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The causes of major depression seem to be biologic, familial, and/or genetic in nature.  Neurotransmitter studies show that deficiencies in norepinephrine and serotonin can cause depressive states.  Familial studies report that major depression in a parent leads to an increased incidence in the offspring.  Genetic studies prove that major depression is more common in pairs of monozygotic twins than dizygotic. 

The lifetime prevalence for major depression is 5-20%.  The incidence is greatest between the ages of 20 and 40 and decreases after the age of 65.  The female to male ratio is 2:1.  Fifteen percent of patients diagnosed with major depression commit suicide at some point in their lifetime.

The established criteria for diagnosing major depression are:

Five or more of the following symptoms have been present during the same 2-week period; at least one of the symptoms must be either a depressed mood or a loss of interest or pleasure.

1. depressed mood for most of the day
2. markedly diminished interest or pleasure in all, or almost all, activities most of the day
3. significant weight loss, or a decrease or increase in appetite nearly every day
4. insomnia or hypersomnia nearly every day
5. psychomotor agitation or retardation nearly every day
6. fatigue or loss of energy nearly every day
7. feelings of worthlessness or excessive guilt nearly every day
8. diminished ability to think or concentrate nearly every day
9. recurrent thoughts of death or recurrent suicidal ideation with or without a specific plan

In addition, the symptoms must cause clinically significant distress or impairment in social, occupational, or other important areas of functioning, are not due to the direct physiologic effects of a substance or a general medical condition, and are not explained by normal bereavement.