Manic episodes rarely appear by themselves; rather, they usually alternate with depression. Such a mood disorder, in which both mania and depression are present, is known as bipolar disorder. In bipolar disorder, periods of mania alternate with periods of depression (each lasting a few days to a few months), sometimes with periods of normal mood intervening. Occasionaly, bipolar disorder is seen in a mild form: The person has alternating moods of unrealistically high spirits followed by moderate depression. Research indicates that bipolar differs in several ways from unipolar depression. Bipolar disorder is much less common and, unlike depression, which occurs two to three times more frequently in women than men, bipolar disorder is equally prevalent in men and women. Bipolar disorder also seems to have a stronger biological component than depression: It is more strongly linked to heredity and is more often treated by drugs.

Genetic factors play a pivotal role in the development of depression, particularly in bipolar disorder. The strongest evidence implicating genetic risk factors comes from studies of twins. If one identical twin is clinically depressed, the other (genetically identical) twin is at substantial risk of becoming clinically depressed as well. Among fraternal twins (who share only about half the same genes), if one twin is clinically depressed, the likelihood is much less that the second twin will also be clinically depressed.

Promising research has linked mood disorders to certain chemical imbalances in the brain – principally an oversupply or undersupply of certain neurotransmitters, chemicals involved in the transmission of nerve impulses from one cell to another. For example, medications that are effective in treating mood disorders are known to alter neurotransmitter levels. Moreover, some animal researchers have found that apparently “depressed” behaviour in rats (such as decreased effort) can be brought about by lowering their levels of the neurotransmitter norepinephrine. Still other evidence ties depression to high levels of hormones released by the endocrine system. Yet no conclusive evidence has established a correlation between high or low levels of neurotransmitters and an increased genetic risk for mood disorders. In fact, the chemical imbalance in the brain associated with depression could be caused by stressful life events. Biology affects psychological experience, but psychological experience also alters biological functioning.

Reference:

1. Understanding Psychology: 3^rd Edition. Charles G. Morris.