The biological link in the association between stress and depression

While this NEJM study is about how to differentiate the hypercortisolism found in depression versus Cushing's Syndrome, the study implies that high levels of cortisol is associated with depression. Is it the cause?

Actually, another study by Robert Sapolsky says that it's probably the other way around -- depression or its inducing stressor causes an elevation in cortisol, which in turn causes many of the affect changes associated with depression. That's how I interpreted this line:

An extensive literature demonstrates that such hypercortisolism can be both a response to the stressors preceding depression (arrow 2) and to depression itself (arrow 3), and can, in turn, contribute to the affective symptomology (arrow 4).