CAPM Market Portfolio Weights and Security Price Adjustment
Summary
The document explains a passage about the market portfolio in the Capital Asset Pricing Model (CAPM), focusing on why an omitted risky investment may fall in price and how its expected return can then rise. The accepted response emphasizes that the argument relies on CAPM assumptions and points to the relationship between an asset’s price and expected return, though it does not derive that relationship in detail.
It also interprets the claim that each investment’s market portfolio weight is proportional to its amount available in the economy. Under the model’s assumptions, the market portfolio aggregates investors’ risky holdings; lending and borrowing offset across investors. If investors hold the same risky portfolio in the same proportions, aggregate security weights match those proportions and thus reflect the available market value of each security. This is a conceptual clarification of the model, not an empirical account of real investor portfolios or a discussion of deviations from CAPM assumptions.
Key ideas
- The explanation of market portfolio formation is based on CAPM assumptions.
- A fall in a security’s price corresponds to a rise in its expected return, supporting its inclusion in the risky portfolio.
- The market portfolio aggregates risky assets held across the economy.
- If investors hold the same risky portfolio in the same proportions, aggregate weights reflect the available amounts of securities.
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# Clarification of The Market Portfolio # Clarification of The Market Portfolio I am currently reading John C. Hulls' "Risk Management and Financial Institutions" and came across the following passage related to the efficient frontier and combinations of risky and risk-free assets: > It is a short step from here to argue that the portfolio of risky investments represented by M must be the portfolio of all risky investments. Suppose a particular investment is not in the portfolio. No investors would hold it and its price would have to go down so that its expected return increased and it became part of portfolio M. In fact, we can go further than this. To ensure a balance between the supply and demand for each investment, the price of each risky investment must adjust so that the amount of that investment in portfolio M is proportional to the amount of that investment available in the economy. The investment represented by point M is therefore usually referred to as the market portfolio. The following part confuses me: > No investor would hold it and its price would have to go down so that its expected return increased and became part of portfolio M. This makes little sense to me at the moment. I understand that by excluding a particular investment (say $I$) in $M$, its demand will decrease as all risk-adverse investors will purchase portions of $M$ to produce a linear efficient frontier. This will decrease the price of $I$ but I am not sure why this will increase its expected value, and thereafter be inserted into the market portfolio. I am also confused about the proportions part of the passage. Any help would be truly appreciated. Thank you ## Answer by Neeraj (score 2, accepted) https://quant.stackexchange.com/a/23219 First understand that following proposition are based on CAPM. To understand how price and expected returns are related you can read following answer. It answer to different question but example describe in this answer will let you to understand how both are related to each other. Now come to your second confusion > .....that the amount of that investment in portfolio M is proportional to the amount of that investment First focus what is market portfolio under CAPM? Market portfolio is sum over, or aggregate, of the portfolios held by each individual in the economy. Lending and borrowing will cancel out (because each lender has a corresponding borrower), and market portfolio will comprise only the value of the aggregate risky portfolio, equal to the entire wealth of the economy. CAPM assumes each investor hold similar portfolio i.e. market portfolio. So each investor hold same proportion of every security in their portfolio. This means that if the weight of Wipro stock in each common risky portfolio is 1%, then Wipro also will constitute 1% of the market portfolio (common sense). This is the same thing that author want to convey from his statement.
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