Judgments of the rationality of beliefs must take the costs of acquiring and possessing beliefs into consideration. In that case, certain false beliefs, especially those that are associated with the benefits of a cohesive community, can be seen to be useful for an agent and perhaps instrumentally rational to hold. A distinction should be made between excusable misbeliefs, which a rational agent should tolerate, and misbeliefs that are defensible in their own right because they confer benefits on the agent. Likely (...) candidates for such misbeliefs are to be found in the realm of nationalism and religion, where the possession costs of true beliefs are high, and where collective beliefs in falsehoods may allow for a cohesive community. We discuss the paradoxes of reflective awareness involved in the idea of deliberately embracing falsehoods. More rigorous, fully reflective concepts of rationality would still disallow false beliefs, but such demanding versions of rationality would commit agents to pay large costs, thereby weakening the motivation for acquiring true beliefs. (shrink)