Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Economics, 2007.Includes bibliographical references.This dissertation consists of three chapters on financial market imperfections, in particular, information imperfections. Chapter 1 studies how the existence of a fixed cost per transaction faced by uninformed investors hampers information revelation through price and exacerbates adverse selection. The exacerbated adverse selection explains one long-standing puzzle in finance - the momentum anomaly. Properly adjusting stock returns for adverse selection by using data on trading volume substantially mitigates momentum-based arbitrage profits for the sample period from 1983 to 2004. Chapter 2 studies how information asymmetry p...