What causes mental illness?

There is no simple answer because no single factor causes mental illness. Mental illnesses are complex diseases, meaning that they are caused by a combination of different factors.

Researchers believe that in most cases of mental illness, both genes and environmental factors play a role. For instance, if a person is at increased risk of having a mental illness because of genetic factors (for example, mental illness runs in the family), he may only develop an illness if a certain combination of stressful situations occurs (environmental factors). Environmental factors can be anything that causes major stress and can occur at any point in life, e.g., death of a loved one, abuse, use of street drugs, loss of income.