Within abnormal psychology, there are many theories and theorists who have influenced the growth of the subject through time. In the past, abnormality was seen as something more spiritual and supernatural, such as witchcraft or evil spirits. As medicine advanced, more modern perspectives arose, including biological, psychological, and social and interpersonal theories.
Biological theories in relation to abnormality examine behaviour and classify it as being caused by biological factors, including defective genes, neurotransmitter and hormone imbalances, and structural brain abnormalities. The famous case of Phineas P. Gage demonstrates how a brain injury can drastically alter an individual's personality and behaviour. These theories are much more medicalized and focus on the disorder as a disease rather than caused by the environment.
Issues related more to the environment, such as a person's upbringing being maladaptive, chronic stress in life, and lack of social support, are included in the social approach to abnormal behaviour. One theory under the social approach to mental disorders is the interpersonal theory, which discusses how mental illness can arise due to negative relationships and the maladaptive behaviours that result from them. The social structural theory discusses how society causes such great stress and restriction on people that it can influence the development of a mental disorder, and family systems theory zeros in on how the family can influence and maintain a mental disorder.
Lastly, a psychological approach to abnormality focuses on how a person's belief system, early childhood experiences, skills, cognitions, and inner conflicts can influence abnormal behaviour and the development of disorders. Some of the areas under the psychological approach include psychodynamic theories (conflicts in the mind), behavioural theories (conditioning effects), cognitive theories (self-concepts and thoughts), and humanistic and existential theories (outward demands and pressure).
In many situations, it is not just one factor leading to the development of a mental disorder; therefore, the diathesis-stress model was created, which lists that an individual may have a vulnerability in either biological, social, and/or psychological factors, and then a trigger in any of these same areas in life could lead to the development of a mental disorder.