There appear to be no anomalies in the aftermarket of a sample of 4,848 U.S. IPOs over the period 1975 to 1995, except issues offered below $6. Risk is priced in the aftermarket in accordance with Rubin-stein's asset-pricing model. Unlike under the efficient markets hypothesis (EMH), however, market priors about the probability of future default are not unbiased at the IPO date. Still, subsequent learning is rational: the market uses Bayes' law with a correct-likelihood function (of news given the eventual fate of an issue). That is, the hypothesis of an efficiently learning market (ELM) cannot be rejected. We produce direct evidence in support of these statements, based on a new class of tests. We also provide indirect evidence, by documenting a gradual convergence of IPO prices towards EMH as issues mature.

We attempt to translate Neo‐Austrian ideas about the workings of financial markets, as originally advanced by F. A. Hayek, into the standard probabilistic language of modern finance. We focus on an apparent paradox, namely the insistence of Neo‐Austrians on order (i.e., stationarity) together with ever‐reemerging inefficiencies . The paper's findings have implications beyond Neo‐Austrian theory: They demonstrate how easy it is to reject market efficiency, but how much more difficult it is to discern the nature of the inefficiency . We illustrate our findings with price data from the U.S. Treasury bill market over the period 1962 to 1999. There is ample evidence that the price of a three‐month Treasury bill is not a random walk, yet the sign of the average price change is erratic, so that inference about the nature of the inefficiency is unreliable.