Depression, like many chronic illnesses (such as diabetes or heart disease), tends to run in families. For most diseases with a heritable component, having a close family member with the illness does not necessarily mean that one will develop the illness oneself. Nonetheless, having a close family member with the disease often does confer an increased likelihood that one will develop the disease in question.
For example, women are generally felt to have a lifetime risk for major depressive disorder of 10 to 25 percent (with men having approximately half that lifetime risk). Individuals who have a first degree relative (parent, child, or sibling) with major depression are thought to be 1½ to 3 times more likely to develop this disorder themselves - which would bring the lifetime risk, for women, to somewhere in the neighborhood of 50 percent. While having a close relative with depression by no means ensures that one will develop depression, it clearly increases the likelihood.
As is the case for all chronic diseases, it is useful to know one's family history with regard to depression. This knowledge allows us to avoid additional risk factors (besides family history, which we cannot avoid), and helps us to become more vigilant with respect to early indications of disease onset.
With respect to additional risk factors, individuals with a family history of depression may wish to avoid excessive use of alcohol or exposure to psychoactive drugs - both of which activities are likely to increase the chance of developing clinical depression.
Individuals with a family history of depression should be especially attentive to periods of depressed mood and to other symptoms commonly associated with clinical depression (e.g., suicidal thinking, loss of interest in usual activities, feelings of worthlessness or guilt, loss of energy, inability to concentrate, appetite changes, or sleep disturbance). Those who notice such symptoms in themselves should consult their physician.
Chronic illnesses are, unfortunately, common. All of us have a family history for some of them. While this knowledge can be unsettling, it can also be empowering. Knowing your family history can help you to avoid risk factors that are within your control - and can also help you, in collaboration with your physician, to recognize and to treat illnesses more promptly. Diseases that are treated at an early stage can often be mitigated, such that they have a less severe course. This is true for depression as well: depression that is treated early is often less profound, and runs a shorter course, than depression treated at a later stage.