Of the behavioral recommendations garnered from modern capital market theory, few, if any, generalizations have been documented as convincingly as the simple advice to hold several assets in one's portfolio. Sharpe made such a conclusion perfectly clear when he stated [27, p. 184]:If the market is efficient and if an investor is privy to no special information or predictive power, what should he do? First, and most important: diversify.

In this paper the traditional capital asset pricing model is reformulated as a system of simultaneous equations in which returns on similar securities are treated as endogenous variables and in which pertinent financial data for particular firms and a market factor are treated as exogenous variables. Such a system is estimated, and serious questions are raised concerning the tenability of the simple linear model so often used to explain capital asset prices under uncertainty.