The exact cause of most mental illnesses is not known, but research suggests that a combination of factors, including heredity, biology, psychological trauma, and environmental stress, may be involved.
Some mental illnesses, such as bipolar disorder and major depression, tend to run in families, which means the likelihood to develop a mental disorder may be passed on from parents to their children.
Some mental disorders have been linked to special chemicals in the brain called neurotransmitters. Neurotransmitters help nerve cells in the brain communicate with each other. If these chemicals are out of balance or not working properly, messages may not make it through the brain correctly, leading to symptoms. Defects in or injury to certain areas of the brain have also been linked to some mental illnesses.
Some mental illnesses may be triggered by psychological trauma, such as severe emotional, physical, or sexual abuse; an important early loss, such as the loss of a parent; or neglect.
Stressful or traumatic events can trigger a mental illness in a person with a vulnerability to a mental disorder.