The Evolution of Portfolio Rules and the Capital Asset Pricing Model

The aim of this paper is to test the performance of the standard version of CAPM in an evolutionary framework. We imagine a heterogeneous population of long-lived agents who invest their wealth according to differential porfolio rules and ask what is the fate of those who happen to behave as prescribed by CAPM. In a complete securities market with aggregate uncertainty, it is shown that traders who either believe' in CAPM and use it as a rule of thumb, or are endowed with genuine mean-variance preferences, under some very weak conditions, vanish in the long run. A sufficient condition to drive CAPM or mean-variance traders' wealth shares to zero is shown to be that an investor endowed with a logarithmic utility function enters the market. Finally, the robustness of the results is checked, allowing for different kinds of heterogeneity among traders.