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Gene-Environment Interactions and Depression

It is artificial and even somewhat misleading to discuss the genetic and psychological causes of Depression as if they were separate domains.  This is because it is becoming increasingly recognized that many genes only become expressed in the presence of certain environmental triggers.  An environmental trigger can be anything that a person experiences in the course of their lifetime, be it the foods one eats, being part of a certain socioeconomic class, the kind of attachments one had with one's parents, or having been traumatized as a child.

One important example of this phenomenon is the 5-HTTLPR gene, where the S type of the gene seems to increase the risk of developing Depression, but mainly when a person with this gene type experiences certain life stressors [ref].  It has been found that the S type of the gene contributes to Neuroticism, which is a personality trait that leads an individual to react more intensely, with more anxiety and worry, to stressors [ref].  This gene has also been found to influence the anatomy of the brain, particularly the amygdala and cingulate gyrus, which are regions that mediate a person's stress response [ref].  Thus, it seems that the S type of 5-HTTLPR puts a person at risk for reacting negatively to a life stressor, which in turn can lead to a Depressive Episode once a stressful event is experienced.   The kinds of stresses that most commonly contribute to a Depression are discussed here.

Even events that occur while a person is still a fetus in their mother's womb can also have a lasting, life-long effect on how the person's genes will be expressed, which is a phenomenon called fetal programming.  Mothers who experience high levels of stress, anxiety or depression during pregnancy will tend to have high levels of stress hormones circulating in their blood system, including Cortisol.  These stress hormones can permanently alter the child's genetic structure, essentially programming¬Ě the child's own neuro-endocrine system to be more reactive to stress and for the child to be at greater risk for developing a Depressive Disorder in his or her lifetime [ref]. 

A similar programming¬Ě process is thought to occur based on how a mother bonds to her child during infancy [ref], and also on if a child is abused or neglected [ref].

 

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Psychological Roots

Medical Causes