The most common view is that disorders tend to result from genetic vulnerabilities and environmental stressors combining to cause patterns of dysfunction or trigger disorders (Diathesis-stress model). A practical mixture of models may often be used to explain particular issues and disorders, although there may be difficulty defining boundaries for indistinct psychiatric syndromes.
The primary model of contemporary mainstream Western psychiatry is the biopsychosocial (BPS), which merges together biological, psychological and social factors. It may be commonly neglected or misapplied in practice due to being too broad or relativistic, however, and biopsychiatry has tended to follow a biomedical model focused on organic" or "hardware" pathology of the brain.
Psychoanalytic theories, focused on unresolved internal and relational conflicts, have been posited as overall explanations of mental disorder, although today most psychoanalytic groups are said to adhere to the biopsychosocial model and to accept an eclectic mix of subtypes of psychoanalysis.
Evolutionary psychology (or more specifically evolutionary psychopathology or psychiatry) has also been proposed as an overall theory, positing that many mental disorders involve the dysfunctional operation of mental modules adapted to ancestral physical or social environments but not necessarily to modern ones. Attachment theory is another kind of evolutionary-psychological approach sometimes applied in the context for mental disorders, which focuses on the role of early caregiver-child relationships, responses to danger, and the search for a satisfying reproductive relationship in adulthood.
An overall distinction is also commonly made between a "medical model" (also known as a biomedical or disease model) or a "social model" (also known as an empowerment or recovery model) of mental disorder and disability, with the former focusing on hypothesized disease processes and symptoms, and the latter focusing on hypothesized social constructionism and social contexts.
People with developmental disabilities, such as mental retardation, are more likely to experience mental illness than those in the general community.
A number of psychiatric disorders have often been tentatively linked with microbial pathogens, particularly viruses; however while there have been some suggestions of links from animal studies, and some inconsistent evidence for infectious and immune mechanisms (including prenatally) in some human disorders, infectious disease models in psychiatry are reported to have not yet shown significant promise except in isolated cases. There have been some inconsistent findings of links between infection by the parasite Toxoplasma gondii and human mental disorders such as schizophrenia, with the direction of causality unclear. A number of diseases of the white matter can cause symptoms of mental disorder.
Poorer general health has been found among individuals with severe mental illnesses, thought to be due to direct and indirect factors including diet, bacterial infections, substance use, exercise levels, effects of medications, socioeconomic disadvantages, lowered help-seeking or treatment adherence, or poorer healthcare provision. Some chronic general medical conditions have been linked to some aspects of mental disorder, such as AIDS-related psychosis.
The current research on Lyme's Disease caused by a deer tick, and related toxins, is expanding the link between bacterial infections and mental illness.
Abnormal levels of dopamine activity have been implicated in a number of disorders (e.g., reduced in ADHD, increased in Schizophrenia), thought to be part of the complex encoding of the importance of events in the external world. Dysfunction in serotonin and other monoamine neurotransmitters such as norepinephrine and dopamine has also been centrally implicated in mental disorders, including major depression as well as obsessive compulsive disorder, phobias, posttraumatic stress disorder, and generalized anxiety disorder, although the limitations of a simple "monoamine hypothesis" have been highlighted and studies of depleted levels of monoamine neurotransmitters have tended to indicate no simple or directly causal relation with mood or major depression, although features of these pathways may form trait vulnerabilities to depression. Dysfunction of the central gamma-aminobutyric (GABA) system following stress has also been associated with anxiety spectrum disorders and there is now a body of clinical and preclinical literature also indicating an overlapping role in mood disorder.
Findings have indicated abnormal functioning of brainstem structures in disorders such as schizophrenia, related to impairments in maintaining sustained attention. Some abnormalities in the average size or shape of some regions of the brain have been found in some disorders, reflecting genes and/or experience. Studies of schizophrenia have tended to find enlarged ventricles and sometimes reduced volume of the cerebrum and hippocampus, while studies of (psychotic) bipolar disorder have sometimes found increased amygdala volume. Findings differ over whether volumetric abnormalities are risk factors or are only found alongside the course of mental health problems, possibly reflecting neurocognitive or emotional stress processes and/or medication use or substance use. Some studies have also found reduced hippocampal volumes in major depression, possibly worsening with time depressed.
Relationship issues have been consistently linked to the development of mental disorders, with continuing debate on the relative importance of the home environment or work/school and peer group. Issues with parenting skills or parental depression or other problems may be a risk factor. Parental divorce appears to increase risk, perhaps only if there is family discord or disorganization, although a warm supportive relationship with one parent may compensate. Details of infant feeding, weaning, toilet training etc do not appear to be importantly linked to psychopathology. Early social privation, or lack of ongoing, harmonious, secure, committed relationships, have been implicated both in childhood (including in institutional care) and also through the lifespan.in social relations and the experience of loneliness, particularly during adolescence, and the development of mental disorder.
Mental disorder has also been linked to the overarching social, economic and cultural system. A value system that promotes individualism, weakens social ties, and creates ambivalence towards children, is being spread or imposed via globalization, yet could adversely affect children's mental health.