Markets have an allocational role; even in the absence of news about payoffs, prices change to facilitate trade and allocate resources to their best use. Allocational price changes create noise in the signal extraction process, and markets where such trading is important are markets in which we may expect to find a failure of informational efficiency. An important source of allocational trading is the use of dynamic trading strategies caused by the incomplete equitization of risks. Incomplete equitization causes trade. Trade implies the inefficiency of passive strategies, thus requiring investors to determine whether price changes are informational or allocational.

Markets have an allocational role; even in the absence of news about payoffs, prices change to facilitate trade and allocate resources to their best use. Allocational price changes create noise in the signal extraction process, and markets where such trading is important are markets in which we may expect to find a failure of informational efficiency. An important source of allocational trading is the use of dynamic trading strategies caused by the incomplete equitization of risks . Incomplete equitization causes trade. Trade implies the inefficiency of passive strategies, thus requiring investors to determine whether price changes are informational or allocational.

A martingale approach is used to characterize general equilibrium in the presence of portfolio insurance. Insurers sell to non-insurers in bad states, and general equilibrium requires that the risk premium rises to induce non-insurers to increase their holdings. We show that portfolio insurance increases price volatility, causes mean reversion in asset returns, raises the Sharpe ratio and volatility in bad states, and causes volatility to be correlated with volume. We also explain why out-of-the-money S&P 500 put options trade at a higher volatility than do in-the-money puts.

A martingale approach is used to characterize general equilibrium in the presence of portfolio insurance. Insurers sell to noninsurers in bad states and general equilibrium requires that the risk premium rises to induce noninsurers to increase their holdings. The authors show that portfolio insurance increases price volatility, causes mean reversion in asset returns, raises the Sharpe ratio and volatility in bad states, and causes volatility to be correlated with volume. They also explain why out-of-the-money S&P 500 put options trade at a higher volatility than do in-the-money puts.

A martingale approach is used to characterize general equilibrium in the presence of portfolio insurance. Insurers sell to noninsurers in bad states, and general equilibrium requires that the risk premium rises to induce noninsurers to increase their holdings. We show that portfolio insurance increases price volatility, causes mean reversion in asset returns, raises the Sharpe ratio and volatility in bad states, and causes volatility to be correlated with volume. We also explain why out‐of‐the‐money S&P 500 put options trade at a higher volatility than do in‐the‐money puts.

Over the past decade, Sanford Grossman's contributions to the economics of information have significantly altered the way economists think about rational expectations. Here his articles are collected in one place, providing a uniform framework for understanding how prices convey information in securities markets. Grossman elaborates a new model of economic equilibrium that casts a dual role for prices both as constraints that affect the immediate costs or benefits of acts and as conveyers of information about the probable future costs and benefits of those acts. He points to the Wall Street panic of October 1987 as an example of the informational role of prices where volatility actually represented sophisticated trading strategies by relatively uninformed individuals.

Market liquidity is modeled as being determined by the demand and supply of immediacy. Exogenous liquidity events coupled with the risk of delayed trade create a demand for immediacy. Market makers supply immediacy by their continuous presence and willingness to bear risk during the time period between the arrival of final buyers and sellers. In the long run the number of market makers adjusts to equate the supply and demand for immediacy. This determines the equilibrium level of liquidity in the market. The lower is the autocorrelation in rates of return, the higher is the equilibrium level of liquidity.