Causes of Clinical Depression: Genetic/Biological Factors
Clinical depression often runs in families, and researchers have identified several genes that may interact to influence the likelihood that depression will develop. It is unlikely that a single gene causes depression, but rather a combination of genes creates a greater risk for the disorder.
A 2003 study identified a serotonin transporter gene (5-HTT) that influenced the rate of depression in response to stress (Caspi et al., 2003). Each person receives two copies of this gene (one from each parent) in either a short or long variant. The study tracked 800 young adults over five years, and results showed that 33% of people with at least one "short" variant became depressed after stressful experiences such as job loss or divorce. In contrast, only 17% of people with two "long" variants became depressed when faced with the same circumstances. People with two "short" variants were the worst off, and their risk of becoming depressed increased steadily with more stressful life events.
In 2008, a protective gene (CRHR1) was identified that when present, reduced the risk of depression following childhood abuse (Bradley et al., 2008). Study researchers found that the CRHR1 gene is the blueprint for corticotrophin-releasing hormone (CRH) - a hormone involved in dealing with stress. Interviews were conducted with 621 adults, and among those who had experienced childhood abuse, people with protective variants of the CRHR1 gene experienced half the amount of depressive symptoms as those without the protective version.
In addition to identifying genes that create a risk for depression, brain chemistry and brain structures have been shown to play a role. Improper functioning of the neurotransmitters serotonin, norepinephrine, and dopamine have been linked to depression; many antidepressant medications act by restoring imbalances and aiding in the generation of new nerve connections in the brain. Differences in brain structures such as the amygdala and hippocampus have also been observed in those with clinical depression.