Theories of liquidity

We survey the theoretical literature on market liquidity. The literature traces illiquidity, i.e., the lack of liquidity, to underlying market imperfections. We consider six main imperfections: participation costs, transaction costs, asymmetric information, imperfect competition, funding constraints, and search. We address three questions in the context of each imperfection: (a) how to measure illiquidity, (b) how illiquidity relates to underlying market imperfections and other asset characteristics, and (c) how illiquidity affects expected asset returns. We nest all six imperfections within a common, unified model, and use that model to organize the literature.