Recent developments in financial economics have included many explorations into market microstructure, that is, the internal functioning of markets and the ways in which they provide liquidity to traders. An important contribution of this literature is that prices can deviate from their fundamental values. This paper describes models of imperfect liquidity and improperly processed information in financial markets, focusing on the noise trader and investor herding literature. The motivations for this line of research are presented, followed by a description of some of the major contributions and tests of some of their empirical implications.

We use the revised estimates of U.S. GNP constructed by Christina Romer (1989) to assess the time-series properties of U.S. output per capita over the past century. We reject at conventional significance levels the null that output is a random walk in favor of the alternative that output is a stationary autoregressive process about a linear deterministic trend. The difference between the lack of persistence of output shocks either before WWII or over the entire century, on the one hand, and the strong signs of persistence of output shocks found by Campbell and Mankiw (1987) and by Nelson and Plosser (1982) for more recent periods is striking. It suggests to us a Keynesian interpretation of the large unit root in post-WWII U.S. output: perhaps post-WWII output shocks appear persistent because automatic stabilizers and other demand-management policies have substantially damped the transitory fluctuations that made up the pre-WWH Bums-Mitchell business cycle.