It's a problem of testing the efficacy of going through treatment (with any substance, inert or active) vs testing the efficacy of a certain active drug. In dou

It's a problem of testing the efficacy of going through treatment (with any substance, inert or active) vs testing the efficacy of a certain active drug. In double-blind experiments, where placebo effects are attempted to be disentangled from actual drug effects, such a baseline would introduce too many variables to be taken in account. There are psychological factors related to the participants response ("Hey, I joined this clinical trial, but I'm not being given anything, what's up with that?") that make it difficult to conclude anything from the results. The participant might even be adversely afflicted by this seeming "lack of care" from the experimenters side. Next to that, experiments might experience observer-expectancy effects, where the expectations of experimenters cause the participants to behave in certain ways. Also, if a positive result was found in the "not given any drug" vs "given the active drug" trial, one would still not be sure if this was due to the participant going through the treatment, or due to the treatment + the effects of the drug.