A great many are puzzled by the difference in returns to equities and bonds. Some have tried to explain the difference in terms of risk.
Bonds provide a steady income while equities are volatile, hence the risk premium.
Others have correctly pointed out that the magnitude of the difference significantly exceeds any reasonable estimate of risk. Glassman and Hasset apparently believe that the difference is accounted for by investors irrational behaviour in demanding an excessive risk premium. It follows from this view that markets have been undervalued for years because the excessive estimate of risk by investors has kept stock prices below their true value. Therefore, once this fact is realised the risk premium will fall and stock prices will surge. Now according to the efficient market hypothesis markets have near-perfect knowledge.