The Economist reports on the pseudo placebo effect in randomised control trials.
"In a new paper, Erwin Bulte of Wageningen University and his colleagues conduct a double-blind test of an agricultural intervention—that is, the treated don’t know whether they are receiving the treatment or the placebo. The treatment is a modern seed of cowpeas, the placebo is the traditional seed. As a second experiment in a different set of villages, they do a normal RCT where the treated know that they are receiving the modern seed. Comparing the results of both experiments reveals some striking results. When the farmers don’t know which seed they are planting, there is no difference between the modern and the traditional seed in terms of yield. When they do know that they are being treated, the modern seeds yield considerably more. What the authors call the “pseudo-placebo effect” therefore accounts for the whole effect that a typical RCT would have found."