By considering investor order placement strategy, this paper demonstrates that transaction costs cause bid-ask spreads to be an equilibrium property of asset markets. With transaction costs, the probability of a limit order executing does not go to unity as the order is placed infinitesimally close to a counterpart market quote; thus, with certainty of execution at the counterpart market quote, a "gravitational pull" is generated that keeps counterpart quotes from being placed infinitesimally close to each other. An equilibrium spread is defined and its size linked to market thinness; implications are noted for the design of a trading system.

This paper considers how estimates of the market model beta parameter can be biased by friction in the trading process (information, decision, and transaction costs) that (a) leads to a distinction between observed and ‘true’ returns; (b) causes observed returns to be generated asynchronously for a set of interdependent securities; and (c) thereby introduces serial cross-correlation into security returns. Several propositions are derived from which consistent estimators of beta are obtained, and the effect of differencing interval length on beta estimates is specified. The formulation is contrasted with the related analyses of Scholes-Williams (1977) and Dimson (1979).