Many people are pre-disposed to depression because of their own biological makeup making their risk of suffering from this condition much greater then those who do not share these risk factors. There are four main biological factors that can increase a persons risk for depression and these include (1) genetic factors, (2) biochemical factors, (3) alterations in hormonal regulation and, (4) sleep abnormalities.

Genetic Factors

Studies done with twins have shown that genetic factors play a role in the development of depressive disorders. There have been a number of studies done that show that the average rate of shared mood disorders among identical twins is 45% to 60%. This means if one twin suffers from depression, or any other mood disorder, there is a 45% to 60% chance that the other twin will also be affected. Contrast this with fraternal twins where the percentage falls of drastically to only 12%.

Moods disorders are inheritable for some people. This also means that those who are genetically susceptible to mood disorders can have an earlier age of onset, a greater rate of suffering other disorders in addition to depression and an increased risk of recurrent illnesses. However, any genetic factors that are present must interact with environmental factors for depression to develop.

Biochemical Factors

The brain contains billions of neurons and is a highly complex organ. There is a lot of evidence that points to the idea that depression is a biological or chemical disorder where central nervous system neurotransmitter abnormalities are a probable cause of clinical depression. These neurotransmitter abnormalities may be the result of inherited or environmental factors, or even of other medical conditions, such as cerebral infarction, hypothyroidism, AIDS, or substance abuse.

Specific neurotransmitters in the brain are believed to be related to mood altered states. It was initially believed that the two main neurotransmitters involved were serotonin and norepinephrine. With new research though it is now thought that depression results from the dysregulation of a number of neurotransmitter systems in addition to serotonin and norepinephrine. The dopamine, acetylcholine, and GABA systems are also believed to be involved in the pathophysiology of major depression.

Alterations in Hormonal Regulation

Although there is still inconclusive evidence that hormones can play a role in depression. The most studied neuroendocrine characteristic that relates to depression has been hyperactivity of the hypothalamic-pituitary-adrenal cortical axis. Evidence of increased cortisol secretion is apparent in 20% to 40% of depressed outpatients and 40% to 60% of depressed inpatients. Results of a dexamethasone suppression test are abnormal in about 50% of patients with depression, which indicates hyperactivity of the hypothalamic-pituitary-adrenal cortical axis. However, the findings of this test may also be abnormal in people with obsessive-compulsive disorders and other medical conditions. Significantly, patients with psychotic major depression are among those with the highest rates of nonsuppression of cortisol on the dexamethasone suppression test.

Sleep Abnormalities

Sleep electroencephalogram abnormalities may be evident in 40% to 60% of outpatients and up to 90% of inpatients during a major depressive episode. People prone to depression tend to have a pre-mature loss of sleep, slow delta wave sleep and altered rapid eye movement (REM) latency. The phase of REM sleep associated with dreaming occurs earlier in two thirds of people with bipolar and major depressive illnesses. This sign is referred to as reduced REM latency and is consistent with the expected manifestation of an inherited trait. Reduced REM latency and deficits in slow-wave sleep typical persist following recovery from a depressed episode. Data also suggests that depressed patients without this sign are not likely to respond to treatment with tricyclic antidepressants, which suppress early REM sleep.

There are many factors that contribute to depression, many of which are biochemical in nature. Those that are biologically induced can be treated with different prescription drugs but as with anything relating to human emotion and chemical makeup answering one question as to why brings up more questions that remain unanswered.