The story
Proven by philosopher Allan Gibbard in 1973.
Etymology from Wiktionary, CC BY-SA 4.0.
Gibbard's theorem
Today it means: A theorem stating that, for any deterministic process of collective decision, at least one of the following three properties must hold: (i) the process is dictatorial, i.e. there exists a distinguished agent who can impose the outcome; (ii) the process limits the possible outcomes to two options only; (iii) the process encourages agents to think strategically: once an agent has identified their preferences, they have no action at their disposal that would best defend their opinions in any situation. Full definition →