Altered neurotransmitter (serotonin, norepinephrine, and dopamine) levels are responsible for clinical depression symptoms in many people. Diets low in omega-3 and other fatty acids, folic acid, and vitamin B-12 appear to be risk factors for development of depression. Epidemiological research has confirmed a genetic link but particular causative genes have not been identified. Social or environmental factors are also implicated in depression.
Clinical depression affects women (up to 20% of women will be affected at some point) at twice the rate of men, while gender does not influence the incidence in children. Adolescent girls are more likely to develop clinical depression than boys. The onset of clinical depression symptoms usually occurs between the ages of 20 to 50, but people over age 65 appear to be at risk also.
The incidence of manic depression is not affected by gender in adults or children. A person with an immediate relative with bipolar disorder is 8 to 18-fold more likely to be afflicted. Research employing brain imaging indicates that the bipolar brain differs from the normal brain.
Other conditions associated with depression are thyroid dysfunction, anxiety disorders, and substance abuse. If untreated, these conditions may cause onset of depression. Heart disease, stroke, diabetes, cancer and neurological diseases are also closely associated with depression. A person with a negative and dependent personality type is more likely to develop clinical depression than is an optimistic person.