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Abnormal psychology is the branch of psychology that studies unusual patterns of behavior, emotion and thought, which may or may not be understood as precipitating a mental disorder. There is a long history of attempts to understand and control behavior deemed to be aberrant or deviant (statistically, morally or in some other sense), and there is often cultural variation in the approach taken. The field of abnormal psychology identifies multiple causes for different conditions, employing diverse theories from the general field of psychology and elsewhere, and much still hinges on what exactly is meant by "abnormal". There has traditionally been a divide between psychological and biological explanations, reflecting a philosophical dualism in regard to the mind body problem, as well as different approaches to the classification of mental disorders. Abnormal includes three different categories, they are subnormal, supernormal and paranormal. 
The science of abnormal psychology studies two types of behaviours: adaptive and maladaptive behaviours. Behaviours that are maladaptive suggest that some problem(s) exists, and can also imply that the individual is vulnerable and cannot cope with environmental stress, which is leading them to have problems functioning on a daily basis. Clinical psychology is the applied field of psychology that seeks to assess, understand and treat psychological conditions in clinical practice. The theoretical field known as 'abnormal psychology' may form a backdrop to such work, but clinical psychologists in the current field are unlikely to use the term 'abnormal' in reference to their practice. Psychopathology is a similar term to abnormal psychology but has more of an implication of an underlying pathology (disease process), and as such is a term more commonly used in the medical specialty known as psychiatry.
People have tried to explain and control abnormal behavior for thousands of years. Historically, there have been three main approaches to abnormal behavior: the supernatural, biological, and psychological traditions. In order to understand abnormal psychology, it is essential to first understand what we mean by the term "abnormal"? On the surface, the meaning seems obvious: something that is outside of the norm. But are we talking about the norms of a particular group, gender or age? Many human behaviors can follow what is known as the normal curve. Looking at this bell-shaped curve, the majority of individuals are clustered around the highest point of the curve, which is known as the average. People who fall very far at either end of the normal curve might be considered 'abnormal'.
In the supernatural tradition, also called the demonological method, abnormal behaviors are attributed to agents outside human bodies. According to this model, abnormal behaviors are caused by demons, spirits, or the influences of moon, planets, and stars. During the Stone Age, trephining was performed on those who had mental illness to literally cut the evil spirits out of the victim's head. Conversely, Ancient Chinese, Ancient Egyptians, and Hebrews, believed that these were evil demons or spirits and advocated exorcism. By the time of the Greeks and Romans, mental illnesses were thought to be caused by an imbalance of the four humors, leading to draining of fluids from the brain. During the Dark Ages, many Europeans believed that the power of witches, demons, and spirits caused abnormal behaviors. People with psychological disorders were thought to be possessed by evil spirits that had to be exorcised through religious rituals. If exorcism failed, some authorities advocated steps such as confinement, beating, and other types of torture to make the body uninhabitable by witches, demons, and spirits. The belief that witches, demons, and spirits are responsible for the abnormal behavior continued into the 15th century. Swiss alchemist, astrologer, and physician Paracelsus (1493–1541), on the other hand, rejected the idea that abnormal behaviors were caused by witches, demons, and spirits and suggested that people's mind and behaviors were influenced by the movements of the moon and stars.
This tradition is still alive today. Some people, especially in the developing countries and some followers of religious sects in the developed countries, continue to believe that supernatural powers influence human behaviors. In Western academia, the supernatural tradition has been largely replaced by the biological and psychological traditions.
In the biological tradition, psychological disorders are attributed to biological causes and in the psychological tradition, disorders are attributed to faulty psychological development and to social context. The medical or biological perspective holds the belief that most or all abnormal behavior can be attributed to a medical factor; assuming all psychological disorders are diseases.
The Greek physician Hippocrates, who is considered to be the father of Western medicine, played a major role in the biological tradition. Hippocrates and his associates wrote the Hippocratic Corpus between 450 and 350 BC, in which they suggested that abnormal behaviors can be treated like any other disease. Hippocrates viewed the brain as the seat of consciousness, emotion, intelligence, and wisdom and believed that disorders involving these functions would logically be located in the brain.
These ideas of Hippocrates and his associates were later adopted by Galen, the Roman physician. Galen extended these ideas and developed a strong and influential school of thought within the biological tradition that extended well into the 18th century.
