The subjects were told they would receive various doses of caffeine before each trial, but they wouldn’t be told which dose they had received. As expected, the cyclists rode 1.3 percent faster when they thought they had received a moderate dose, 3.1 percent faster after a high dose, and 1.4 percent slower when they thought they got the placebo.10 In reality, all the pills were placebos. The performance boost, and associated changes in how much pain or effort they perceived during the rides, were entirely fueled by their own expectations.
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