Abnormal psychology is the study of abnormal behavior in order to explain, foretell, explicate, and change abnormal patterns of implementation. Abnormal psychology studies the natural history of psychopathology and its causes, and this knowledge is applied in clinical psychology to take care of a patient with psychological disorders.

In the study of abnormal activities, it can be complex to define the line between which behaviors are considered normal and which are not. In wide-ranging, abnormal behaviors must be maladaptive and cause a subjective anxiety (signs of emotional distress). In general, abnormal behaviors are classified as:

• Abnormal as in "irregular" in relative to the overall population.
• The behavior fails to endorse well being, growth, and accomplishment of a person.
• Abnormal as in "unexpected". The behavior is not communally acceptable.
• Abnormal as in "indefensible". The behavior that cannot be streamlined.