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The Influence of Environmental and Genetic Factors on Depression

Updated on May 25, 2012
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The Influence of Environmental and Genetic Factors on Depression

Abstract

This essay investigated the question: To what extent do environmental and genetic factors play a role in the development of depression? Several different environmental and genetic interactions in depressed individuals were examined. Hammen (2005) states that “improved methods of assessment and research design have established a robust and causal association between stressful events and major depressive episodes.” Other studies (Nemande, Reiss, Dombeck, 2007) have shown that depression may be a response to stress in and of itself. The effect of gender on depression was also examined, as women are more likely to become depressed, and several theories were postulated for why this might be in terms of environmental stressors, hormones, and other biological factors. The effects of physical health on depression were also examined, and studies (Nemade, Reiss, Dombeck, 2007) have shown that being physically unhealthy contributes to the rates of depression. The influence of culture was examined, and studies (Nemande, Reiss, Dombeck, 2007) show that rigid gender roles may significantly impact the incidence of depression in individuals who lose a partner. Childhood abuse was also studied, and studies (Nemande, Reiss, Dombeck, 2007) show that childhood abuse has an impact upon the development of depression later in life. The effect of genetic inheritance was studied, and studies (Caspi, Sugden, Moffitt, Taylor, Craig, Harrington, McClay, Mill, Martin, Braithwaite, Poulton, 2003) found that the 5HTT gene may moderate the influence of stressful life events on depression. The influence of brain chemistry was examined, and studies (Delgado, Moreno, 2000) found “abnormal levels of noradrenaline and serotonin in patients suffering from major depression” (Crane, Hannibal, 2009, p. 152). In conclusion, it was found that environmental and genetic factors may both play a significant role in the development of depression.


To What Extent Do Environmental and Genetic Factors Cause Depression?

Introduction

It has long been believed that environmental and genetic factors both contribute to the development of depression. After all, sometimes individuals with no known family history of depression develop depression and sometimes individuals with a known family history of depression do not (Women and Depression, 2009). So to what extent do environmental and genetic factors play a role in the development of depression? It is known that stressful life events tend to occur before a depressive episode (Depression, 2008). Nemande, Reiss, and Dombeck (2007) found that “many people with major depression report that a stressful life event triggered their first or second depressive episode.” Caspi et. al (2003) found that a certain serotonin transporter (5HTT) gene may moderate the influence of stress on depression. Thus it is clear that genetic and environmental factors may both play a role in an individual’s development of depression.

Depression affects 16.5 percent of adults in the United States at some point in their lives and two percent of adults have depression that can be classified as ‘severe’ (Kessler, Chiu, Demler, Walters, 2005). It affects 11.2 percent of adolescents (aged 13-18) at some point in their lives and 3.3 percent of adolescents (aged 13-18) have depression that can be classified as ‘severe’ ( Merikangas, He, Burstein, Swanson, Avenevoli, Cui, Benjet, Georgiades, Swendsen, 2010). These statistics demonstrate a need to understand the causes of depression, so that it may be prevented and treated more effectively.

Past studies have demonstrated that both environmental and genetic factors may play a role in the development of depression, but not many studies have examined the combined interaction of both factors at once. The purpose of this essay is to examine existing research on the topic and to identify how stressful or traumatic life events and genetic or biological makeup combine and interact to cause depression.


Extended Essay

Depression is widely understood to be caused by the interaction of environmental and genetic, or biological, factors. Genetics may play a role in influencing brain chemistry and functioning, which, when abnormal, can lead to depression and other disorders. Genetics may also influence how an individual responds to stressful events in their life. Stressful or traumatic life events are widely considered to be environmental causes of depression. Often a combination of genetic and environmental factors are believed to have caused depression, as stressful events may occur in an individual’s life and genetics may influence how the individual psychologically responds to these events.

However, some individuals who develop depression have no known family history of depression, and some individuals with a known genetic predisposition to depression do not develop the disorder themselves. Thus it is clear that neither genetics nor the environment is solely responsible for the development of depression. Either may be able to cause depression in an individual in the absence of the other, and both may combine to cause depression. This essay will examine how individuals can develop depression through genetic influences, environmental influences, or a combination of the two.

