First off, the reason double-blind testing is so effective is that it reduces subconscious biases in behaviour. If the person administering drugs knows its a pl

First off, the reason double-blind testing is so effective is that it reduces subconscious biases in behaviour. If the person administering drugs knows its a placebo, he/she will consciously, or subconsciously, behave differently. All sorts of unwanted effects come into play, most importantly Observer-expectancy (For a good example, https://en.wikipedia.org/wiki/Clever_Hans https://en.wikipedia.org/wiki/Clever_Hans) Double-blind testing allows one to test the effects of administering anything (the placebo effect), versus the effects of the actual drugs. By administering an inert substance, the baseline placebo effect can be tested, as no actual medicative effects are expected from this inert substance. When testing placebo vs nothing, in the nothing case a lot changes. The participants will no longer be administered anything, the experimenters will be aware of not administering anything, etc. etc. Because so many variables change in such a set-up, you can no longer actually tell which of these variables changing caused the effect and which of them didn't. Thus making any results and conclusion derived from the suspect.