The group of symptoms which doctors and therapists use to diagnose depression ("depressive symptoms"), which includes the important proviso that the symptoms have manifested for more than a few weeks and that they are interfering with normal life, are the result of an alteration in brain chemistry. This alteration is similar to temporary, normal variations in brain chemistry which can be triggered by illness, stress, frustration, or grief, but it differs in that it is self-sustaining and does not resolve itself upon removal of such triggering events (if any such trigger can be found at all, which is not always the case.)

Instead, the alteration continues, producing depressive symptoms and through those symptoms, enormous new stresses on the person: unhappiness, sleep disorders, lack of concentration, difficulty in doing one's job, inability to care for one's physical and emotional needs, strain on existing relationships with friends and family. These new stresses may be sufficient to act as triggers for continuing brain chemistry alteration, or they may simply prevent the resolution of the difficulties which may have triggered the initial alteration, or both.

The depressive brain chemistry alteration seems to be self-limiting in most cases: after one to three years, a more normal chemistry reappears, even without medical treatment. However, if the alteration is profound enough to cause suicidal impulses, a majority of untreated depressed people will, in fact, attempt suicide, and as many as 17% will eventually succeed. Therefore, depression must be thought of as a potentially fatal illness.

Suicidal Ideation

Friends and relatives may be deceived by the casual way that profoundly depressed people speak of suicide or self-mutilation. They are not casual because they "don't really mean it"; they are casual because these things seem no worse than the mental pain they are already suffering. Any comment such as, "You'd be better off if I were gone," or "I wish I could just jump out a window," is the equivalent of a sudden high fever; the depressed person must be taken to a doctor or licensed therapist who can monitor their danger. A formulated plan, such as, "I'm going to jump in front of the next car that comes by," is the equivalent of sudden unconsciousness: an immediate medical emergency which may require hospitalization.

Depression can shut down the survival instinct or temporarily suppress it. Therefore, depressed suicidal thinking is not the same as the suicidal thinking of normal people who have reached a crisis point in their lives. Depressive suicides give less warning, need less time to plan, and are willing to attempt more painful and immediate means, such as jumping out of a moving car. They may also fight the impulse to suicide by compromising on self-injury -- cutting themselves with knives, for example, in an attempt to distract themselves from severe mental pain. Again, relatives and friends are likely to be astonished by how quickly such an impulse can appear and be acted upon.

What initiates the alteration in brain chemistry?

It can be either a psychological or a physical event. On the physical side, a hormonal change may provide the initial trigger: some women dip into depression briefly each month during their premenstrual phase; some find that the hormone balance created by oral contraceptives disposes them to depression; pregnancy, the end of pregnancy, and menopause have also been cited. Men's hormone levels fluctuate as deeply but less obviously.

It is well known that certain chronic illnesses have depression as a frequent consequence: some forms of heart disease, for example, and Parkinsonism. This seems to be the result of a chemical effect rather than a purely psychological one, since other, equally traumatic and serious illnesses don't show the same high risk of depression.

The typical chemical changes that characterize depression can also be caused by psychosocial factors.

Is a tendency towards depression inherited?

It seems there are some people whose brain chemistry is predisposed to the depressive response, and others who are at much lower risk of depression even if exposed to the same physical or psychological triggers. The close relatives of manic-depressives are at a higher risk for unipolar depression than the population at large or their adopted/by marriage relations.

There seems to be a link between high creativity and the gene for manic-depression: artists and writers often are not manic-depressive themselves, but have a family member who is. Studies of families in which members of each generation develop manic-depressive illness found that those with the illness have a somewhat different genetic make-up than those who do not get ill. However, the reverse is not true: not everybody with the genetic make-up that causes vulnerability to manic-depressive illness has the disorder. Apparently additional factors, possibly a stressful environment, are involved in its onset.

Major depression also seems to occur, generation after generation, in some families. However, depression can occur in people with no family history of any form of mental illness. And there probably is no human who is entirely immune to depression if stressed enough.