The first topic of investigation was genetic and biological factors, in the absence of environmental stressors. How might individuals develop depression without any environmental stressors, simply due to genetic or biological factors? Some researchers believe that serotonin, a neurotransmitter in the brain, may have an influence over the development of depression. Duenwald (2003) suggested that a short variant of the 5HTT gene, which plays a role in serotonin pathways in the brain, may be correlated with a higher risk of developing depression (Crane, Hannibal, 2009, p. 151). Similarly, Kendler, Kuhn, Vittum, Prescott, and Riley (2005) found that individuals with two short alleles (a form of a gene at a specific location on a specific chromosome) at the 5HTT locus were more sensitive to the depressive effects of severe life events than those with one or two long alleles. Caspi et. al (2003) postulate that these results could indicate that genetics control responses to environmental stressors (Crane, Hannibal, 2009, p. 151). More recently, however, Risch, Herrell, Lehner, Liang, Eaves, Hoh, Griem, Kovacs, Ott, and Merikangas (2009) performed a meta-analysis of published data and found no association between the 5HTT gene and the development of depression.

These findings seem to suggest that regardless of the control this gene may or may not have over responses to environmental factors, environmental factors would still have to be present to develop depression. Thus it may be that an individual could not develop depression solely due to genetic or biological factors, at least in the case of their 5HTT gene.

However, there is more evidence that neurotransmitters may have an influence on the development of depression. Delgado and Moreno (2000) found “abnormal levels of noradrenaline and serotonin in patients suffering from major depression” (Crane, Hannibal, 2009, p. 152). Some researchers speculate, however, that this finding may indicate correlation, not causation. After all, it’s possible that depression could cause the abnormal levels, not the other way around (Crane, Hannibal, 2009, p. 152).

Rampello, Ferdinando, and Francesco (2000) studied neurotransmitters in the brains of patients with major depressive disorder, and they found that patients with the disorder had an imbalance of several neurotransmitters, including noradrenaline, serotonin, dopamine, and acetylcholine (Crane, Hannibal, 2009, p. 152). This may also point to correlation, not causation, but it provides further support for the theory of imbalances in neurotransmitters. Burns (2003), however, states that despite spending many years of his life researching serotonin metabolism in the brain, he has never seen “any convincing evidence that depression results from a deficiency of brain serotonin” (Crane, Hannibal, 2009, p. 152).

It is clear that although there is some support for the theory of imbalances of neurotransmitters causing depression, depressed patients should be wary of taking medications which alter serotonin levels in the brain, because it is not yet known for sure that abnormal serotonin levels are a cause of depression, and that the medications will treat this effectively.

There is definitely some evidence to suggest depression can develop without environmental causes. So can environmental factors cause depression in an individual without any known genetic factors? It has been suggested that depression may be a natural response to stress, regardless of an individual’s genetic makeup. “It is clear that some people develop depression after a stressful event in their lives” (“Environmental Causes,” 2010). Nemande, Reiss, and Dombeck (2007) found that “many people with major depression report that a stressful event triggered their first or second depressive episode. Interestingly, research suggests that later depressive episodes (starting with the third) tend to develop spontaneously (in the absence of any particular stressor). Some scientists call this phenomenon the ‘kindling effect,’ or ‘kindling-sensitization hypothesis’” (“Social and Relational Factors,” 2007).

The kindling-sensitization hypothesis surmises that initial depressive episodes may spark changes in the brain’s chemistry and limbic system that make it more prone to developing future episodes of depression (“Environmental Causes,2010). Thus, subsequent depressive episodes could occur with virtually no environmental stressors at all. This hypothesis would seem to downplay the importance of environmental stressors. However, an environmental stressor would have to occur to set off the first depressive episodes, and subsequent episodes may be sparked by small, daily hassles, so environmental factors would still be present.

