ABSTRACT Imagine, as most economists do, that financial-market participants understand the basic structure of the world: While they cannot predict the future with certainty, they are endowed with knowledge of the possible outcomes of their actions and the probability that each of those outcomes will occur. Given these assumptions, if bankers, regulators, investors, and rating agencies were rational, we may conclude that the financial crisis was caused by poor incentives: These actors must have knowingly jeopardized their institutions and the global (...) economy to pursue private gain. Alternatively, market participants, caught up in the mania of crowds, may have irrationally ignored risks of which they were aware and of which they knew the correct probability. Neither of these explanations for the financial crisis stands up to the evidence, since both explanations assume that the participants knew the U.S. housing market was about to crash. A fresh look at the data five years later suggests that they were ignorant of this fact, and that they acted rationally (or at least reasonably) in light of what they (thought that they) knew. This allows a more nuanced understanding of how financial institutions ended up in bedlam, why credit markets collapsed, and why they took so long to recover. (shrink)