The occurrence of mental illness can be caused by different factors or a combination of these several factors. Different explanations are offered by different schools of thought, such as biological, psychological or social explanations for causing mental illness. Current theories however, have mostly acknowledged that all three contribute in varying amounts to a person's mental ill-health. Many psychiatric diseases can be classified as syndromes, or the association of several clinically recognizable features that do not always have a single cause.

The currently most popular explanation for mental illness is biological explanations. It believes that a person suffering from mental illness may exhibit a difference in brain structure or function or in neurochemistry, either through genetic or environmental vulnerabilities. Research findings for example, have shown that people with schizophrenia have enlarged ventricles and reduced gray matter in the brain. In addition, other studies have also found that imbalance in neurotransmitters could also play a factor in causing mental illness. Studies have focused on the neurotransmitters dopamine, nor epinephrine, and serotonin. If these chemicals are out of balance or may not be working properly, messages sent to the brain may not be sent correctly leading to symptoms of mental illness.

Problems in mental conditions may also be caused by injury to certain areas of the brain.

Moreover, many genetic studies have showed strong evidence that mental illness can be inherited, such as in the cases of schizophrenia and bipolar disorder. Many mental illnesses have been found to run in a family, which suggests that people with a mentally ill family member are more likely to develop mental illness as well. Experts link mental illness to abnormalities in many genes, and not just one. It is possible that a person who has inherited a susceptibility to this illness may not develop the illness. The occurrence of mental illness comes from the interaction of multiple genes and other factors such as stress, traumatic event, or abuse, which can trigger an illness in a person who has inherited genes susceptible to it.

Brain damage and the development of mental illness had also been found to have been caused by certain infections. For instance, a condition known as pediatric autoimmune neuropsychiatric disorder (PANDA) has been closely associated with the Streptococcus bacteria, linked to the development of certain mental illnesses in children.

Prenatal damage caused by disruption of early fetal brain development such as loss of oxygen to the brain, may be a factor in the development certain mental illness. Other factors such as poor nutrition and exposure to toxins could also be responsible or contributory to insanity.

Psychological explanations suggest that individual conflict, stress, crises, or traumatic experiences could trigger mental illness in especially vulnerable individuals. The early loss of a significant person/s such as a parent; neglect; and the lack of ability to relate to others may be other factors (See ?Mental Health: Causes of Mental Illness?).

Social theorists suggest that mental illness may be caused by the conditions of the person's environment. For example, there has been significant finding of higher incidences of psychological disorder in areas where people have suffered a major natural or man-made disaster. People involved in civil or military actions have also shown higher occurrences of this illness. Areas that are more affluent or stable may have lesser probability of mental illness compared to places that suffer from endemic poverty, few resources and support.

In so far as major psychiatric disorders are concerned, the nature versus nurture debate has generally been settled. It has been widely accepted that both play an important role in the sufferer's life.

It is has been known though, that mental illness is NOT caused by: weakness of character or personality, or sinful behavior (although stress arising from guilt may cause psychological disorder).

Psychiatric disorders may differ from one individual to another. Degree may range from mild, severe, or anything in between. Symptoms can also vary even in one person over time --- from severe to complete remission and back. Such incidence or ?flare-ups? may be triggered by stress or other factors.

Symptoms usually vary and every person with mental illness is different from one another. Arizona NAMI had developed a list of several warning signs of mental illness. The presence of a single symptom does not necessarily indicate mental illness but a severe symptom or multiple signs are strong indication for a need to seek medical evaluation.