Tease Me, Placebo
Proof that placebo effect is strongest when you’re told you *might* be getting real medicine peaking at somewhere around 3/4 chance of thinking you’ll get the non-placebo. Being told you’re definitely getting real medicine is no better than being told that you have a 1/4 chance of the real thing.
Likely the same cause as the greater reinforcement effect of intermittent random rewards. It seems that the ambiguous interpretation of “maybe the drug doesn’t work, or maybe I got the placebo - I hope I got the real medication!” inspires more hope than “hm, I haven’t noticed much difference. does this drug even work?”
Lidstone et al (2010) took some patients with Parkinson’s Disease, which is associated with low levels of the neurotransmitter dopamine. Then they told them they mightgive them medications that would raise their dopamine. In fact, they told the patients that they had a 25%, 50%, 75%, or 100% of receiving this medication; otherwise they would receive a placebo. In fact this was all a total lie; everyone got the placebo. Then they pumped everyone’s blood full of radioactive chemicals and stuck them into a PET scanner, which measured the amount of dopamine released in their brains. They found that the higher the probability of receiving real medication, the higher the amount of dopamine released (trend did not reach significance) except in the 100% case, where dopamine was lowest of all. These are pretty weird results.
via (link has other, larger, more powerful studies; this is just the most amusing one)
(increased dopamine probably also feeds back by improving the placebo effect itself; if you ran this study with a medication supposed to improve some other factor, you might find that dopamine is also increased as part of the placebo mechanism)