CAPM Equilibrium and Why Investors Hold the Market Portfolio
Summary
The document explains CAPM equilibrium as a market-clearing condition: aggregate demand for securities matches their supply. Under the model’s assumptions, investors choose combinations of the risk-free asset and the market portfolio, with their allocation between the two reflecting their preferences and risk tolerance.
It clarifies why CAPM can still estimate an individual stock’s expected return even though the stock is already included in the market portfolio. The estimate describes the return consistent with the model; it does not imply that an investor should overweight that stock. Doing so would depart from the model’s prescribed portfolio. The explanation depends on assumptions such as mean-variance preferences, shared expectations about returns and correlations, and borrowing and lending at the risk-free rate. It offers a conceptual answer rather than empirical evidence or guidance for settings where those assumptions fail.
Key ideas
- CAPM equilibrium means the market’s aggregate demand for securities matches their supply.
- Under CAPM assumptions, investors combine the risk-free asset with the market portfolio.
- The model can estimate a stock’s expected return without recommending an overweight in that stock.
- Departing from the market portfolio conflicts with the model’s prescribed allocation.
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Full text
# Assumptions of the CAPM # Assumptions of the CAPM As to my understanding, the CAPM assumes that all investors behave as described in the portfolio theory. Consequently, all investors hold a combination of the risk-free investment and the efficient portfolio (the portfolio with the highest Sharp ratio). I have two questions: - It is said that CAPM is an equilibrium model. What exactly does that mean in this context? - If it is assumed that all investors hold the efficient portfolio (only with the distinction of how large the share of the portfolio is compared to the risk-free investment), why should an investor use the model to calculate the expected return on an individual stock? The stock is already contained in the Market Portfolio, which is held by every investor. Investing a higher amount in a stock shpuld consequently lead to a deviation from the market portfolio(?) Wouldn't the use of the model then argue against its assumption? ## Answer by Xiaohuolong (score 1, accepted) https://quant.stackexchange.com/a/60539 - It means the supply of all securities equals the demand of all securities, so the market clears and is in equilibrium as in standard economics language. - Indeed, all investors will hold some combination of the market portfolio and risk-free security and will not deviate from this, assuming the assumptions of the CAPM are satisfied, namely that investors have mean-varaince preference, that they agree on the means/sds of returns as well as correlations, and can fund and lend at the risk-free rate. An investor can use the model to calculate the expected return on an individual stock, but again, they should still not deviate from holding the market portfolio and risk-free investment.
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