Abnormal psychology or psychopathology is the study, classification and treatment of mental disorders.

In ancient times mental disorders were believed to be caused by demonic possession. The treatment as such was an extremely brutal process called trephaning. In this process, a hole was drilled into the skull believing the demon inside will escape through it.

During the Vedic period in ancient India, a holistic system of medicine was developed known as Ayurveda. The ayurvedic model explained mental disorders as an imbalance of the three doshas(vata, pitta and kapha) in the human body.

During the 550BC-500AD in ancient Greece, greek physician Hippocrates came up with his own explanation of mental disorders. He proposed a model of four bodily fluids called humours whose imbalance led to mental disorders. These four humours were blood, phlegm, black bile and yellow bile. He also classified all known mental disorders of that time into three categories: mania(excessive emotion), melancholia(depression) and phrenitis(brain fever). He was the first to propose a biological model for the explanation of abnormal behaviour and disregard the importance of superstition in mental disorders.

During the dark ages, the Christian church rose to power and the demonology doctrine was followed. People showing abnormal behaviour were considered to be followers of Satan and were crucified. This belief persisted into the late middle ages till the Christian spirit of charity prevailed and Saint Augustine wrote extensively about mental conflict, feelings and emotions.

Compared to Europe, the Middle East fared quite better. The aspects of Greek science survived with them eventually leading to the establishment of the first mental hospital in Baghdad in 792 AD.

Johanne Weyer(1515-1588), a german physician and writer was deeply disturbed by the imprisonment, torture and burning of the mentally ill. He exposed the fallacies of attributing abnormal behaviour to witchcraft and demons. Along with Paracelsus, his contributions led to the rise of asylums.

In the 18th century, Phillipe Pinel spoke against the  maltreatment patients received at asylums across Europe and America. His views led to development of moral therapy. He also advocated meticulous maintenance of records of the the first arrival of the patient, the first onset of symptoms etc. At the same time, Benjamin Rush became the first president of the American Psychological Association. He advocated a biological model for mental disorders. However his method of treatment(bleeding, water therapy and surgery) were ineffective and produced little results.

Then came along Franz Mesmer, a german physician who is best known for his theory animal magnetism also called as mesmerism. His theory states that there is natural energetic transference that occurred between all animated and inanimate objects. Imbalance or disruption leads to metal disorders.

The most notable contribution in the 19th century in the field of abnormal psychology was given by Sigmund Freud. He theorized that all mental disorders were caused by unresolved conflicts stemming from infancy and childhood in the unconscious mind. He developed a technique to treat mental disorders effectively called psychoanalysis.

Dorothea Lynde Dix (1802–1887) was an American activist on behalf of the indigent insance who, through a vigorous program of lobbying state legislatures and the United State Congress, created the first generation of American mental asylums. During the Civil War, she served as a Superintendent of Army Nurses. Her contributions spread awareness all across the world about humanitarian treatment of mental patients. This led to Mental Hygiene Movement.

The 20th century witnessed many schools of though like Behaviourism, Humanitarianism, Cognitivism, Existentialism etc. Each explained abnormal behaviour and treatment in different ways.

Today the most popular form of treatment of metal disorders is Cognitive Behavioural theory that accounts for biological, psychological and social factors influencing behaviour.