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The causes of depression are not fully known. Most likely a combination of genetic, biologic, and environmental factors play a role.

Genetic Factors

Because depression often runs in families, it may have a genetic component. Data from family, twin, adoption, and genetic studies strongly indicate a genetic factor. Studies have found that close relatives of patients with depression are two to six times more likely to develop the problem than individuals without a family history.

Biologic Factors

Evidence supports the theory that depression has a biologic basis. The basic biologic causes of depression are strongly linked to abnormalities in the delivery of certain key neurotransmitters (chemical messengers in the brain). These neurotransmitters include:

  • Serotonin. Perhaps the most important neurotransmitter in depression is serotonin. Among other functions, it is important for feelings of well-being. Imbalances in the brain’s serotonin levels can trigger depression and other mood disorders.
  • Other Neurotransmitters. Other neurotransmitters possibly involved in depression include acetylcholine and catecholamines, a group of neurotransmitters that consists of dopamine, norepinephrine, and epinephrine (also called adrenaline). Corticotropin-releasing factor (CRF), a stress hormone and neurotransmitter, may be involved in depression and anxiety disorders.

The degree to which these chemical messengers are disturbed may be affected by other factors such as genetic susceptibility. For example, researchers have identified a defect in the gene known as SERT, which regulates serotonin and has been linked to depression.

Reproductive Hormones. In women, the female hormones estrogen and progesterone may play a role in depression.
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