Almost anybody can become depressed if subjected to enough emotional stress, but some persons seem to become depressed more easily than others. A personal trauma or stressful event may trigger a depressive episode. Such psychological stress accounts for about half of all cases of severe depression. Other causes include side effects of a surgical operation or medication, alcohol or drug abuse, or withdrawal from drugs. Some types of depression, such as premenstrual syndrome or menopausal depression, are apparently related to a person's metabolism and age.
Depression can also result from an underlying physical problem, such as thyroid disease.
Many behavioral scientists believe that some types of depression may have roots in the experiences of a person's childhood. Studies undertaken by researchers indicate that some types of depression may run in families. There is growing evidence that vulnerability to depression may be genetically transmitted. This means that severe depression may result from biochemical imbalances in brain function. Hereditary origins of depression may be suspected in persons who persistently become depressed for no apparent reason.