Unfounded self-confidence (or any unfounded belief) is very harmful.
Citation needed. Bluff (i.e. unfounded confidence) seems to be a very efficient strategy in many games. Apparently, even in chess:
UNIDENTIFIED MALE #2: Rook to D1.
CAMPBELL: And this particular move was really bad, and so it caused us to give up the game right away.
FOO: This really bad move confused Kasparov. Murray says he heard Kasparov’s team stayed up that night trying to analyze the logic behind that move—what it meant. The only thing was—there was no logic.
Citation needed. Bluff (i.e. unfounded confidence) seems to be a very efficient strategy in many games. Apparently, even in chess: