the three-factor model captures variations in asset returns that the CAPM misses. Behavioralists argue that violations of the CAPM reflect irrational pricing that three-factor model catches with the Book-to-Market. In fact, it is impossible to tell whether the problem in explaning 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.