Nemande, Reiss, and Dombeck (2007) also state that “the relationship between stress and depression is not surprising. Many people who are depressed show high levels of cortisol (a stress hormone) in their blood, leading some researchers to speculate that depression may be a stress response in and of itself” (“Social and Relational Factors,” 2007). This finding would seem to suggest that depression can happen to anyone, regardless of their genetic makeup, simply as a response to stress.

However, doesn’t an individual’s genetic and biological makeup control the production and regulation of cortisol levels in their blood? Could it be that their biological makeup is abnormal, and this causes them to be depressed and to produce too much cortisol? It’s also possible that this finding could simply be a case of correlation, and not causation. The relationship between two variables can be determined to be a positive correlation without an increase or decrease in one variable actually causing the other to increase or decrease.

Furthermore, Caspi et al. (2003) found that the 5HTT gene may moderate the influence of stressful life events on depression. Individuals with one or two copies of the short allele exhibited more depressive symptoms, diagnosable depression, and suicidal tendencies in relation to stressful life events than individuals homozygous for the long allele. Thus it would seem that genetics play an important role in how stress can cause depression, by controlling how stressful events influence an individual psychologically. This is perhaps the most important evidence to suggest how genetics and the environment may combine to form depression.

This finding does not refute, however, the postulation that stressful events are crucial to initiating depression, regardless of whether genetics may control the psychological outcome. Besides, even if individuals with the short alleles exhibited more depressive symptoms than their long allele counterparts, this study does not state that individuals with the long allele did not become depressed. Therefore, it is reasonable to assume that while genetics may play an important role, even those without such a genetic predisposition, or vulnerability to developing the disorder, may develop depression regardless, as a response to stressful or traumatic life events.

Another way to examine the influence of environmental and biological factors is through gender differences. We know that women are more likely than men to be diagnosed with depression in any given year (Women and Depression, 2009). The question becomes, then, why are women more likely to be diagnosed as depressed? Aside from the fact that women may be more likely to seek psychological help for depression (and thus be officially diagnosed as depressed more often), it is possible that it is due both to differing life stressors between genders as well as hormonal and biological differences (Crane, Hannibal, 2009, p. 156).

There are many environmental stressors that are unique to women that one must keep in mind when examining gender differences and depression. For example, “Girls may undergo more hardships, such as poverty, poor education, childhood sexual abuse, and other traumas than boys” (Women and Depression, 2009). One must also take into account how females are more often the target of unwanted sexual attention—rape or child abuse—both of which may lead to later depression among its victims. According to Levitan, Parikh, Lesage, Hegadoren, Adams, Kennedy, and Goering (1998) there was a significant correlation between female gender and the risk of early sexual abuse. Also, according to Nemande, Reiss, and Dombeck (2007), “Many people who develop depression previously experienced traumatic situations during their childhood years in the form of sexual, emotional, and/or physical abuse” (“Social and Relational Factors,” 2007).

Females are also the unfortunate target of many Western culture media images of what beautiful is—most teenage girls and young women are taught by their culture and by the media that skinny is pretty. This social standard can cause a lot of emotional stress for girls who do not or believe they do not fit their society’s standards of beautiful. With their self-esteem and self-efficacy damaged by feelings of being fat or ugly, adolescent and young women may develop depression more easily. It is simply a fact that boys more often do not have these kinds of stressors in their lives.

There are also biological factors that women have that boys do not. For example, “Brain chemistry appears to be a significant factor in depressive disorders. Scientists are studying the influence of female hormones, which change throughout life. Researchers have shown that hormones directly affect the brain chemistry that controls emotion and mood” (Women and Depression, 2009). Also, some women seem to develop depression the week before menstruation (premenstrual dysphoric disorder), just after giving birth (postpartum depression), and during the transition into menopause (Women and Depression, 2009). All of this would seem to suggest that female hormones may play an important role in the development of depression—although it is important to remember that these turning points in life (giving birth and entering menopause) could also be classified as environmental stressors, as they both involve major changes in a woman’s life. This could be another example of how biological and environmental factors may combine to form depression in an individual.

