This paper offers an epistemological discussion of self-validating belief systems and the recurrence of ?epistemic defense mechanisms? and ?immunizing strategies? across widely different domains of knowledge. We challenge the idea that typical ?weird? belief systems are inherently fragile, and we argue that, instead, they exhibit a surprising degree of resilience in the face of adverse evidence and criticism. Borrowing from the psychological research on belief perseverance, rationalization and motivated reasoning, we argue that the human mind is particularly susceptible to belief (...) systems that are structurally self-validating. On this cognitive-psychological basis, we construct an epidemiology of beliefs, arguing that the apparent convenience of escape clauses and other defensive ?tactics? used by believers may well derive not from conscious deliberation on their part, but from more subtle mechanisms of cultural selection. (shrink)