The version of the CAPM developed by Sharpe (1964) and Lintner (1965) has
never been an empirical success. In the early empirical work, the Black (1972)
version of the model, which can accommodate a flatter trade off of average return
for market beta, has some success. But in the late 1970s,research begins to uncover
variables like size, various price ratios and momentum that add to the explanation
of average returns provided by beta. The problems are serious enough to invalidate
most applications of the CAPM.
For example, finance textbooks often recommend using the Sharpe-Lintner
CAPM risk-return relation to estimate the cost of equity capital. The prescription is
to estimate a stock's market beta and combine it with the risk-free interest rate and
the average market risk premium to produce an estimate of the cost of equity. The
typical market portfolio in these exercises includes just U.S. common stocks. But
empirical work, old and new, tells us that the relation between beta and average
return is flatter than predicted by the Sharpe-Lintner version of the CAPM. As a result, CAPM estimates of the cost of equity for high beta stocks are too high
(relative to historical average returns) and estimates for low beta stocks are too low
(Friend and Blume, 1970). Similarly, if the high average returns on value stocks
(with high book-to-market ratios) imply high expected returns, CAPM cost of
equity estimates for such stocks are too low.'
The CAPM is also often used to measure the performance of mutual funds and
other managed portfolios. The approach, dating to Jensen (1968), is to estimate
the CAPM time-series regression for a portfolio and use the intercept Uensen's
alpha) to measure abnormal performance. The problem is that, because of the
empirical failings of the CAPM, even passively managed stock portfolios produce
abnormal returns if their investment strategies involve tilts toward CAPM problems
(Elton,Gruber, Das and Hlavka, 1993).For example, funds that concentrate on low
beta stocks, small stocks or value stocks will tend to produce positive abnormal
returns relative to the predictions of the Sharpe-Lintner CAPM, even when the
fund managers have no special talent for picking u'inners.
The CAPM, like Markowitz's (1952, 1959) portfolio model on which it is built,
is nevertheless a theoretical tour de force. We continue to teach the CAPM as an
introduction to the fundamental concepts of portfolio theory and asset pricing, to
be built on by more complicated models like Merton's (1973) ICAF'M. But we also
warn students that despite its seductive simplicity, the CAPM's empirical problems
probably invalidate its use in applications.