While genetics may indeed play a significant role in the development of depression, is it also possible that children could have ‘learned helplessness’ or learned to be depressed from their depressed parents? After all, it stands to reason that a young child would not know why their parent is always sad. They may even believe it is their fault. Over the years, the child may start to act like the parent. Older siblings and parents are role models for young children. If that role model is depressed, who is to say the child will not learn to be depressed, and to view themselves as worthless, just as their parent might?

The results would be especially potent if this situation were to occur to a single parent with a single child. With no other parent or sibling around to be a positive role model, the child would have no one to learn from except the depressed parent. Also, the parent’s moods may make them lash out at the child and blame the child for their situation, such as a woman who was raped and became pregnant because of it. If the mother blames the child for her situation, there is no reason the child will not start to blame themselves for it also. This guilt and shame, along with the parent’s blame and depression, may make the child much more vulnerable to depression. Of course, going to school, making friends, and interacting with teachers would give the child some positive role models, but the parent’s influence could be a lot more potent because of their close relationship and the fact that they live together.

Furthermore, research into mirror neurons has shown that infants as young as six months old will imitate what they see. If this research could be applied to a child growing up with a depressed mother, it stands to reason that the child could learn to be depressed by imitating the mother. Field (1992), in a study of “the impact of maternal depression on early infant interactions and development” found that “infants of depressed mothers developed a depressed mood as early as three months which persisted for the first year if the mother’s depression persisted” (Field, 1992, pg. 1). This finding may provide further support for the theory of learned depression.

Another factor in the development of depression may be physical health, which is often tied to psychological health. Can being physically unhealthy also contribute to the development of depression? According to Nemande, Reiss, and Dombeck (2007), “People who are not physically healthy are at an increased risk for developing mental illnesses such as depression” (“Lifestyle Factors,” 2007). Also, individuals who engage in an unhealthy lifestyle have more difficulty overcoming depressive episodes. Some negative lifestyle practices that could contribute to this are: abuse of drugs or alcohol; overwork; poor diet, including excess caffeine or sugar; lack of exercise; poor sleep; and lack of leisure time (“Lifestyle Factors,” 2007).

Other studies also show that “physical health problems were demonstrated to be a predictor of both the onset and the persistence of depression” (Geerlings, Beekman, Deeg, Tilburg, 2000). “In later life, declining physical health is often thought to be one of the most important risk factors for depression. In multivariate analyses minor depression was related to physical health, while major depression was not. General aspects of physical health had stronger associations with depression than specific disease categories” (Beekman, Penninx, Deeg, Braam, Tilburg, 1997).

Thus, declining physical health, especially in later life, can also be a risk factor for development of depression, although the latter study would seem to suggest this would be more common for minor depression than for major depression.

Some may argue, however, that the correlation between physical health and depression does not point to causation. It could be that an individual is depressed and eating is their method of coping. Thus, obesity could occur as a side effect of depression, not the other way around. However, even this scenario does not rule out the possibility that declining health, due to old age or a chronic illness such as leukemia or AIDs, could potentially have negative effects upon an individual psychologically, and cause them to develop depression. Also, the effects on the psyche of being obese were already examined previously regarding young women and cultural values about body image.

Another environmental factor may be examined in relation to the incidence of depression: social culture. Can culture also contribute to depression? It would seem that sometimes even culture can become a stressor. Aside from the previous example of women and body image, cultural practices may define society in such a way that an individual may become depressed. For example, “some cultures have rigid gender roles that define expected behavior. Men’s lives exist primarily outside the home, while women’s roles are specifically in the home. In these cultures, women may not even leave their homes unless escorted by a male family member; conversely, men never enter the kitchen. If someone from this type of culture encounters a social stressor which forces a change in roles or a challenge to the status quo (i.e., the death of a spouse), such stress can cause this person to become depressed. For instance, if a husband loses a wife, he will not know how to care for his children’s' day to day needs such as feeding, bathing, etc. Similarly, if a wife loses her husband, she will not know how to provide financial support for her family and may even be prevented from trying. Both individuals may start to think of themselves as worthless or useless if they cannot meet the needs of their children” (“Sociology of Depression,” 2007).

