Its advantage, however, is that it is a clean benchmark that allows me to sidestep the messy problem of deciding what are reasonable information and trading costs.
Since there are surely positive information and trading costs, the extreme version of the market efficiency hypothesis is surely false. A weaker and economically more sensible version of the efficiency hypothesis says that prices reflect information to the point where the marginal benefits of acting on information (the profits to be made) do not exceed the marginal costs ( Jensen (1978)). A precondition for this strong version of the hypothesis is that information and trading costs, the costs of getting prices to reflect information, are always 0 ( Grossman and Stiglitz (1980)). I take the market efficiency hypothesis to be the simple statement that security prices fully reflect all available information. Instead, I discuss the work that I find most interesting, and I offer my views on what we have learned from the research on market efficiency.
The literature is now so large that a full review is impossible, and is not attempted here. The task is thornier than it was 20 years ago, when work on efficiency was rather new. S equels are rarely as good as the originals, so I approach this review of the market efficiency literature with trepidation.
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