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James Tobin (1958) added the notion
of leverage to
portfolio theory by incorporating into the analysis an
asset which pays a
risk-free rate. By combining a risk-free
asset with a portfolio on the
efficient
frontier, it is possible to construct portfolios whose
risk-return profiles are superior to those of portfolios on the efficient
frontier. Consider Exhibit 1:
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The capital market line is the tangent
line to the efficient frontier that passes through the risk-free
rate on the expected return axis. |
In Exhibit 1, the risk-free rate is assumed to be 5%, and
a tangent line—called the capital market
line—has been drawn to the efficient frontier passing through the
risk-free rate. The point of tangency corresponds to a portfolio on the
efficient frontier. That portfolio is called the
super-efficient portfolio.
Using the risk-free asset, investors who hold the
super-efficient portfolio may:
leverage their position by
shorting the risk-free
asset and investing the proceeds in additional holdings in the
super-efficient portfolio, or
de-leverage their position by selling some of their
holdings in the super-efficient portfolio and investing the proceeds in
the risk-free asset.
The resulting portfolios have risk-reward profiles which
all fall on the capital market line. Accordingly, portfolios which combine
the risk free asset with the super-efficient portfolio are superior from a
risk-reward standpoint to the portfolios on the efficient frontier.
Tobin concluded that portfolio construction should be a
two-step process. First, investors should determine the super-efficient
portfolio. This should comprise the risky portion of their portfolio.
Next, they should leverage or de-leverage the super-efficient portfolio to
achieve whatever level of risk they desire. Significantly, the composition
of the super-efficient portfolio is independent of the investor's appetite
for risk. The two decisions:
the composition of the risky portion of the investor's
portfolio, and
the amount of leverage to use, are entirely independent of one another. One decision has
no effect on the other. This is called Tobin's
separation theorem.
William Sharpe's (1964)
capital
asset pricing model (CAPM) demonstrates that, given strong simplifying
assumptions, the super-efficient portfolio must be the
market portfolio.
From this standpoint, all investors should hold the market portfolio
leveraged or de-leveraged to achieve whatever level of risk they desire.
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beta—a
measure of the systematic risk of a portfolio.
capital asset pricing
model—a model for asset pricing and portfolio construction.
efficient frontier—a
set of portfolios that each maximize expected return for a given
level of risk.
leverage Debt financing or anything that can similarly magnify the risk
and reward of an investment.
market risk Exposure to the uncertain market value of a portfolio.
portfolio theory—a body of
theory for how risk averse investors construct portfolios. |
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The following books offer different
perspectives on portfolio theory and the capital market line. Bernstein (1993)
is a must-read history of finance during the 20th century. Body,
Kane and Marcus (2004)
is the standard university text on finance. For the practitioner's
perspective, see Fabozzi and Markowitz (2002).
All three books are exceptional.
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Zvi Bodie, Alex Kane, Alan J. Marcus |
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2004 |
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Sharpe, William
F. (1964). Capital asset prices: A theory of market equilibrium
under conditions of risk, Journal of Finance, 19 (3),
425-442.
Tobin,
James (1958). Liquidity preference as behavior towards risk,
The Review of Economic Studies, 25, 65-86. |
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