Depressive Disorder: Is it Caused

¶ … depressive disorder: is it caused by biological (nature) or social (nurture) factors?

Major depressive disorder: Nature and nurture debates

Perhaps no disorder better exemplifies the nature vs. nurture debate than major depressive disorder. While virtually every human being feels some degree of depression in his or her lifetime, not everyone experiences the core features of major depressive disorder. Major depressive disorder is one of the most serious 'mood' disorders, according to the Diagnostic Manuel (DSM IV) criteria. To meet the DSM criteria, a subject must be in a depressed mood most of the day, have a decreased ability to experience pleasures, exhibit a change of more than 5% of body weight, show insomnia or hypersomnia nearly every day, feel a lack of energy or too much energy, feel fatigued, and be plagued by feelings of worthlessness, feel excessive or inappropriate guilt, poor concentration and frequent thoughts of death. As for the source of major depression, some psychiatric professionals do contend that individuals can develop what is called "endogenous depression" or depression symptoms without any clearly identified psychological cause or trigger -- i.e. The person just 'wakes up depressed' (Franklin 2003). "An endogenous depression is a biologically caused depression, due presumably to either genetic causes or a malfunction in the brain chemistry. But, all depression involves some changes in brain chemistry, even when the cause is clearly a psychological trauma" (Franklin 2003).

Supporting the biological basis for major depressive disorder is the fact that some physically-related diseases, such as multiple sclerosis and heart disease seem to have some physical components that can be isolated from the obviously depressive aspects of having a major illness. Because there is a presumption that individuals with chronic ailments are 'naturally' depressed, the comorbidity between depression and diseases like MS may go underdiagnosed, particularly in lower-income individuals not receiving adequate psychological support for their illness, due to financial concerns, or individuals for whom depression is not a culturally acceptable 'state' to be in. A study of low-income MS patients without a formal diagnosis of depression, found at least 20% met the clinical criteria "and 15% of those who did not report any mental comorbidity had CESD scores above the threshold for probable major depression; this suggests that depression frequently remains undiagnosed. Similarly, an Irish community-based study reported that 23% of MS patients with moderate or severe depressive symptoms were undiagnosed" (Marrie, 2009, p. 390). Depression can also exacerbate existing illnesses: "depression is a risk factor for morbidity and mortality in coronary heart disease. A meta-analysis of 22 studies found that major depression more than doubles the risk of mortality after an acute myocardial infarction…depression increased the risk of nonfatal myocardial infarction or cardiac death more than fourfold after an episode of unstable angina…depression is a significant risk factor in both forms of acute coronary syndrome" (Carney 2009, p.410).

This demonstrates that the ability to separate biological from purely mental causes of depression is nearly impossible -- the body affects the mind and vice versa. Even individuals without specific psychological triggers likely to precipitate depression in their environment like a loss of a loved one can respond to biologically-based treatments like medication and "after psychological treatment and recovery from depression, the brain chemistry returns to normal, even without medication. To date, there is no hard research evidence to support the notion of endogenous depression" (Franklin 2003). But these responses to medication and treatment vary wildly with the individual, and arguing against a strong genetic component for depression is that fact that individuals within the same family may respond very differently to one type of treatment, even different types of antidepressants.

Also in favor of a more environmental explanation are the fact that there are certain lifestyle correlatory factors with major depression that have biological implications. A lack of sunlight affects certain individuals more severely, but a lack of light and exercise can cause almost all individuals to feel mildly depressed. In another example, one study of Canadian smokers found a surprisingly high prevalence of major depression amongst current smokers in the general Canadian population, even when controlled for other factors that could have an impact upon the study results, such as gender. "This prevalence was especially elevated among smokers in the younger age groups. This may have important public health implications. Specifically, identification of adolescents at risk for smoking may allow more efficient targeting of intensive education resources. Further, effective management of depression in these people may contribute to reduced rates of smoking, a possibility that deserves exploration in future longitudinal studies (Khaled 2009, p. 208). Smoking may be used as a form of 'self-medication' for the depressed, and a lack of motivation for self-care can lead to decreased desire to quit smoking, but once again, a neurochemical component cannot be discounted -- the chemicals involved in smoking may also increase the tendency to feel depression.

The complex interplay between brain and environment is what makes depression so difficult to study: because the body affects the brain and the brain affects the body it is almost impossible to entirely screen out one factor, biology or environment. Nature determines the individual's complex neurological makeup, but environmental interactions alter that makeup. Additionally, unless children are adopted, parents may suffer from depression, too, which creates a 'chicken and egg' debate as to what happened first -- does being raised by depressed parents cause children to feel depressed, or is a higher rate of depression due to genetic factors? A study of maternal depression with the emotional and behavioral problems and adaptive skills of four- to ten-year-old children as rated by their mothers, fathers, and teachers found that children of mothers with depression "had significantly poorer adaptive skills than children of sociodemographically similar mothers without depression; according to the reports of mothers and fathers, these children also had more emotional and behavioral problems," (Riley 2009). Interestingly, the study also found that of the children with depressed mothers, teacher interventions remained non-significant for children with behavior problems, in contrast to the children of non-depressed mothers. But whether this is due to the impact of the children's mother or purely biological factors, it is difficult once again to determine, given the data.

The most reliable way to control for the 'chicken and egg' problem would seem to be twin studies, which do support a strong genetic causative factor for depression, at face value. But twin studies have the problem of being somewhat limited, given that the available supply of twins with and without depression is relatively small. Still, heritability of depression based on twin studies is 40% to 50% higher than in the general population (Levinson 200, p.1). Adoption studies also provide some support for a role for genetic factors and the relative risk ratio is around 2 to 3 times higher if one or more of the adoptive child's parents was depressed. However, adopted children have higher environmental risk factors, including childhood abuse and neglect -- this type of life stress compromises the reliability of such studies (Levinson 2005, p.1).