Behavioral: The behavioral approach to abnormal psychology focuses on observable behaviors. In behavioral therapy, the focus is on reinforcing positive behaviors and not reinforcing maladaptive behaviors. This approach targets only the behavior itself, not the underlying causes.
Medical: The medical approach to abnormal psychology focuses on the biological causes on mental illness. This perspective emphasizes understanding the underlying cause of disorders, which might include genetic inheritance, related physical disorders, infections and chemical imbalances. Medical treatments are often pharmacological in nature, although medication is often used in conjunction with some other type of psychotherapy. http://psychology.about.com/od/abnormalpsychology/f/abnormal-psychology.htm
Throughout time, societies have proposed several explanations of abnormal behavior within human beings. Beginning in some hunter-gatherer societies, animists have believed that people demonstrating abnormal behavior are possessed by malevolent spirits. This idea has been associated with trepanation, the practice of cutting a hole into the individual's skull in order to release the malevolent spirits. Although it has been difficult to define abnormal psychology, one definition includes characteristics such as statistical infrequency. 
A more formalized response to spiritual beliefs about abnormality is the practice of exorcism. Performed by religious authorities, exorcism is thought of as another way to release evil spirits who cause pathological behavior within the person. In some instances, individuals exhibiting unusual thoughts or behaviors have been exiled from society or worse. Perceived witchcraft, for example, has been punished by death. Two Catholic Inquisitors wrote the Malleus Maleficarum (Latin for 'The Hammer Against Witches'), that was used by many Inquisitors and witch-hunters. It contained an early taxonomy of perceived deviant behavior and proposed guidelines for prosecuting deviant individuals.
The act of placing mentally ill individuals in a separate facility known as an asylum dates to 1547, when King Henry VIII of England established the St. Mary of Bethelem asylum. Asylums remained popular throughout the Middle Ages and the Renaissance era.
The number of different theoretical perspectives in the field of psychological abnormality has made it difficult to properly explain psychopathology. The attempt to explain all mental disorders with the same theory leads to reductionism (explaining a disorder or other complex phenomena using only a single idea or perspective). Most mental disorders are composed of several factors, which is why one must take into account several theoretical perspectives when attempting to diagnose or explain a particular behavioral abnormality or mental disorder. Explaining mental disorders with a combination of theoretical perspectives is known as multiple causality.
The diathesis–stress model emphasizes the importance of applying multiple causality to psychopathology by stressing that disorders are caused by both precipitating causes and predisposing causes. A precipitating cause is an immediate trigger that instigates a person's action or behavior. A predisposing cause is an underlying factor that interacts with the immediate factors to result in a disorder. Both causes play a key role in the development of a psychological disorder.
Abnormal psychology revolves around two major paradigms for explaining mental disorders, the psychological paradigm and the biological paradigm. The psychological paradigm focuses more on the humanistic, cognitive and behavioral causes and effects of psychopathology. The biological paradigm includes the theories that focus more on physical factors, such as genetics and neurochemistry.
The standard abnormal psychology and psychiatry reference book in North America is the Diagnostic and Statistical Manual of the American Psychiatric Association. The current version of the book is known as DSM IV-TR. It lists a set of disorders and provides detailed descriptions on what constitutes a disorder such as Major Depressive Disorder or anxiety disorder. It also gives general descriptions of how frequently the disorder occurs in the general population, whether it is more common in males or females and other such facts. The diagnostic process uses five dimensions called "axes" to ascertain symptoms and overall functioning of the individual. These axes are as follows
The major international nosologic system for the classification of mental disorders can be found in the most recent version of the International Classification of Diseases, 10th revision (ICD-10). The ICD-10 has been used by World Health Organization (WHO) Member States since 1994. Chapter five covers some 300 mental and behavioral disorders. The ICD-10's chapter five has been influenced by APA's DSM-IV and there is a great deal of concordance between the two. WHO maintains free access to the ICD-10 Online. Below are the main categories of disorders:
Psychologists may uses different perspectives to try to get better understanding on abnormal psychology. Some of them may just concentrate on a single perspective. But the professionals prefer to combine two or three perspectives together in order to get significant information for better treatments.