The three-factor model captures variations in asset returns that the
CAPM misses. Behavioralists argue that violations of the CAPM
reflect irrational pricing that the three-factor model catches with the
B/M factor. In fact, it is impossible to tell whether the problem in
explaining returns is because of irrational pricing or rational pricing in
an incomplete model. Testing the CAPM is difficult because of lack
of theoretical or empirical clarity on what constitutes the market
portfolio. Some argue that it is impossible to test the CAPM because empirical results test whether the market portfolio proxy is efficient
but tell nothing about the CAPM. Efforts to find a reasonably efficient
proxy have extended the market portfolio to include assets other than
stocks and international assets. Still, the market proxy is ineffective
because adding the B/M and other variables in regressions effectively
annuls the CAPM-predicted beta–expected return relationship.
The authors conclude that the power of variables other than beta to
explain average returns invalidates most CAPM applications. They
specifically reject using the CAPM to estimate the cost of equity
capital and to evaluate performance of mutual fund managers.