There is no single known cause of depression. Rather, it likely results from a combination of genetic, biochemical, environmental, and psychological factors. Trauma, loss of a loved one, a difficult relationship, or any stressful situation that overwhelms the ability to cope may trigger a depressive episode. Subsequent depressive episodes may occur with or without an obvious trigger.
Research with brain-imaging technologies, such as magnetic resonance imaging (MRI), shows that the brains of people who have depression look different than those of people without depression. The parts of the brain responsible for regulating mood, thinking, sleep, appetite, and behavior appear to function abnormally. In addition, the chemicals that brain cells use to communicate—neurotransmitters—may also be affected. It is not clear which changes seen in the brain may be the cause of depression and which ones the effect.
Some types of depression tend to run in families, suggesting there may be some genetic vulnerability to the disorder.