The three-factor model captures variations in asset returns that the CAPM misses. behaviourist 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 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 stock and international assets. Still, the market proxy is ineffective because adding the B/M and other variables in regressions effectively annals the CAPM-predicted beta-expected return relationship.
The authors conclude that power of variable 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.