The causes of mental disorders are complex, and interact and vary according to the particular disorder and individual. Genetics, early development, drugs, disease or injury, neurocognitive and psychological mechanisms, and life experiences, society and culture can all contribute to the development or progression of different mental disorders.

General theories

There are a number of theories or models seeking to explain the causes (etiology) of mental disorder. They may differ in their classification of mental disorders.

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.

Genes

Family-linkage and twin studies have indicated that genetic factors often play an important role in the development of mental disorders. The reliable identification of specific genetic susceptibility to particular disorders, through linkage or association studies, has proven difficult. This has been reported to be likely due to the complexity of interactions between genes, environmental events, and early development or to the need for new research strategies. The heritability of behavioral traits associated with mental disorder may be greater in permissive than in restrictive environments, and susceptability genes probably work through both "within-the-skin" (physiological) pathways and "outside-the-skin" (behavioral and social) pathways. Investigations increasingly focus on links between genes and endophenotypes — more specific traits (including neurophysiological, biochemical, endocrinological, neuroanatomical, cognitive, or neuropsychological) - rather than disease categories.

Pregnancy and birth

Environmental events surrounding pregnancy and birth have been linked to an increased development of mental illness in the offspring. This includes maternal exposure to serious psychological stress or trauma, conditions of famine, obstetric birth complications, infections, and gestational exposure to alcohol or cocaine. Such factors have been hypothesized to affect specific areas of neurodevelopment within the general developmental context and to restrict neuroplasticity.

People with developmental disabilities, such as mental retardation, are more likely to experience mental illness than those in the general community.

Disease, injury and infection

Higher rates of mood, psychotic, and substance abuse disorders have been found following traumatic brain injury (TBI). Findings on the relationship between TBI severity and prevalence of subsequent psychiatric disorders have been inconsistent, and occurrence has been linked to prior mental health problems as well as direct neurophysiological effects, in a complex interaction with personality and attitude and social influences.

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.

Individual characteristics

Mental characteristics of individuals, as assessed by both neurological and psychological studies, have been linked to the development and maintenance of mental disorders. This includes cognitive or neurocognitive factors, such as the way a person perceives, thinks or feels about certain things; or an individual's overall personality, temperament or coping style or the extent of protective factors or 'positive illusions' such as optimism, personal control and a sense of meaning.

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.

Life events, stresses and relationships

It is reported that there is good evidence on the importance of psychosocial influences on psychopathology in general, although less known about the specific risk and protective mechanisms. Maltreatment in childhood and in adulthood, including sexual abuse, physical abuse, emotional abuse, domestic violence and bullying, has been linked to the development of mental disorder, through a complex interaction of societal, family, psychological and biological factors. Negative or stressful life events more generally have been implicated in the development of a range of disorders, including mood and anxiety disorders. The main risks appear to be from a cumulative combination of such experiences over time, although exposure to a single major trauma can sometimes lead to psychopathology, including PTSD. Resilience to such experiences varies, and a person may be resistant to some forms of experience but susceptible to others. Features associated with variations in resilience include genetic vulnerability, temperamental characteristics, cognitive set, coping patterns, and other experiences.

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.

Neighbourhoods, society and culture

Problems in communities or cultures, including poverty, unemployment or underemployment, lack of social cohesion, and migration, have been implicated in the development of mental disorder. Stresses and strains related to socioeconomic position (socioeconomic status (SES) or social class) have been linked to the occurrence of major mental disorders, with a lower or more insecure educational, occupational, economic or social position generally linked to more mental disorder. There have been mixed findings on the nature of the links and on the extent to which pre-existing personal characteristics influence the links. Both personal resources and community factors have been implicated, as well as interactions between individual-level and regional-level income levels. The causal role of different socioeconomic factors may vary by country. Socioeconomic deprivation in neighbourhoods can cause worse mental health, even after accounting for genetic factors. In addition, minority ethnic groups, including first or second-generation immigrants, have been found to be at greater risk for developing mental disorders, which has been attributed to various kinds of life insecurities and disadvantages, including racism. The direction of causality is sometimes unclear, and alternative hypotheses such as the Drift Hypothesis sometimes need to be discounted.

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.

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