One former professional poker player, now retired, recently mucked a pair of aces against a bluff. We all muck our cards from time to time -particularly when we either don't have a decent starting hand, or even more often, if we think another player has a better hand. It's a mind game. Basically, it's a mental condition that causes us to muck our mental confidence. This condition is either, as psychologists would say, a reflex-reaction, or one brought about by the appearance of strength from an opponent.
Poker is a game of cards - but in reality it's a game of the mind.
Perhaps not so much in lower limits, but certainly so in higher limit games, and even more profoundly in professional high-stakes games and tournaments. Many notable professionals and highly successful tournament players, such as the late Stu Ungar, have been known to put the man off his hand. This happened during the last World Series Ungar won, when he was seen putting his opponent off his starting hand of J-9 offsuit. At a recent poker game, one of the players at my table made another player fold aces-full. It was a classic example of a mind muck.