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In the case of a psychological disorder such as depression, the cause is never solely genetic or environmental, but a complex interaction between the two. The diathesis-stress model is a way to understand the interaction between genetics and environment in causing mental illness. According to this model, people inherit a genetic tendency toward a mental disorder — diathesis — which is acted upon by environmental stimuli — stress — to cause the disorder to appear.
The diathesis-stress model shows the inheritance of vulnerability to a specific condition such as depression, anxiety, or addiction. For a more vulnerable person, less environmental stress is necessary to trigger a mental illness. A person with very little vulnerability can endure high amounts of stress without developing a mental disorder.
For example, many soldiers go to war and experience traumatic events, however, not all develop a mental illness like post-traumatic stress disorder (PTSD). This is because some soldiers have a genetic predisposition toward PTSD, while some do not. Those with a genetic vulnerability are much more likely to have PTSD triggered by a traumatic event. Soldiers with a lesser genetic vulnerability can experience the same event without developing the same disorder.
The diathesis-stress model shows how genetic inheritance can have a large influence on behavior. Researchers estimate that around half of a person’s personality traits and cognitive abilities are due to genetics. The rest may be due to a variety of environmental and developmental factors.
In the case of mental illness, it is often much more complicated than isolating a single gene that is responsible for the disorder. For most mental disorders, many genes have a small effect, and the cumulative influence of all these genes together is what leads to a mental illness. This can make it very difficult to research the genetic background behind a psychological disorder.
The diathesis-stress model shows that genes are only a part of the picture in the case of a psychological disorder. Environmental effects are just as important in causing mental illness. These environmental stressors can vary greatly from person to person, making it difficult to isolate one specific cause or factor leading to mental illness.
Biological influences such as the arousal of the nervous system in a stressful situation can be one type of environmental stress referred to in the diathesis-stress model. Social and developmental influences, such as the way a person was raised and how the people around them respond to a stressor can also play a large role.