The quality-stress model predicts that people will develop when their own tendencies and stress are added together above a certain level.
The causes of major depressive disorder are not known. However, a biopsychosocial model has been proposed, which suggests that biological, psychological and sociological factors all play an important role in causing depression. The quality-stress model suggests that depression can occur when a pre-existing vulnerable body or physical quality is activated by an extremely stressful life event. Pre-existing vulnerable physical qualities can be genetic, implying an interaction between nature and nurture, or due to childhood perceptions of the world. Childhood abuse, whether physical, sexual or psychological, is a risk factor for depression. Other psychiatric problems such as anxiety and substance abuse are others that can co-occur. Childhood trauma is also associated with the severity of depression, lack of response to treatment and length of illness. Psychiatric disorders such as depression are more likely to occur after childhood trauma. Various genes have also been proposed to control susceptibility.
The quality-stress model suggests that depression is a result of a patient's pre-existing susceptibility (quality) being activated by stressful events in their life. This susceptibility of the patient can be due to genetics, thus involving an interplay of innate and acquired qualities, or a schema, a specific cognitive pattern acquired by the patient through learning as a child. Both of these models of interaction have been supported by experiments. For example, researchers in New Zealand used a predictive approach to study depression. In the experiment the researchers followed a group of otherwise healthy people over a long period of time and recorded the conditions associated with depression. Ultimately the researchers concluded that the 5-hydroxytryptamine transporter (5-HTT) gene in the body influences whether people consistently experience depression in response to stressful events. In particular, they noted that depression is more likely to occur in people with one or two short alleles of the 5-hydroxytryptamine transporter.
A Swedish study estimated the genetic predisposition to depression to be around 40% for women and 30% for men. Evolutionary psychologists believe that the genes that make people suffer from depression have long been present in the history of natural selection. Patients can also develop symptoms similar to those of major depression if they take long-term psychotropic medication, such as long-term sedatives and sleeping pills. These symptoms are caused by side effects of the medication or withdrawal reactions to the medication and are known as psychoactive substance-induced mental disorders, rather than major depressive disorder.
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