Scientific Beta

Proponents of cap-weighted stock market indices often argue that such indices provide efficient risk/return portfolios. This paper reviews the evidence in the academic literature and concludes that only under very unrealistic assumptions would such indices be efficient investments. In the presence of realistic constraints and frictions, cap-weighted indices cannot, according to the academic literature, be expected to be efficient investments.

Proponents of cap-weighted stock market indices often argue that such indices provide efficient risk/return portfolios. This paper reviews the evidence in the academic literature and concludes that only under very unrealistic assumptions would such indices be efficient investments. In the presence of realistic constraints and frictions, cap-weighted indices cannot, according to the academic literature, be expected to be efficient investments. 

The three main conclusions of the research are the following:

• A cap-weighted stock market index is not the market portfolio of financial theory (the Capital Asset Pricing Model (CAPM) theory is often evoked to show that cap-weighted stock market indices are efficient portfolios and attractive investments). That it is not is clear from the choices made in empirical studies that attempt to come up with reasonable proxies for the market portfolio. These studies attach great importance to including many more stocks than indices do, and their proxies of the market portfolio include bonds, real estate, and non-tradable assets such as human capital.
   
• Even if it were possible to construct and hold the market portfolio, the theory does not predict that the market portfolio is efficient unless we make highly unrealistic assumptions. In fact, the authors of the seminal academic research in the 1950s and 1960s, Harry Markowitz and William Sharpe, have themselves emphasised (Sharpe (1991) and Markowitz (2005)) that the market portfolio may not be efficient in a more realistic setting.
   
• In view of these arguments, financial theory alone does not justify the current practice of capweighting. In fact, from a theoretical perspective, cap-weighted stock market indices seem to offer no particular advantage.

A revisited version of this paper was published in the Fall 2011 issue of the Journal of Index Investing.