One of the major controversies in considering the causes of mental disorders is that of the nature-nurture debate. Proponents of the "nature" school of thought believe that the ultimate cause of psychopathology is biological and tend to emphasize the role of genes, infectious diseases, physical diseases, and malfunctions of the brain and nervous system in the onset of mental disorders. Individuals who place more emphasis on nurture, however, stress that the ultimate cause of psychopathology is experience. These scientists are more interested in how role learning, methods used to adapt to one's environment, societal demands, and parent-child relationships play in the onset and expression of psychological disorders.

One system that has developed that incorporates theories of both camps is that of the diathesis-stress model of abnormal behavior. This view assumes that the development of any disorder is the result of the interaction between some precipitating event and the predisposition of the individual. Psychopathology, then, can be multiply determined in that there is a predisposition, or vulnerability factor, for developing a disorder--the nature element--but it must be combined with some triggering event in the environment, such as stress or a challenging experience--the nurture factor--before abnormal behavior is expressed. One useful and common analogy used, therefore, is that genes load the gun, but it is the environment that pulls the trigger. The biopsychosocial systems approach, then, embodies the concept of a diathesis- stress model in that it proposes that the etiology of mental disorders is best understood as an interaction between biological, psychological, and social factors. Biological factors focus on the role that genes and neurophysiology play in the development of mental disorders. This category uses such techniques as adoption studies, twin studies, and the efficacy of drug treatments to understand the biological bases of psychopathology.

One major component of the psychological category is behavioral theories. The premise of these theories is that just as all normal behaviors are learned, so are abnormal behaviors. Proponents of this theory consider such issues as classical conditioning, operant conditioning, and modeling. Attachment theory is also an important factor of the psychological perspective. It is based on the assumptions that we all have a central need to establish a healthy relationship with our caregiver and that mental disorders arise when that core relationship has been violated, through such means as neglect and abuse. Motivation, temperament, and emotion also play a very important role within the psychological construct. Abraham Maslow, a humanist psychologist, for example, proposed that there is a hierarchy of basic human needs ranging from survival needs, such as the need for food, to self-actualization needs, such as the need to fulfill one's potential.

Mental disorders, then, arise from unmet or frustrated needs. One final category within the psychological approach to the etiology of mental disorders is the psychoanalytic approach, which emphasizes the importance of stages of development. Stages of psychological development are important especially because the stress associated with developmental transitions can serve as precipitators, or triggers, of mental disorders if there already exists a predisposing diathesis. The two main theories are that of Erik Erikson's theory of psychosocial development, emphasizing social conflicts and tasks, which continues throughout the life span, and Sigmund Freud's theory of psychosexual development, which emphasizes struggles regarding sexuality, where more emphasis is placed on internal conflicts of the child. Freud, especially, placed great emphasis on the struggles and conflicts between the unconscious and conscious mind. According to Freud, conflicts between the id, the pleasure-seeking drive, and the ego, the reality principle, lead to neurotic anxiety, while conflicts between the ego and the superego, which represents our internalization of societal rules and values, are manifested as moral anxiety.

The social category of the biopsychosocial model argues that there is something in society that fosters the mental disorder, such as assumed social roles. Although all mental disorders are shaped to some extent by culture, no mental disorder can be entirely attributed to cultural or social factors. (Psychotic disorders, such as schizophrenia, are less influenced by culture than non-psychotic disorders.) This field of research is especially interested in such characteristics as interpersonal relationships, and gender and race differences within disorders. More importantly, cross-cultural studies are very important in their assessment and demonstration of culture-bound syndromes. One theory of the social category that is very intriguing is the theory of cultural relativity. This theory argues that there is a full spectrum of behavior for human kind, but the role of culture precisely is to make parts of the spectrum allowable and other parts punishable. While this method maintains cohesiveness, identity, and unity amongst groups, it also artificially limits normal behavior within the human race.