It’s difficult to determine what the biological causes for depression might be, although a host of theories and hypotheses have been formed over the years. Although researchers have yet to identify a singular biological, genetic, or environmental cause for depression, there is a general consensus among psychologists and biologists alike that all three of the possible factors listed can be influences determining an individual’s propensity to depression and other psychological traits. Interestingly, although the three can be examined independently, genetic and environmental causes (such as stress) of depression can also be classified as biological as well as more broadly, in the fields of psychology and physiology.

Certainly, the issue of genetics is an aspect that influences an individual’s biological composition, thus his or her predisposition to inheriting the same biological anomalies from one or both parents becomes both a genetic and biological matter. Environmental causes, such as high levels of stress caused by life-altering events, for instance, are not exclusive to the outside world; these environmental influences produce biological changes and can significantly alter the chemistry of the brain and the way neurotransmitters function. In short, depression is a mental illness that is distinctly biological and there are several aspects of the biological causes that will be discussed. In determining the biological roots of depressive disorders, certain imaging techniques, such as MRI, can yield powerful insight into how our brains function and how depression, on a chemical and biological level, operates.

One of the most frequently investigated and cited biological predictors of an individual’s propensity toward developing depression can be found in the “serotonin (5_HT) system, which is a neurotransmitter system that has been shown to be involved in modulating neural circuits that regulate emotion and mood in humans, and that apparently plays a key role in the pathophysiology of depression” (Neumeister 2004). In short, the transmission of serotonin, when altered in terms of amount, in particular, can be a critical determinant in the development of depression. As a result of this connection, many pharmacological treatments for depression work in a directly upon the serotonin (5-HT) system in a regulatory function to correct the imbalances. This can be reflected in stress levels, thus creating a cycle between biological and stress related causes.

The chemical problem these medications seek to correct is that “serotonin type 1a receptors and the serotonin transporters are reduced in depression, and recent genetic research in animals humans has implicated both in depression” (Neumeister 2004). Problems with this system of neurotransmitters can be influenced by a number of factors, including the important predictor of having a genetic disposition to depression, interactions with medications being taken, high levels of long-term stress (which involves another chemical, as discussed below) and illness or conditions that cause biological and  hormonal changes, such as pregnancy–another cause of depression, for instance.

In addition to the widely-accepted links between serotonin and depression, other chemical fluctuations can cause depression. For example, another enzyme that influences balance in brain chemistry, tryptophan, can be a biological cause of depression. For instance, tryptophan reduction can put individuals with a greater propensity toward depression at a much higher risk of developing major depression (Neumeister 2004).

Additionally, genetic factors outside of mere inheritance can have an impact on the presence of depressive disorders as there are some gene-influenced biological anomalies that can occur that can dramatically alter delicate brain chemistry. For example, the gene for tryptophan hydroxylase-2 (Tph2) which is an enzyme that regulates the secretion of serotonin, if mutated, can significantly repress serotonin production, the lack of which can lead to depression that will not respond to serotonin-level-related antidepressants as the existing levels of serotonin are too low to be affected (Holden 2004). The study that produced these results represents the discovery of “the first-gene linked to unipolar depression that has a documented effect on brain chemistry” (Holden 2004). This evidence suggests that there are other genetic factors outside of those inherited (as depression is generally characterized as a condition that can be genetic) that can influence serotonin and brain chemistry in general, thus leading to clinical depression.

Environmental factors are another aspect of depression and the various motivators behind it. While some issues can be classified as general life stress, these factors alone are not necessarily part of the cause for the resulting depression. In fact, there is also a biological result of these environmental elements in the development of major depression. “There is an increased focus in recent research that integrates the connection between the 5-HT (serotonin) system and another biological factor in depression which manifests in the transmission of corticosteroids (Porter et al 2004). In response to stress, in particular, long-term stress, a biological root cause of depression has been linked to the production of cortisol, which is released in stressful situations or times when alertness is required. This is the case in certain types of depressive disorders and is especially linked to cases of post-traumatic stress disorder.

Although it is present in the body and most prevalent in the early hours of the day, those suffering from depression only experience a small amount, if any, reduction throughout the course of the day. The implications are two-fold and causal. For those living under a significant amount of stress on a daily basis (long-term stress) the amount of cortisol remains high which interferes with brain chemistry and the transmission of serotonin. This biological shift as a result of high stress levels causes fluctuations in the amount of corticotrophin that is transported (Neumeister 2004).

More generally speaking, another significant biological contributor to the development of depression is genetics. There are numerous studies that examine, with due credit to the possibilities of environmental factors integrated and some that use twins or adopted persons to stabilize the results, the prominent association between genetic inheritability of depression. “Taking into account evidence from twin, adoption and family studies, we have to acknowledge the strong genetic contribution to the pathophysiology of depression” (Neumeister 2004).

The genetic propensity to depression is one of the most researched and well-documented issues in studies attempting to find the root biological causes for depression. As a result of this correlation, for several years researchers have attempted to isolate genes that are key in identifying depression and although some success has been achieved, there are still other ways researchers look for depression and its biological base, one of which involves the imaging technique known as MRI.

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Holden, C. (2004). Mutant Gene Tied to Poor Serotonin Production and Depression. Science306(5704), 2023.

Neumeister. (2004). Implications of genetic research on the role of the serotonin in depression: emphasis on the serotonin type 1A receptor and the serotonin transporter.Psychopharmacology174(4), 512-524.

Porter, R. J., Gallagher, P., Watson, S.,  & Young, A. H. (2004). Corticosteroid-serotonin interactions in depression: a review of the human evidence. Psychopharmacology,173(1/2), 1-17.