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There are many different potential causes of mental illness. Some people are born with risk factors such as genetic predisposition or chemical imbalances that trigger their condition. For example, a person with a family history of depression may develop this disorder even if they have not experienced a distressing event.  

Mental functions are controlled and interpreted by the brain. For people with mental health disorders, the brain undergoes changes in structure and function which influence behavior.

Research has also shown that the incidence level of major mental illnesses such as schizophrenia and major depression is relatively consistent across diverse societies throughout the world. This lends additional support to the notion that the basis for these disorders is biological in part.

However, factors other than biology can also trigger mental illness, such as a stressful event or series of events. For example, some people with no history of or risk factors for mental illness may develop post-traumatic stress disorder (PTSD) following a tragic event, such as the death of a loved one, combat, rape or child abuse.

Scientists do not fully understand how most mental illnesses are triggered, and research continues in an attempt to further understand how these illnesses unfold.