This is what the book The Emperor's New Drugs is about. The idea of the book is that there was some well-known research showing that anti-depressants are effective for severe depression, but not mild or moderate cases. But if you actually only look at studies that used active placebos as controls, then there is no evidence clinical efficacy even for severe depression. Meaning that the nominal benefit for anti-depressants for severe depression that's seen in many studies is in fact probably due to people thinking they are better due to the side effects of the drug, rather than due to true drug benefits.
It could also be that participating in the study addresses sources of depression by giving the subject a way that they are 'helping out' society, or are 'cared about' etc.
There are probably many factors to depression which may include chemical or structural causes; however the human brain is a complex self adapting analog computer. It would be foolish to think that such a system can be reduced a single or even simple set of controlled variables.
