Why do well-functioning psychological systems sometimes give rise to absurd beliefs that are radically misaligned with reality? Drawing on signalling theory, I develop and explore the hypothesis that groups often embrace beliefs that are viewed as absurd by outsiders as a means of signalling ingroup commitment. I clarify the game-theoretic and psychological underpinnings of this hypothesis, I contrast it with similar proposals about the signalling functions of beliefs, and I motivate several psychological and sociological predictions that could be used to distinguish it from alternative explanations of irrational group beliefs.absurd belief, irrationality, motivated reasoning, self-deception, signalling, socially adaptive belief "Apparently irrational cultural beliefs are quite remarkable: they do not appear irrational by slightly departing from common sense, or timidly going beyond what the evidence allows. They appear, rather, like down-right provocations against common sense rationality" (Sperber, 1985, p. 85).