A group of soldiers go on a 40 km full-gear march. A portion are told by researchers that it’s only 30km, another portion that it’s 40km, and the last group that it’s 60km.
After the march, hormone testing reveals that their stress levels are consistent with their expectations, not the reality.
The lesson? Hope and despair are self-fulfilling prophecies.
(Paraphrased from Erik Barker)