Psychiatrists in the past tended to emphasize the distinction between depressions triggered by stressors and those that spring unbidden out of a disordered brain.
But there is likely a continuum with some depressions following the onslaught of massive stressors, others occurring in the midst of placid lives, and still others somewhere in between.
Stress presumably interacts with genetic vulnerability, mediated by the stress hormone system.
One easy way to think about this is to think about an individual's resilience to stress as a dam that holds back water.
The height of the dam represents an individual's genetic resistance to stress. The higher the dam, the better that an individual can adapt to stressful situations.
Stress can be thought of as the water added by rainstorms. The stress hormone system is the drainage system that may be more or less efficient at siphoning off water as the need arises.
Just as rain is good for filling a dry reservoir, arousal can have positive psychological effects. Research has even shown that performance increases as arousal increases -- up to a certain point, at least.
Think about a student who might nod off in class during a mid-semester lecture, but writes a long and clever essay at semester's end when a course grade is on the line.
But as arousal increases further, it becomes stressful, as it creates a mismatch between demands and ability to meet those demands, which leads to decreasing performance.
If the same student has four papers and four final exams to prepare for, and his parents want to talk and e-mail extensively with him about their impending separation, he might become anxious and overwhelmed, and produce rushed, sloppy work.
There is extensive literature examining the relationship of stressors to depression.
One early study found that in the six months preceding a depression, people tended to have increased arguments with their spouse, marital separation, new jobs, changes in work conditions, serious personal illnesses, serious illness or death of family members, or family members leaving home.
While some of these could have been the result of depression, others, such as the death of a family member, are almost certainly independent of it.
Later studies have generally confirmed a relationship between stressors and depression, though the relationship is not a simple one. For example, one research group found that severe stressors often preceded depression in women; however, only about one out of five women experiencing such a stressor went on to develop depression.
And there is evidence that genetic factors and stressors interact in causing depression. A study of several thousand female twins found that the four stressors that most strongly predicted depression were death of a close relative, assault, serious marital problems, and divorce or breakup.
In people at lowest genetic risk, the likelihood of depression was low for those who had not experienced a severe stressor, and medium for those who did.
In people at highest genetic risk, the probability was low in the one group and high in the other. The high risk represents the interactive risk of depression in those with both genetic vulnerability and exposure to high stress.