In contrast to the biopsychosocial model, which describes the interdependence of depression causes, the diathesis-stress model talks about the relationship between potential causes of depression, and the degree to which people may be vulnerable to react to those causes. The diathesis-stress model suggests that people have, to different degrees, vulnerabilities or predispositions for developing depression. In the language of this model, these vulnerabilities are referred to as diatheses. Diatheses include some of the biological and psychological factors mentioned in the previous discussion of the biopsychosocial model. Some people may have more of these diatheses for developing depression than other people. However, this model suggests that having a propensity towards developing depression alone is not enough to trigger the illness. Instead, an individual's diathesis must interact with stressful life events (of a social, psychological or biological nature) in order to prompt the onset of the illness.
According to the model, the greater a person's inherent vulnerability for developing depression, the less environmental stress will be required to cause him or her to become depressed. In contrast, if someone has a smaller amount of vulnerability for becoming depressed, it will take greater levels of environmental stress in order to produce the disorder. Until this critical amount of stress has been reached, people will generally function normally, and their vulnerabilities are considered to be "latent" or hidden.
The impact of particular stressors varies across different people. Death or other losses such as a job layoffs; relationship difficulties like divorce; normal milestones such as puberty, marriage, or retirement; alcoholism or drug abuse; neurochemical and hormonal imbalances; and infections can all be powerful enough to cause depressive symptoms in someone with a diathesis for this illness. However, each of these events will impact individuals in a unique manner. A significant loss may be enough to trigger depression in one person, while a very similar loss experienced by another person might not faze them all that much.
According to both the Biopsychosocial Model and the Diathesis-Stress Theory, Unipolar Depression is caused by numerous psychological, social, and biological factors that interact with one another, and with a given individual's unique vulnerabilities. Depression is thus a very complex sort of condition that really demands to be thought about in a wholistic manner. Since no one factor causes depression, it is probably unreasonable to expect that only one type of treatment can fix the problem.