Humans can be both rational and irrational. The term stupid perhaps is attempting to get at irrationality. Perhaps. Whenever I hear the term stupid, it sounds very inflammatory and accusatory for not seeing the world as I do. But I do find humans extremely irrational given both our emotions and our stern commitments to our beliefs. Rationality involving the discovery and consideration of alternatives demands much from us, so that irrationality, or accepting what is readily at hand, is the default position for most of us.