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In one of the last Harry Potter books, Ron must play keeper during a very important game of Quiddich. He is very nervous, so Hermione gives him a potion that will give him extraordinary ability. Ron plays great and helps his team win the game. Later, Hermione reveals that the potion was only juice. How can we account for why Ron played so well?
This is a primary example of the placebo effect? The only thing that actually was improved was Ron's confidence, not his actual physical ability.