the deviations of the P/E from its historical average are in fact quite modest. But suppose that we see them as significant, that we believe they indicate the expected return on stocks is unusually low relative to history. Is it low with respect to the expected return on other assets? A central aspect of the crisis has been the decrease in the interest rate on bonds, short and long. According to the yield curve, interest rates are expected to remain quite low for the foreseeable future. The expected return on stocks may be lower than it used to be, but so is the expected return on bonds.
Pointer from Mark Thoma.
Their point is that when interest rates are low, you can justify exceeding historical norms for the price-earnings ratio on stocks. I made a similar point about the price-earnings ratio for real estate relative to interest rates during the housing bubble.