Another environmental factor that may have a significant influence over the development of depression is early sexual or physical abuse. Can childhood abuse lead to depression later in life? In a study of depressed adults, Brodsky, Oquendo, Ellis, Haas, Malone, and Mann (2001) found that childhood abuse played a role in the severity of depression, as shown in the likelihood of a suicide attempt. “In 136 depressed adult inpatients, the authorsassessed trait impulsivity, aggression history, and number oflifetime suicide attempts as well as the medical lethality and the intent to die associated with the most lethal attempt. Thesevariables were then compared between those with and those without a reported history of childhood physical or sexual abuse.Subjects who reported an abuse history were more likely to havemade a suicide attempt and had significantly higher impulsivityand aggression scores than those who did not report an abusehistory. A logistic regression analysis revealed that abuse history remainedsignificantly associated with suicide attempt status after adjustmentfor impulsivity, aggression history, and presence of borderlinepersonality disorder. Thus, abuse in childhood may constitute an environmentalrisk factor for the development of trait impulsivity and aggressionas well as suicide attempts in depressed adults” (Brodsky, Oquendo, Ellis, Haas, Malone, Mann, 2001, pg 1).

Furthermore, according to Levitan et al. (1998), “Numerous studies have linked childhood trauma withdepressive symptoms over the life span. In a large community sample from Ontario, the authorsexamined whether a history of physical or sexual abuse in childhoodwas associated with particular neurovegetative symptom clustersof depression, with mania, or with both” (Levitan et. al, 1998, pg 1).They found that “ahistory of physical or sexual abuse in childhood was associatedwith major depression with reversed neurovegetative features” (Levitan et. al, 1998, pg 1).

Also, according to some researchers, “Adolescents and young adults with a history of childhood maltreatment were three times more likely to become depressed or suicidal compared with individuals without such a history. The effects of childhood sexual abuse were largest and most independent of associated factors. Risk of repeated suicide attempts was eight times greater for youths with a sexual abuse history” (Brown, Cohen, Johnson, Smailes, 1999, pg 1).

Thus, childhood abuse may definitely play a significant role in the development and severity of depression. Some researchers might debate the implications of the previous findings upon the actual probability of developing depression after childhood abuse, but even common sense argues that an individual who experiences physical or sexual abuse in childhood would be likely to be negatively influenced psychologically as a result.

Conclusion: After much research and examination of findings, it certainly seems that environmental and genetic factors both contribute heavily to the development of depression. There is evidence to suggest that individuals can develop depression with solely environmental or genetic factors, but it is also clear that both may combine in the development of depression. There is support for the theory that certain neurotransmitters in the brain may have an influence over depression, although it is unknown if this may simply be a case of correlation, not causation. Studies have also shown that the 5HTT gene may have an influence over the development of depression.

It has been demonstrated that depressed individuals tend to have suffered a severe life stressor before becoming depressed, and it has been suggested that depression may be a stress response in and of itself. The higher depression rate for women may suggest hormones and biological factors as causes, although this phenomenon could also be because women have stressors in their lives that men do not have. Also, although depression may have significant genetic influences, it also seems possible that children may learn to be depressed from their depressed parents, although the exact role of mirror neurons in the stages of early development and their relation to the development of depression have yet to be fully established.

Another contributing factor to depression seems to be physical health, which has been demonstrated to be correlated with an individual’s psychological health. However, it is possible that depression may contribute to ill physical health just as much as ill physical health may contribute to depression. Culture may also become an environmental stressor, if the death of a loved one prevents an individual from being able to meet the needs of his or her family. Last but not least, childhood abuse has long been shown to have a negative effect upon psychological health, and to contribute to the development of depression.

With all of the various environmental stressors that may arise in life, it is clear that depression is not a genetic phenomenon alone. Research has shown, however, that genetic and biological factors may indeed play a significant role in the development of depression. Thus, environmental stressors and genetic or biological makeup both appear to be crucial to the development of depression.


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