Abnormal psychology focuses on describing patterns of behavior that deviate from accepted social norms. There is no single, simple definition of abnormality.
Four broad definitions of behavior that deviates from the norm have been described in the literature. 
- The first definition is norm violation. This view of abnormality views humans in the context of their inability to function within the norms of their culture, and may have negative consequences for the individual. Each culture has a set of rules that define right and wrong, which are called norms. These norms guide many aspects of a person's interpersonal-relationships, and can widely differ from one culture to another. For example, it is polite in some cultures for people to stand close together and maintain eye contact during a conversation. Other cultures may have norms where distance and humbleness are accepted as the norm. In either culture, most who do not conform to the accepted standards, are considered abnormal.
- The second definition is statistical rarity. Abnormality is calculated as a substantial deviation from the population average. Those people who are statistically rare are considered abnormal. For example, a person who washes their hands excessively each day, is considered obsessive-compulsive.
- The third definition is maladaptive behavior. Societies expect their members to meet the demands of everyday life, and not engage in harmful behavior. If a person cannot pay their bills, hold down a job, or socialize with friends or family, then their behavior could be considered maladaptive, if the individual choses the behavior, and it is not a product of hardships that effect the individual through social and economic hardships in the society as a whole. Managing one's life successfully is generally a norm, and it takes a person's lifestyle into consideration, if the norms are not met.
- The fourth definition is personal discomfort, when a person is distressed over their own thoughts, feelings, behaviors or social and economic situations, which can lead to developing a negative schema about themselves, and create deviant behaviors.
- Harmful behavior to self or others
- Poor contact with reality
- Emotional reactions unsuitable to the situation
- Erratic, unpredictable behavior
Early views of abnormal behavior were explained by remote and supernatural forces, or as a battle between good and evil. Possession by evil spirits was treated by exorcism or trephination. Exorcisms were performed to make the body uncomfortable for an evil spirit to inhabit. These commonly included prayer, noisemaking, and special potions, although some possessed people were submerged in water, whipped, or starved. Trephination is a form of surgery in which a hole is drilled in the skull. This was performed because it was believed inhabiting spirits caused abnormality, and by creating holes it would release the spirits.
A major breakthrough occurred in 1897 when Krafft-Ebing made the discovery that general paresis was caused by syphilis. This was the first mental disorder found to have a biological cause, and advanced the medical model of psychopathology.
- Behavioral psychology - this paradigm views behaviour as a result of interaction with the environment. Abnormal behaviour may be the result of causes or circumstances in the environment. Much of this field is based on Pavlov's classical and B. F. Skinner's operant conditioning.
- Cognitive psychology - this paradigm has a strong focus on understanding the mechanisms of the mind in order to explain behavior. This perspective views abnormal behaviour as the product of mental processing, much in the same vein as computer programming.
- Neuroscience - is a medical view that seeks to understand abnormality by studying the neurological bases of behaviour. It focuses on the interaction between behaviour and biological functions of the brain, para-sympathetic nervous system, limbic system of which the Amygdala the fright-flight mechanism that monitor's an individuals internal and external dangers within the skull, which is located between the optic nerves and the brain, and if damaged, or diseased, may create a host of neurological symptoms that may include, panic attacks, depression, hyper-vigilance and other perceived mental disorders. Some fields are looking at classifying some known disorders associated with the Amydala as somatic dis-orders rather than mental disorders, though the distinction's require furture research and qualified study groups.
Mental health professionals classify mental disorders with by category. The use of labels allows communication between clinicians. Labels are valuable for statistical purposes in research, and for bridging the gap between research and treatment. Categories contribute to planning of treatment programs and facilities.
The use of classification has not gone without controversy for several reasons. Labels of mental illness carry social stigma. The current diagnostic categories are imperfect since mental disorders rarely conform to exactly one label and may overlap. A different system of dimensional classification has been presented as an alternative. This is based on dimensions of pathology, for example, a person would have several ratings of severity scales such as depression, anxiety, and obsessions.
In North America, the main diagnostic text is the Diagnostic and Statistical Manual of Mental Disorders of the American Psychiatric Association. The current version of the handbook is known as the DSM IV-TR. It lists a categories of disorders and provides a detailed description of each one. It also gives general descriptions of incidence, prevalence, and duration. The diagnostic process uses five dimensions called 'axes' to ascertain symptoms and overall functioning of the individual.
Internationally, the system for the classification of mental disorders is the 10th edition of the International Classification of Diseases (ICD-10). The ICD-10 has been used by WHO Member States since 1994. Chapter five covers about 300 mental and behavioral disorders, and forms the basis of the APA's DSM-IV-TR. The ICD-10 is available Online.
- Alloy, Lauren B.; John H. Riskind, Margaret J. Manos (2004). Abnormal Psychology: Current Perspectives. McGraw-Hill. ISBN 007242298X.
- Maher, W. B. & Maher, B. A. (1985). Psychopathology: I. From ancient times to the eighteenth century. In G. A. Kimble & K. Schlesinger (Eds.), Topics in the history of psychology (Vol. 2). Hillsdale, NJ: Erlbaum.