Mental illnesses, in general, are thought to be caused by a variety of genetic and environmental factors:
Inherited traits. Mental illness is more common in people whose biological (blood) relatives also have a mental illness. Certain genes may increase your risk of developing a mental illness, and your life situation may trigger the actual mental illness.
Environmental exposures before birth. Exposure to viruses, toxins, alcohol or drugs while in the womb can sometimes be linked to mental illness.
Negative life experiences. Situations in your life, such as the loss of a loved one, financial problems and high stress, can play a role in triggering mental illness. So can an upbringing that leads to poor self-esteem or a history of sexual or physical abuse. Life experiences can lead to unhealthy patterns of thinking linked to mental illness, such as pessimism or distorted ways of thinking.
Brain chemistry. Biochemical changes in the brain are thought to affect mood and other aspects of mental health. Naturally occurring brain chemicals called neurotransmitters play a role in some mental illnesses. In some cases, hormonal imbalances affect mental health. It's thought that inherited traits, life experiences and biological factors can all affect brain chemistry linked to mental illnesses.