social rules and laws, is often violent, takes advantage of others, and feels little guilt about it.

Two important societal issues that have important implications for psychology are homelessness and physician-assisted suicide.