Tom J. Johnsen and Oddgeir Friborg say:
The metaregressions examining the temporal trends indicated that the effects of CBT have declined linearly and steadily since its introduction, as measured by patients’ self-reports (the BDI, p < .001), clinicians’ ratings (the HRSD, p < .01) and rates of remission (p < .01). Subgroup analyses confirmed that the declining trend was present in both within-group (pre/post) designs (p < .01) and controlled trial designs (p < .02). Thus, modern CBT clinical trials seemingly provided less relief from depressive symptoms as compared with the seminal trials.
It has been proposed that major depressive disorder is near-identical with sickness behavior, so raising the possibility that it is a maladaptive manifestation of sickness behavior due to abnormalities in circulating cytokines.
Scott Alexander has recently blogged about this, or closely related stuff, here
. I have not read his post yet.