Environmental

Mental illness can develop after exposure to events or stressors, called "triggers," such as the death of a loved one, a divorce, dysfunctional family or changing schools.

Genetic

Illnesses such as schizophrenia, depression, bipolar disorder tend to run in families and may be linked to gene abnormalities. A person can inherit the tendency to develop mental illness, but until he experiences a stressor, he doesn't develop symptoms of the disease.

Biochemical

A person diagnosed with bipolar disorder may have had physical changes in her brain---not producing enough of a necessary neurotransmitter, for example---that led to her illness.

Infections

Strep infections appear to influence the development of pediatric autoimmune neuropsychiatric disorder associated with Streptococcus bacteria (PANDAS). This condition has been linked to obsessive-compulsive disorder (OCD) and other mental illnesses that develop in childhood, according to WebMD.com.

Psychological

Mental illness can develop after psychological trauma, such as childhood abuse or neglect, or a loss early in life, such as the death of a parent. These persons may also not relate very well to others, leading to social isolation.

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Causes of Mental Illness

Mental Illness: Causes